block, a lower consumption of local anesthetics postoperatively, and less need for rescue pain
medication.58 Several other studies, however, have found no significant differences in local anesthetic
required, speed of onset for motor and sensory block, visual analog scores, or opioid consumed
postoperatively when comparing stimulating with nonstimulating catheters.59–62 The most recent metaanalysis investigating 649 patients comparing stimulating and nonstimulating catheters from 11 trials
showed a statistically significant benefit in analgesic effect from stimulating catheters.63 Significantly,
these studies did not use ultrasound guidance and only used nerve stimulation for localization of the
nerve and confirmation of the catheter position. The position of a nonstimulating catheter can be
confirmed by bolusing local anesthetic or saline through the catheter and visualizing the spread under
ultrasound. If normal saline or local anesthetic is bolused through the catheter, it must be noted that
should replacement of the catheter be necessary, electrical nerve stimulation will no longer be
possible.64 A nonconducting solution, such as dextrose 5% in water, may be used to ascertain the
catheter tip location and preserve the ability to stimulate.65 Any advantage of stimulating over
nonstimulating catheters placed with ultrasound guidance may be even further diminished because the
spread of the local anesthetic solutions injected through the catheter (as evidenced by ultrasound) is
the gold standard of documenting proper catheter placement, rather than a specific motor response to
nerve stimulation (Figures 3-12 and 3-13).
FIGURE 3-12. Continuous nerve block set with stimulating catheter, including lidocaine 1%,
lidocaine 1% with epinephrine, needles, syringes, and gauze, sterile sponges with iodine solution,
stimulating needle, securement device, stimulating catheter, sterile drape and swabstick pack, and
adaptor.
FIGURE 3-13. Nonstimulating catheter set, including nonstimulating catheter, extension tubing,
clamp-style catheter connector, 2-inch stimulating Tuohy needle, 4-inch stimulating Tuohy needle, and
label.
Securing Perineural Catheters
Dislodgement of a catheter is relatively common and leads to ineffective analgesia and requires
reinsertion of the catheter. There are a variety of methods and devices for securing indwelling
continuous catheters, most of which incorporate some means of fixing the device and/or catheter to the
skin via adhesive tape on one side of the device.
Some practitioners tunnel the indwelling catheters to better secure them, although there is no data
documenting that tunneling a catheter decreases the incidence of dislodgement. The benefits of
tunneling should be weighed against the potential for dislodging the catheter in the process of needle
insertion. If the decision is made to tunnel the catheter, then application of a topical skin adhesive to
the puncture site that the catheter passes through can help secure the catheter and prevent leakage of
local anesthetic. This is due to the fact that the puncture sites produced by catheters have larger
diameter than the catheters themselves. The catheter should be covered with a transparent, sterile
occlusive dressing to allow daily inspection of the catheter exit site. This allows for monitoring
catheter migration and early signs of infection.66
Infusion Pumps
Patients are increasingly being sent home with peripheral nerve catheters attached to portable infusion
pumps that ensure the accurate and reliable delivery of local anesthetic. The pumps can be either
elastomeric or electronic. The elastomeric pumps use a nonmechanical balloon mechanism to infuse
local anesthetics and consist of an elastomeric membrane within a protective shell. The pressure
generated on the fluid when the balloon is stretched is determined by the material of the elastomer
(e.g., latex, silicon, or isoprene rubber) and its shape.67 These pump sets typically contain an
elastomeric pump with a fill port, a clamp, an air-eliminating filter, a variable controller, a flow rate
dial, a rate-changing key, and a lockable cover. Most electronic pumps can hold 400 mL of local
anesthetic, and the anesthesiologist can easily program the concentration, rate, and volume. These
pumps are lightweight, typically come with carrying cases, and do not impose any limitations on
mobility for the patient. One study found that the elastomeric pumps were as effective as electronic
pumps in providing analgesia following ambulatory orthopedic surgery; however, the elastomeric
pumps led to higher patient satisfaction scores due to fewer technical problems.68 However,
underfilling the elastomeric pump results in a faster flow rate, whereas overfilling results in a slower
rate. The elastomeric pump flow rate is also affected by changes in temperature that affect the solution
viscosity. Recently, the elastomeric pump was shown to have technical difficulties with 20.5% not
deflating correctly after being attached to the catheter resulting in insufficient analgesia.69 The patient
should be given emergency contact information and be informed of the signs and symptoms of
excessive local anesthetic absorption. Typically the catheter remains in place for 2 to 3 days
postoperatively, and an anesthesiologist or another health care worker guides the patient through the
removal of the catheter over the phone.
Nerve Stimulators
The advent of nerve stimulation has been a great advance in the performance of regional anesthesia.
Because the electrical properties of a nerve stimulator contribute to the performance of a successful
PNB, practitioners should be familiar with the model used in their institution. Past models of
electrical nerve stimulators have used a constant voltage system. However, the current, not the
voltage, stimulates a nerve. Therefore, the amplitude of those nerve stimulators required constant
adjustment to maintain a desirable current output. Ideally, the current output of a nerve stimulator
should not change as the needle is being advanced through various resistances encountered from the
tissue, needle, and connectors. Resistance is a measure of the resistance to flow of alternating current
through tissue, and there is an inverse relationship between resistance and current thresholds
necessary to elicit a motor response.70 Most modern models deliver a constant current output in the
presence of varied resistance. Settings that can be altered on these models include frequency, pulsewidth, and current milliamperes. Nerve stimulators are described in a greater detail in Chapter 4.
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4
Electrical Nerve Stimulators and Localization of Peripheral Nerves
Martin Simpel and Andre van Zundert
History of Electrical Nerve Stimulation
Quick Facts
1780: Galvani1
was the first to describe the effect of electrical neuromuscular stimulation
1912: Perthes2
developed and described an electrical nerve stimulator
1955: Pearson3
introduced the concept of insulated needles for nerve location
1962: Greenblatt and Denson4
introduced a portable solid-state nerve stimulator with variable current
output and described its use for nerve location
1973: Montgomery et al5
demonstrated that noninsulated needles require significantly higher current
amplitudes than the insulated needles
1984: Ford et al6
reported a lack of accuracy with noninsulated needles once the needle tip passed the
target nerve
Ford et al suggested the use of nerve stimulators with a constant current source, based on the
comparison of the electrical characteristics of peripheral nerve stimulators7,8
The use of nerve stimulation became commonplace in clinical practice only in the mid- to late
1990s. Research on the needle–nerve relationship and the effect of stimulus duration ensued.9–11
More recently, the principles of electrical nerve stimulation were applied to surface mapping of
peripheral nerves using percutaneous electrode guidance (PEG)12–15 for confirmation and epidural
catheter placement16–18 and peripheral catheter placement.19 This chapter discusses the
electrophysiology of nerve stimulation, electrical nerve stimulators, various modes of localization of
peripheral nerves, and integration of the technology into the realm of modern regional anesthesia.
What Is Peripheral Electrical Nerve Stimulation?
Nerve stimulation is a commonly used method for localizing nerves before the injection of local
anesthetic. Electrical nerve stimulation in regional anesthesia is a method of using a low-intensity (up
to 5 mA) and short-duration (0.05–1 ms) electrical stimulus (at 1–2 Hz repetition rate) to obtain a
defined response (muscle twitch or sensation) to locate a peripheral nerve or nerve plexus with an
(insulated) needle. The goal is to inject a certain amount of local anesthetic in close proximity to the
nerve to block nerve conduction and provide a sensory and motor block for surgery and/or,
eventually, analgesia for pain management. The use of nerve stimulation can also help to avoid an
intraneural intrafascicular injection and, consequently, nerve injury.
Electrical nerve stimulation can be used for a single-injection technique, as well as for guidance
during the insertion of continuous nerve block catheters. More recently, ultrasound (US) guidance and,
in particular, the so-called dual guidance technique in which both techniques (peripheral nerve
stimulation [PNS] and US) are combined, has become a common practice in many institutions.
Indications for the Use of PNS
In principle, almost all plexuses or other larger peripheral nerves can be located using PNS.20 The
goal of nerve stimulation is to place the tip of the needle (more specifically, its orifice for injection)
in close proximity to the target nerve to inject the local anesthetic in the vicinity of the nerve. The
motor response (twitch) to PNS is objective and reliable and independent from the patient’s
(subjective) response. Nerve stimulation is often helpful to confirm that the structure imaged with
ultrasound (US) is actually the nerve that is sought. This is because the needle–nerve relationship may
not always be visualized on US; an unexpected motor response can occur, alerting the operator that
the needle tip is already in close proximity to the nerve. Likewise, the occurrence of a motor response
at a current intensity of <0.3-0.2 mA can serve as an indicator of an intraneural needle placement.
Although this response may not always be present even with an intraneural needle position (low
sensitivity), its presence is always indicative of intraneural placement (high specificity).
The disadvantages of PNS are the need for additional equipment (nerve stimulator and insulated
needles), the greater cost of insulated needles, and abnormal physiology or anatomy where it may be
difficult to elicit a motor response.
TIPS
PNS is an adjunct to and not a substitute for knowledge of anatomy.
Presence of neurologic disorders (e.g., polyneuropathy) can result in difficulties in obtaining a motor
response. The use of a longer pulse duration (0.3 or 1.0 ms, instead of 0.1 ms), may be helpful in
these cases.
PNS is not reliable in a patient receiving muscle relaxants.
PNS can be used in patients who have received central neuraxial blocks.
Basics of Neurophysiology and Electrophysiology
Membrane Potential, Resting Potential, Depolarization, Action Potential, and
Impulse Propagation
All living cells have a membrane potential (a voltage potential across their membrane, measured
from the outside to the inside), which varies (depending on the species and the cell type) from about
−60 mV to −100 mV. Nerve and muscle cells in mammals typically have a membrane potential
(resting potential) of about −90 mV.
Only nerve and muscle cells have the capability of producing uniform electrical pulses, the socalled action potentials (also called spikes), which are propagated along their membranes,
especially along the long extensions of nerve cells (nerve fibers, axons). A decrease in the electric
potential difference (e.g., from −90 mV to −55 mV, or depolarization) elicits an action potential. If
the depolarization exceeds a certain threshold, an action potential or a series of action potentials is
generated by the nerve membrane (also called firing) according to the all-or-nothing rule, resulting in
propagation of the action potential along the nerve fiber (axon). To depolarize the nerve membrane
from outside the cell (extracellular stimulation), the negative polarity of the electrical stimulus is
more effective in removing the positive charge from the outside of the membrane. This in turn
decreases the potential across the membrane toward the threshold level.
There are several types of nerve fibers. Each fiber type can be distinguished anatomically by their
diameter and degree of myelinization. Myelinization is formed by an insulating layer of Schwann cells
wrapped around the nerve fibers. These characteristics largely determine the electrophysiologic
behavior of different nerve fibers, that is, the speed of impulse propagation of action potentials and
the threshold of excitability. Most commonly, the distinguishing features are motor fibers (e.g., Aα,
Aβ) and pain fibers (C). The Aα motor fibers have the largest diameter and highest degree of
myelinization and therefore the highest speed of impulse propagation and a relatively low threshold
level to external stimulation. C-fibers (which transmit severe, dull pain) have very little to no
myelinization and are of smaller diameter. Consequently, the speed of propagation in these fibers is
relatively low, and the threshold levels to external stimulation, in general, are higher.
There are several other, efferent fibers, which transmit responses from various skin receptors or
muscle spindles (Aδ). These are thinner than Aα fibers and have less myelinization. Some of these
(afferent) sensory fibers, having a relatively low threshold level, transmit the typical tingling
sensation associated with a lower level of pain sensation when electrically stimulated. Such sensation
can occur during transcutaneous stimulation before a motor response is elicited.
The basic anatomic structure of myelinated Aα fibers (motor) and nonmyelinated C fibers (pain) is
shown schematically in Figure 4-1. The relationship between different stimuli and the triggering of the
action potential in motor and pain fibers is illustrated in Figures 4-2A, B.
FIGURE 4-1. (A) Schematic anatomic and electrophysiologic structure of nerve fibers of myelinated
and (B) unmyelinated nerve fibers.
FIGURE 4-2. (A) Action potential, threshold level, and stimulus. Motor fibers have a short chronaxy
because of the relatively low capacitance of their myelinated membrane (only the area of the nodes of
Ranvier count; see Figure 4-1), therefore, it takes only a short time to depolarize the membrane up to
the threshold level. (B) Pain fibers have a long chronaxy because of the higher capacitance of their
nonmyelinated membrane (the entire area of the membrane counts); therefore, it takes a longer time to
depolarize the membrane up to the threshold level. Short impulses (as indicated by the vertical dotted
line) would not be able to depolarize the membrane to its threshold level.
Threshold Level, Rheobase, Chronaxy
A certain minimum current intensity is necessary at a given pulse duration to reach the threshold level
of excitation. The lowest threshold current (at infinitely long pulse durations) is called rheobase. The
pulse duration (pulse width) at double the rheobase current is called chronaxy. Electrical pulses with
the duration of the chronaxy are most effective (at relatively low amplitudes) to elicit action
potentials. This is the reason why motor response can be elicited at such short pulse duration (e.g.,
0.1 ms) at relatively low current amplitudes while avoiding the stimulation of C-type pain fibers.
Typical chronaxy figures are 50 to 100 μs (Aα fibers), 170 μs (AΔ fibers), and ≥400 μs (C fibers).
Figure 4-3 illustrates the relationship of the rheobase to chronaxy for motor fibers versus pain nerve
fibers.
FIGURE 4-3. (A) Comparison of threshold curves, chronaxy, and rheobase level of motor (high
speed) and pain fibers (low speed). (B) Experimental data, threshold amplitudes obtained with
percutaneous stimulation (Stimuplex Pen and Stimuplex HNS 12). Stimulation obtained with
percutaneous stimulation of the median nerve near the wrist looking for motor response of the thumb.
(C) Experimental data, threshold amplitudes obtained with percutaneous stimulation (Stimuplex Pen
and Stimuplex HNS 12). Stimulation of the median and radial nerves near the wrist and at the
midforearm looking for electric paresthesia (tingling sensation) in the middle and ring finger (median
nerve) or superficial pain sensation near the wrist (radial nerve), respectively.
Impedance, Impulse Duration, and Constant Current
The electrical circuit is formed by the nerve stimulator, the nerve block needle and its tip, the tissue
characteristics of the patient, the skin, the skin electrode (grounding electrode), and the cables
connecting all the elements. The resistance of this circuit is not just a simple Ohm’s resistor equation
because of the specific capacitances of the tissue, the electrocardiogram (ECG) electrode to skin
interface, and the needle tip, which influence the overall resistance. The capacitance in the described
circuit varies with the frequency content of the stimulation current, and it is called impedance, or a
so-called complex resistance, which depends on the frequency content of the stimulus. In general, the
shorter the impulse, the higher its frequency content, and, consequently, the lower the impedance of a
circuit with a given capacitance. Conversely, a longer pulse duration has a lower frequency content.
As an example, for a 0.1-ms stimulus, the main frequency content is 10 kHz plus its harmonics;
whereas for a 1.0-ms impulse, the main frequency content is 1 kHz plus harmonics. In reality, the
impedance of the needle tip and the electrode to skin impedance have the highest impact. The
impedance of the needle tip largely depends on the geometry and insulation (conductive area). The
electrode to skin impedance can vary considerably between individuals (e.g., type of skin, hydration
status) and can be influenced by the quality of the ECG electrode used.
Because of the variable impedance in the circuit, created primarily by the needle tip and electrode
to skin interface, a nerve stimulator with a constant current source and sufficient (voltage) output
power is important to use to compensate for the wide range of impedances encountered clinically.
Clinical Use of PNS
Proper Setup and Check of the Equipment
The following are a few important aspects for successful electrolocalization of the peripheral nerves
using PNS:
Use a high-quality nerve stimulator and a high-accuracy constant current source.
Use insulated nerve stimulation needles with a small conductive area at the tip. The smaller the
conductive area, the higher the current density is at the tip, and the greater spatial discrimination in the
near field.
Use high-quality skin electrodes with a low impedance.
TIPS
Some lower priced ECG electrodes can have too high of an impedance/resistance. This limits their
suitability for use with nerve stimulation.
Good quality skin electrodes have an impedance of a maximum of a few hundred ohms.
Typically, biomedical engineers use a dummy resistor (e.g., 10 kOhm), which allows them to check
that the nerve stimulator and cables are functioning properly.
Before starting the procedure, check for the proper functioning of the nerve stimulator and the
connecting cables.
During nerve stimulator-assisted nerve localization, the negative pole (cathode) should be connected
to the stimulating electrode (needle) and the positive pole (anode) to the patient’s skin.
The design of the connectors should prevent a faulty polarity connection.
Connect the nerve stimulation needle to the nerve stimulator (which should be turned on), and set the
current amplitude and duration to the desired levels.
For superficial blocks, select 1.0 mA as a starting current intensity.
For deep blocks, select 1.5 mA as a starting current intensity.
Select between 0.1 and 0.3 ms of current duration for most purposes.
For more technical details and how to operate a specific nerve stimulator, refer to the instructions for
use supplied with the stimulator.
Transcutaneous Nerve Mapping
Electrolocalization of peripheral nerves is typically accomplished by inserting a needle into the tissue
and advancing the needle toward the expected location of the nerve(s) of interest. However, a nerve
mapping pen can be used to locate superficial nerves (up to a maximum depth of approximately 3 cm)
with transcutaneous nerve stimulation before the nerve block needle is inserted. Transcutaneous nerve
mapping is particularly useful when identifying the best site for needle insertion in patients with
difficult anatomy or when the landmarks prove difficult to indentify. Figure 4-4 shows three examples
of commercially available nerve mapping pens.
FIGURE 4-4. Tip configuration of several commercially available nerve mapping peripheral nerve
stimulators. From left to right: Stimuplex Pen, B. Braun Melsungen (Germany); nerve mapping pen,
Pajunk (Germany); NeuroMap, HDC (USA).
Nerve mapping is also very useful when training anesthesia residents. It should be noted that
longer stimulus duration (e.g., 1 ms) is needed to accomplish transcutaneous nerve stimulation,
because the energy required to stimulate transcutaneously is larger. The electrode tip of the pen
should have an atraumatic ball-shaped tip. The conductive tip diameter should not be larger than
approximately 3 mm to provide sufficient current density and spatial discrimination, which may not
be the case with larger tip diameters. Some nerve stimulators do not provide the required impulse
duration of 1 ms or a strong enough constant current source (5 mA at minimum 12-kOhm output load)
to perform nerve mapping. Therefore, it is recommended that the mapping pen and the nerve
stimulator be paired, ideally by acquiring them from the same manufacturer.
The transcutaneous stimulation often results in a sensation reported by the patient as tingling,
pinprick, or a slight burning sensation. The perception varies greatly among individuals. Most people
tolerate transcutaneous stimulation with a nerve mapping pen very well; however, some individuals
describe it as uncomfortable or even painful (depending on the stimulus amplitude and duration).
However, the amount of energy delivered by nerve stimulators with a maximum output of 5 mA at 1
msec pulse duration is far too low to create any injury of the skin or the nerves. A moderate
premedication is usually sufficient to make the procedure well tolerated by patients.
Percutaneous Electrode Guidance
PEG10,11 combines the transcutaneous nerve stimulation (nerve mapping) with nerve block needle
guidance (Figure 4-5). In essence, a small aiming device is mounted and locked onto a conventional
nerve block needle, which allows the conductive needle tip to make contact with the skin without
scratching or penetrating the skin. Once the best response is obtained, the needle is advanced through
the skin in the usual fashion and the remainder of the apparatus continues to stabilize the needle and
guide it toward the target. The device also allows the operator to make indentations in the skin and
tissue so the initial distance between the needle tip at the skin level and the target nerve is reduced
and the nerve block needle has less distance to travel through tissue. The technique allows for
prelocation of the target nerve(s) before skin puncture.
FIGURE 4-5. Percutaneous electrode guidance technique using Stimuplex Guide (B. Braun
Melsungen, Germany) during a vertical infraclavicular block procedure.
Operating the Nerve Stimulator
The starting amplitude used for nerve stimulation depends on the local practice and the projected
skin-nerve depth. For superficial nerves, amplitude of 1 mA at 0.1 (or 0.3) ms impulse duration to
start is chosen in most cases. For deeper nerves, it may be necessary to increase the initial current
amplitude between 1.5 and 3 mA until a muscle response is elicited at a safe distance from the nerve.
Too high current intensity, however, can lead to direct muscle stimulation or discomfort for the
patient, both of which are undesirable.
Once the sought-after muscle response is obtained, the current intensity amplitude is gradually
reduced and the needle is advanced further slowly. The needle must be advanced slowly to avoid too
rapid advancement between the stimuli. Advancement of the needle and current reduction are
continued until the desired motor response is achieved with a current of 0.2–0.5 mA at 0.1 ms
stimulus duration. The threshold level and duration of the stimulus are interdependent; in general, a
short pulse duration is a better discriminator of the distance between the needle and the nerve.20 When
the motor twitch is lost during needle advancement, the stimulus intensity first should be increased to
regain the muscle twitch rather than move the needle blindly. Once a proper motor response is
obtained with a current of 0.2–0.5 mA (most nerve blocks), the needle is positioned correctly for an
injection of local anesthetic. A small test dose of local anesthetic is injected, which abolishes the
muscle twitch. Then the total amount of local anesthetic appropriate for the desired nerve block is
injected. Of note, the highly conductive injectate (e.g., local anesthetic or normal saline solution)
short-circuits the current to the surrounding tissue, effectively abolishing the motor response. In such
situations, increasing the amplitude may not bring back the muscle twitch. Tsui and Kropelin21
demonstrated that injection of dextrose 5% in water (D5W) (which has a low conductivity) does not
lead to loss of the muscle twitch if the needle position is not changed.
It should be remembered that the absence of the motor response with a stimulating current even up
to 1.5 mA does not rule out an intraneural needle placement (low sensitivity). However, the presence
of a motor response with a low-intensity current (<0.2–0.3 mA) occurs only with intraneural and,
possibly, intrafascicular needle placement. For this reason, if the motor response is still present at
<0.2–0.3 mA (0.1 ms), the needle should be slightly withdrawn to avoid the risk of intrafascicular
injection. Figure 4-6A–C illustrates the principle of the needle to nerve approach and its relation to
the stimulation.
FIGURE 4-6. (A) Stimulating needle at a distance to the nerve and high stimulus current elicits a
weak evoked motor response. (B) Stimulation needle close to the nerve and high stimulus current
eliciting a strong muscle twitch. (C) Stimulating needle close to the nerve and low (near threshold)
stimulus current elicits a strong evoked motor response.
To avoid or minimize discomfort for the patient during the nerve location procedure, it is
recommended that a too high stimulating current be avoided. The needle should not be advanced too
fast because it can increase the risk of injuries and the evoked motor response may be missed.
The Role of Impedance Measurement
Measurement of the impedance can provide additional information if the electrical circuit is optimal.
Theoretically, impedance can identify an intraneural or intravascular placement of the needle tip. Tsui
and colleagues22 reported that the electrical impedance nearly doubles (12.1–23.2 kOhm), which is
significant, when the needle is advanced from an extraneural to intraneural position in a porcine
sciatic nerve. Likewise, injection of a small amount of (D5W), which has a high impedance, results in
a significantly higher increase of impedance in the perineural tissue than it does within the
intravascular space.23 Thus measurement of the impedance before and after dextrose injection can
potentially detect intravascular placement of the needle tip, thus identifying the placement before the
injection of local anesthetic. In this report, the perineural baseline impedance [25.3 (±2.0) kOhm]
was significantly higher than the intravascular [17.2 (±1.8) kOhm]. Upon injection of 3 mL of D5W,
the perineural impedance increased by 22.1 (±6.7) kOhm to reach a peak of 50.2 (±7.6) kOhm and
remained almost constant at about 42 kOhm during the 30-second injection time. By contrast,
intravascular impedance increased only by 2.5 (±0.9) kOhm, which is significantly less compared
with the perineural needle position. At the present time, however, more data are needed before these
findings can be incorporated as an additional safely monitoring method in clinical practice.
Sequential Electrical Nerve Stimulation
Current nerve stimulation uses stimuli of identical duration (typically 0.1 ms), usually at 1 or 2 Hz
repetition frequency. A common problem during nerve stimulation is that the evoked motor response
is often lost while moving the needle to optimize its position. In such cases, it its recommended that
the operator either increase the stimulus amplitude (mA) or increase the impulse duration (ms), the
latter of which may not be possible. Alternatively, the operator can take a couple of steps, depending
on type of the nerve stimulator used. The SENSe (sequential electrical nerve stimulation) technique
incorporates an additional stimulus with a longer pulse duration after two regular impulses at 0.1
msec duration, creating a 3 Hz stimulation frequency.24 The third longer impulse delivers more charge
than the first two and therefore has a longer reach into the tissue. Consequently, an evoked motor
response often is elicited at 1 Hz, even when the needle is distant from the nerve. Once the needle tip
is positioned closer to the nerve, muscle twitches are seen at 3 Hz. The advantage of the SENSe is
that a motor response (at 1/second) is maintained even when the motor response previously elicited
by the first two impulses is lost due to slight needle movement. This feature helps prevent the
operator from moving the needle “blindly”.24
Figure 4-7 shows examples of the particular SENSe impulse patterns at different stimulus
amplitudes. Eventually the target threshold amplitude remains the same as usual (about 0.3 mA) but at
3 stimuli per second. With the SENSe technique, a motor response at only 1/second indicates that the
needle is not yet placed correctly.
FIGURE 4-7. Sequential electrical nerve stimulation (SENSe) impulse pattern of the Stimuplex HNS
12 nerve stimulator (B. Braun Melsungen, Germany) depending on the actual stimulus amplitude. The
impulse duration of the third impulse decreases with the stimulus amplitude below 2.5 mA from 1.0
ms to a minimum of 0.2 ms compared with the constant impulse duration of 0.1 ms of the first two
impulses. (A) Impulse pattern at 0.3 mA (threshold level). (B) Impulse pattern at 1.0 mA. (C) Impulse
pattern at 2.0 mA.
Troubleshooting During Nerve Stimulation
Table 4-1 lists the most common problems encountered during electrolocalization of the peripheral
nerves and the corrective action.
TABLE 4-1 Common Problems during Electrolocalization of Nerves and Corrective Actions
Characteristics of the Modern Equipment for Nerve Stimulator
Guided Peripheral Nerve Blocks
Most Important Features of Nerve Stimulators20,25
Electrical Features
An adjustable constant current source with an operating range of 10 kOhm, minimally, output load
(impedance) and ideally at ≥15 kOhm.
A precisely adjustable stimulus amplitude (0–5 mA): An analog control dial is preferred over
up/down keys.
A large and easy-to-read digital display of actual current flowing to maintain precise control of the
stimulus.
A selectable pulse duration (width), at least between 0.1 ms and 1.0 ms, to allow the operator to
selectively stimulate motor fibers (0.1 ms) and to stimulate sensory fibers as well (1.0 ms).
A stimulus frequency between 1 and 3 Hz (meaning 1–3 pulses per second) because the use of a too
low frequency can lead to “blind” advancement of the needle in between stimuli. Use of a too high
frequency will lead to superimposing of muscle twitches, which makes the detection of twitches more
difficult.
A monophasic rectangular output pulse to provide reproducible stimuli.
Configurable start-up parameters so the machine will comply with the hospital protocol and to avoid
mistakes when multiple users are working with the same device.
A display of the circuit impedance (kOhm) is recommended to allow the operator to check the
integrity of the electrical circuit and to detect a potential intraneural or intravascular placement of the
needle tip.
An automatic self-check process of the internal functioning of the unit with a warning message if
something is wrong.
An optional remote control (handheld remote control or foot pedal).
Safety Features
Easy and intuitive use
A large and easy-to-read display
Limited current range (0–5 mA) because a too high amplitude may be uncomfortable to the patient
A display of all relevant parameters such as Amplitude (mA) [alternatively stimulus charge (nC)],
stimulus duration (ms), stimulus frequency (Hz), impedance (kOhm), and battery status
Clear identification of output polarity (negative polarity at the needle)
Meaningful instructions for use, with lists of operating ranges and allowed tolerances
Battery operation of the nerve stimulator, as opposed to electrical operation, provides intrinsic safety:
no risk of serious electric shock or burns caused by a short circuit to main supply of electricity
The maximum energy delivered by a nerve stimulator with 5 mA and 95 V output signal at 1 ms
impulse duration is only 0.475 mWs (see Section 7.3).
Combined units for peripheral (for PNB) and transcutaneous (for muscle relaxation measurement)
electrical nerve stimulation are not to be used because the transcutaneous function produces an
unwanted high energy charge
Alarms/warnings:
Open circuit/disconnection alarm (optical and acoustical)
Warning/indication if impedance is too high; that is, the desired current is not delivered
The display of actual impedance appears to be useful and recommendable
Near threshold amplitude indication or alarm
Low battery alarm
Internal malfunction alarm
Table 4-2 provides a comparison of the most important features of commonly used nerve
stimulators.
TABLE 4-2 Comparison of Most Relevant Features of Modern Nerve Stimulators
Stimulating Needles
Needle
A modern stimulating needle should have the following characteristics:
A fully insulated needle hub and shaft to avoid current leakage
The conductive electrode area should be able to accomplish higher current density at the tip for
precise nerve location
Depth markings for easy identification and documentation of the needle insertion depth
Figures 4-8A and B show a comparison of the electrical characteristics of noninsulated and
insulated needles with uncoated bevel (Figure 4-8A) and fully coated needles with a pinpoint
electrode (Figure 4-8B). Even though a noninsulated needle provides for discrimination (change in
threshold amplitude) while approaching the nerve, there is virtually no ability to discriminate once
the needle tip has passed the nerve. The discrimination near the nerve is more precise in needles with
a pinpoint electrode tip (Figure 4-8B) compared with needles with an uncoated bevel (Figure 4-8A).
FIGURE 4-8. (A) Threshold amplitude achieved with an uncoated needle and a coated needle with
an uncoated bevel. (B) Threshold amplitude achieved with a fully coated needle and a pinpoint
electrode.
Connectors
Connectors and cables should be fully insulated and include a safety connector to prevent current
leakage as well as the risk of electric charge if the needle is not connected to the stimulator. Extension
tubing with a Luer lock connector should be present for immobile needle techniques.
Visualization of the Needle Under Ultrasound Imaging
Because US imaging is more in use (in particular with the use of the “dual guidance” technique), the
importance of good visualization of the nerve block needle is becoming an additional important
feature. The visibility (distinct reflection signal) of the needle tip certainly is the most important
aspect because this is the part of the needle that is placed in the target area next to the nerve.
However, in particular when using the in-plane approach, the visibility of the needle shaft is of
interest as well because it helps to align the needle properly with the US beam and to visualize its
entire length up to the target nerve.
Stimulating Catheters
In principle, stimulating catheters function like insulated needles. The catheter body is made from
insulating plastic material and usually contains a metallic wire inside, which conducts the current to
its exposed tip electrode. Such stimulating catheters are usually placed using a continuous nerve block
needle, which is placed by first using nerve stimulation as described and acts as an introducer needle
for the catheter. Once this needle is placed close to the nerve or plexus to be blocked, the stimulating
catheter is introduced through it and the nerve stimulator is connected to the catheter. Stimulation
through the catheter should reconfirm that the catheter tip is positioned in close proximity to the target
nerve(s). However, it must be noted that the threshold currents with stimulating catheters may be
considerably higher. Injection of local anesthetic or saline (which is frequently used to widen the
space for threading the catheter more easily) should be avoided because this may increase the
threshold current considerably and can even prevent a motor response. D5W can be used to avoid
losing a motor response.21 Since the ultimate test for the properly positioned catheter is the
distribution of the local anesthetic, rather than evoked motor response, the role of the catheter
stimulating with US-guided blocks is not clear.
Recommendations for Best Practice
Adequate knowledge of anatomy
Correct patient positioning
Proper technique and equipment
Standard nerve stimulator settings for peripheral nerve blocks:
Stimulus duration: 0.1 ms for mixed nerves
Amplitude range: 0–5 mA or 0–1 mA (sufficient for superficial nerves)
Stimulus frequency: 2 or 3 Hz, or SENSe
Nerve stimulator check:
Check battery status
Check that all connections are placed properly (cable, needle, skin electrode)
Check the entire nerve stimulator function using a test resistor (this automatically checks connectors
and cable as well)
Needles:
Use fully insulated nerve block needles, Figure 4-8.
Use the appropriate gauge size and length (avoid too long needles; see Table 4-3)
TABLE 4-3 Stimulation Needle Sizes Recommended for Various Nerve Blocks
End point of nerve stimulation:
Threshold current 0.2 to 0.3 mA (at 0.1 ms)
Higher threshold current (≥0.5 mA) means the needle tip is too far from the nerve end point and block
failure becomes more likely
Lower threshold current (≤0.2 mA) signals a risk of intraneural/fascicular injection, and consequently,
the risk for neural damage increases
To avoid discomfort for the patient and take precautions for safety:
Use a low-intensity current nerve stimulation
Limit the stimulus energy by limiting the initial stimulus current amplitude: 1 mA (superficial blocks),
2 mA (deeper blocks), maximum 5 mA (e.g., psoas compartment and deep sciatic blocks), and the
stimulus duration; do not use long stimulus duration (1 ms) if it is not needed
Do not inject at exceedingly high pressure or if the threshold current is <0.2–0.3 mA (0.1 ms) to avoid
a intraneural/fascicular injection and subsequent neurologic damage
Apply appropriate anesthesia technique (e.g., infiltration of puncture site, light sedation)
Appendix: Glossary of Physical Parameters
Voltage, Potential, Current, Resistance/Impedance
Voltage (U) is the difference in electrical potential between two points carrying different amounts of
positive and negative charge. It is measured in volts (V) or millivolts (mV). Voltage can be compared
with the filled level of a water tank, which determines the pressure at the bottom outlet (Figure 4-9A).
In modern nerve stimulators using constant current sources, voltage is adapted automatically and
cannot nor needs to be influenced by the user.
FIGURE 4-9. Ohm’s law and principle of a constant current source. Functional principle of a
constant current source. (A) Low resistance R1
requires voltage U1
to achieve desired current I1
. (B)
High resistance causes current I to decrease to if voltage U remains constant
. (C) Constant current source automatically increases output voltage to to
comPNSate for the higher resistance R2
and, therefore, current I increases to the desired level of
.
Current (I) is the measure of the flow of a positive or negative charge. It is expressed in amperes
(A) or milliamperes (mA). Current can be compared with the flow of water.
A total charge (Q) applied to a nerve equals the product of the intensity (I) of the applied current
and the duration (t) of the square pulse of the current: .
The minimum current intensity (I) required to produce an action potential can be expressed by the
relationship
where, t = pulse duration, c = time constant of nerve membrane related to chronaxy.
The electrical resistance R limits the flow of current at a given voltage (see Ohm’s law) and is
measured in ohms (Ω) or kilo Ohms (kΩ).
If there is capacitance in addition to Ohm’s resistance involved (which is the case for any tissue),
the resistance becomes a so-called complex resistance, or impedance. The main difference between
the two is that the value of the impedance depends on the frequency of the applied voltage/current,
which is not the case for an Ohm’s resistor. In clinical practice, this means the impedance of the
tissue is higher for low frequencies (i.e., a long pulse duration) and lower for higher frequencies (i.e.,
a short impulse duration). Consequently, a constant current source (which delivers longer duration
impulses, e.g., 1 ms versus 0.1 ms) needs to have a stronger output stage (higher output voltage) to
compensate for the higher tissue impedance involved and to deliver the desired current. However, the
basic principles of Ohm’s law remain the same.
Ohm’s Law
Ohm’s law describes the relationship between voltage, resistance, and current according to the
equation:
or conversely,
This means that at a given voltage, current changes with resistance. If a constant current must be
achieved (as needed for nerve stimulation), the voltage has to adapt to the varying resistance of the
entire electrical circuit. For nerve localization in particular, the voltage must adapt to the resistance
of the needle tip, the electrode to skin interface, and the tissue layers. A constant current source does
this automatically. Ohm’s law and the functional principle of a constant current source are illustrated
in Figures 4-9A–C.
Coulomb’s Law, Electric Field, Current Density, and Charge
According to Coulomb’s law, the strength of the electric field and, therefore, the corresponding
current density (J) in relation to the distance from the current source is given by:
This means the current (or charge) that reaches the nerve decreases by a factor of 4 if the distance
to the nerve is doubled, or conversely, it increases by a factor of 4 if the distance is divided in half
(ideal conditions assumed).
The charge Q is the product of current multiplied by time and is given in ampere seconds (As) or
coulomb (C). As an example, rechargeable batteries often have an indication of Ah or mAh as the
measure of their capacitance of charge (kilo = 1000 or 103
; milli = 0.001 or 10−3; micro = 0.000001
or 10−6; nano = 0.000000001 or 10−9).
Energy of Electrical Impulses Delivered by Nerve Stimulators and Related
Temperature Effects
According to a worst-case scenario calculation, the temperature increase caused by a stimulus of 5
mA current and 1 ms duration, at a maximum output voltage of 95 V, would be <0.5 C, if all the
energy were concentrated within a small volume of only 1 mm3
and no temperature dissipation into
the surrounding tissue occurred. This calculation can be applied for the tip of a nerve stimulation
needle.
The maximum energy (E) of the electrical impulse delivered by a common nerve stimulator would
be:
The caloric equivalent for water is
One stimulus creates a temperature difference ΔT within 1 mm3
of tissue around the tip of a nerve
stimulation needle. For the calculation that follows, it is assumed that tissue contains a minimum of
50% water and the mass (M) of 1 mm3
of tissue is 1 mg.
That is, the maximum temperature increase in this worst-case scenario calculation is <0.5 C. In
practice, this means the temperature effect of normal nerve stimulation on the tissue can be neglected.
REFERENCES
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. Ford DJ, Pither CE, Raj PP. Electrical characteristics of peripheral nerve stimulators: Implications
for nerve localization. Reg Anesth. 1984;9:73-77.
. Pither CE, Ford DJ, Raj PP. The use of peripheral nerve stimulators for regional anesthesia, a review
of experimental characteristics, technique, and clinical applications. Reg Anesth. 1985;10:49-58.
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Med. 2002;27(2):227-228.
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peripheral nerves to facilitate peripheral plexus or nerve block. Reg Anesth Pain Med. 2002;27:261-
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5
Monitoring and Documentation
Jeff Gadsden
Introduction
The incidence of complications from general anesthesia has diminished substantially in recent
decades, largely due to advances in respiratory monitoring.1
The use of objective monitors such as
pulse oximetry and capnography allows anesthesiologists to quickly identify changing physiologic
parameters and intervene rapidly and appropriately.
In contrast, the practice of regional anesthesia has traditionally suffered from a lack of similar
objective monitors that aid the practitioner in preventing injury. Practitioners of peripheral nerve
blocks were made to rely on subjective end points to gauge the potential risk to the patient. This is
changing, however, with the introduction and adoption of standardized methods by which to safely
perform peripheral nerve blocks with the minimal possible risk to the patient. For example, instead of
relying on feeling “clicks,” “pops”, and “scratches” to identify needle tip position, the
anesthesiologist can now directly observe it using ultrasonography. It follows that advancements such
as this may help in reducing the three most feared complications of peripheral nerve blockade: nerve
injury, local anesthetic toxicity, and inadvertent damage to adjacent structures ("needle
misadventure").
Objective monitoring, and the rationale for its use, is discussed in the first part of this chapter. The
later section focuses on documentation of nerve block procedures, which is a natural accompaniment
to the use of these empirical monitors. The proper documentation of how a nerve block was
performed has obvious medicolegal implications and aids the future practitioner in choosing the best
nerve block regimen for that particular patient.
SECTION I: MONITORING
What Are the Available Monitors?
Monitors, as used in the medical sense, are devices that assess a specific physiologic state and warn
the clinician of impending harm. The monitors discussed in this chapter include nerve stimulation,
ultrasonography, and the monitoring of injection pressure. Each of these has its own distinct set of
both advantages and limitations. For this reason, these three technologies are best used in a
complementary fashion (Figure 5-1), to minimize the potential for patient injury, rather than just
relying on the information provided by one monitor alone. The combination of all three monitors is
likely to produce the safest possible environment in which to perform a peripheral nerve block.
FIGURE 5-1. Three modes of monitoring peripheral nerve blocks for patient injury. The overlapping
area of all three (yellow area) represents the safest means of performing a block.
A fourth monitor that many clinicians use regularly is the use of epinephrine in the local anesthetic.
Good evidence supports this practice as a means of improving safety during peripheral nerve blocks,
particularly in patients receiving higher doses of local anesthetic. First, it acts as a marker of
intravascular absorption. About 10 to 15 μg of epinephrine injected intravenously reliably increases
the systolic blood pressure >15 mm Hg, even in sedated or beta-blocked individuals (whereas a heart
rate increase is not reliable in sedated patients).2,3 The recognition of this increase permits the
clinician to halt the injection promptly and increase his or her vigilance for signs of systemic toxicity.
Second, epinephrine truncates the peak plasma level of local anesthetic, resulting in a lower risk for
systemic toxicity.4,5 Concerns regarding the effects of epinephrine on nutritive vessel vasoconstriction
and nerve ischemia have been unsubstantiated. In contrast, concentrations of 2.5 μg/mL have been
associated with an increase in nerve blood flow, likely due to the predominance of the beta effect of
the drug.6
Therefore, when added to local anesthetics, epinephrine can enhance safety during
administration of larger doses of local anesthetics.
Nerve Stimulation
Neurostimulation has largely replaced paresthesia as the primary means of nerve localization in the
1980s and has only recently been challenged by ultrasound guidance. Its effectiveness as a method of
nerve localization has been challenged since the publication of a series of studies showing that,
despite intimate needle–nerve contact as witnessed by ultrasonography, a motor response may be
absent.7
In some instances, a current intensity as high as >1.5 mA may be necessary to elicit motor
response with needle placement within epineurium of the nerve.8
There are probably multiple factors
that contribute to the explanation of this phenomenon, including the nonuniform distribution of motor
and sensory fibers in the compound nerve and the unpredictable pattern of current dispersion in the
tissue depending on tissue conductances and impedances.
Although this has led some clinicians to de-value nerve stimulation in an era of ultrasound-guided
blocks, a growing body of evidence suggests that the presence of a motor response at a very low
current (i.e., <0.2 mA) is associated with an intraneural needle tip placement (Table 5-1). In a 2005
trial, Voelckel et al conducted percutaneous sciatic nerve blocks in pigs and demonstrated that when
local anesthetic was injected at currents between 0.3 and 0.5 mA, the resulting nerve tissue showed
no signs of an inflammatory process, whereas injections at <0.2 mA resulted in lymphocytic and
granulocytic infiltration in 50% of the nerves.9
Tsai et al performed a similar study investigating the
effect of distance to the nerve on current required; although a range of currents were recorded for a
variety of distances, the only instances in which the motor response was obtained at <0.2 mA was
when the needle tip was intraneural.10
TABLE 5-1 Studies of Intensity of the Current (mA) and Needle Tip Location
More recently, a study was conducted on 55 patients scheduled for upper limb surgery who
received ultrasound-guided supraclavicular brachial plexus blocks. The authors set out to determine
the minimum current threshold for motor response both inside and outside the first trunk
encountered.11 They discovered that the median minimum stimulation threshold was 0.60 mA outside
the nerve and 0.3 mA inside the nerve. Interestingly, stimulation currents of ≤0.2 mA were not
observed outside the nerve, whereas 36% of patients experienced a twitch at currents <0.2 mA while
the needle was intraneural.
Taken together, these data suggest that although the sensitivity of a “low-current” twitch for
intraneural placement is not high, the specificity is. Put another way, the needle tip can be in the nerve
and not elicit a motor response at very low currents; however, if a twitch is elicited at <0.2 mA, it is
certain that the tip is intraneural.
Most regional anesthesiologists agree that injection of local anesthetic into the nerve may be a risk
factor for injury and that extra-neural deposition minimizes the potential for an intrafascicular
injection.12 Ultrasonography is good, but not perfect, at delineating the exact position of the needle
tip. In our attempts to get “close, but not too close” to the nerve so we might have the best block
result, needles occasionally but inevitably cross the epineurium into the substance of the nerve. This
event in and of itself may be of minimal consequence.13 However, injection into a fascicle carries a
high risk of injury.14 It is for this reason that a reliable electrical monitor of needle tip position is a
useful safety instrument. If a motor twitch is elicited at currents <0.2 mA, our approach is to gently
withdraw the needle until the motor response disappears and then attempt to reelicit the twitch at the
more appropriate (0.3–0.5 mA) current.
Overall, nerve stimulation adds little to the cost of a nerve block procedure, in terms of time,
clinician effort, or dollars. It also serves as a useful functional confirmation of the anatomic image
shown on the ultrasound screen (e.g. “Is that the median or ulnar nerve?”). In our practice, nerve
stimulator is routinely used in conjunction with ultrasound guidance as an invaluable monitor of the
needle tip position with respect to the nerve, based on the association of low currents with intraneural
placement. In addition, an unexpected motor response during ultrasound-guided blocks may alert the
operator of the needle-nerve relationship that was missed on ultrasound.
Ultrasonography
The use of ultrasound guidance to assist in nerve block placement has become very popular, for a
number of reasons. First, ultrasound allows visualization of the needle in real time and therefore
quickly and accurately guide the needle toward the target. Multiple injection techniques that were
difficult, or indeed dangerous, to do in the era of nerve stimulation alone are now easy to perform
because the nerves can be seen and injectate carefully deposited at various points around them. Also,
because a motor response is not technically required, blocks can now be performed in amputees who
do not have a limb to twitch. Not surprisingly, ultrasound has the potential to improve the safety of
peripheral nerve blocks for a number of reasons.
First, adjacent structures of importance can be seen and avoided. The resurgence in popularity of
the supraclavicular block is a testament to this. Before ultrasound, the highly effective block was
relatively unpopular as a means of anesthetizing the brachial plexus, for fear of causing a
pneumothorax, despite the paucity of data regarding its actual incidence. However, now that the
brachial plexus and, more importantly, the rib, pleura, and subclavian artery can all be seen at the
supraclavicular level, this block has become common in clinical practice. However, recent reports of
pneumothoraces serve as a reminder that while ultrasound may reduce the incidence of complications
of nerve blocks, it is unlikely to entirely prevent when used as a sole monitor.15,16 Similarly, there are
reports of intravascular and intraneural needle placement witnessed (and despite the use of)
ultrasound, highlighting the need to use care with this technology that is, in the end, a tool that is not
failsafe.17–19
A useful adjunct to the visualization of structures on the ultrasound screen is the ability to measure
the distance from skin to target using electronic calipers (Figure 5-2). This, coupled with needles that
have depth markings etched on the side of the nerve block needles, confers a great safety advantage by
warning the clinician of a “stop distance,” or a depth beyond which he or she should stop, reassess
the needle visualization, and perhaps withdraw and begin again.
FIGURE 5-2. An example of ultrasound being used to determine the depth of a structure of interest.
Another important advantage that ultrasound can confer is the ability to see the local anesthetic
distribution on the screen image (Figure 5-3). If corresponding tissue expansion is not seen when
injection begins, then the needle tip is not where it is thought to be, and the clinician should
immediately halt injection and relocate the tip of the needle. This is particularly worrisome in
vascular areas because the lack of spread can signal the intravascular needle placement. However,
ultrasound has been used successfully to diagnose an intra-arterial needle tip placement when an
echogenic “blush” was noted in the lumen of the artery, allowing for rapid cessation of the block
technique and avoidance of what surely would have been systemic toxicity.20,21
FIGURE 5-3. Axillary block with axillary artery (art), ulnar nerve (u), needle (arrowheads), (A)
before and (B) after injection of small amount of local anesthetic, showing spread of injectate
between artery and nerve.
Similarly, ultrasound may also be able to reduce the likelihood of systemic toxicity by allowing
clinicians to use less local anesthetic. Several authors have published large reductions in the volume
required to affect an equivalent block to standard nerve stimulation techniques. For example, Casati
et al demonstrated a significant reduction in volume required to produce an effective three-in-one
block (22 mL vs. 41 mL).22 Sandhu et al showed in a feasibility study that infraclavicular block was
possible using ultrasound with volumes typically half of what were used with nerve stimulation alone
(16.1 ± 1.9 mL).23 Riazi et al published a study in 2008 showing that ultrasound guidance allowed for
a substantial reduction of volume for interscalene block used for postoperative pain while still
providing a quality block (5 mL vs. 20 mL).24 Interestingly, this low dose also resulted in less
diaphragmatic impairment related to phrenic nerve paresis.
The utility of ultrasound in prevention of nerve injury during peripheral nerve blockade is likely
over-estimated. The problem is threefold: First, observing the needle tip in relation to the nerve is
user dependent, and one can often be fooled by poor technique or simply unfavorable echogenic
characteristics of the tissue–needle interface; second, the current resolution available is not adequate
to distinguish between an intra- versus extrafascicular needle tip location. This difference is crucial
because evidence is mounting that an intraneural (but extrafascicular) injection is likely not
associated with injury, whereas injection inside the fascicles themselves produces clinical and
histologic damage.14,25 Lastly, once injection has begun, even a minuscule amount of local anesthetic
can produce damage if intrafascicular.26 Relying on the visual confirmation of tissue expansion may
result in damage before expansion is detected on the screen. It is, in other words, probably too late.
Injection Pressure Monitoring
How, then, can the clinician distinguish the intrafascicular versus the extrafascicular needle tip
placement, if ultrasound guidance is insufficient? An additional modality to ultrasound and nerve
stimulation is monitoring of injection pressures. In a study of intraneural injections in dog sciatic
nerves, a slow injection of lidocaine while the needle tip was intrafascicular was associated with an
immediate and substantial rise in the pressure of the syringe-tubing-needle system (>20 psi), followed
by return of the pressure tracing to normal (i.e., <5 psi) levels. In contrast, those nerves that
underwent extrafascicular injection showed no high pressure whatsoever.14 Moreover, those limbs in
which the nerves were exposed to high-injection pressures all experienced clinical signs of
neuropathy (muscle wasting, weakness) as well as histologic evidence (inflammation, disruption of
the nerve architecture). The implication is that injection into a low compliance compartment, such as
within the tough, durable perineurium, is likely to result in generation of high pressures that can either
directly damage delicate nerve fibers or rupture the fascicle itself, leading to nerve injury.
The use of “hand feel” to avoid high injection pressure is unfortunately not reliable. Studies of
experienced practitioners blinded to the injection pressure and asked to perform a mock injection
using standard equipment reveals wide variations in applied pressure, some grossly exceeding the
established thresholds for safety.27 Similarly, anesthesiologists perform poorly when asked to
distinguish between intraneural injection and injection into other tissues such as muscle or tendon in
an animal model.28 It is therefore important to use an objective and quantifiable method of gauging
injection pressure.
Although the practice of injection pressure monitoring during peripheral nerve blocks is relatively
young, monitoring options do exist. Tsui et al described a method of “compressed air injection
technique” where 10 mL of air was drawn into the syringe along with the local anesthetic.29 Holding
the syringe upright, it is then possible to avoid exceeding a maximum threshold of 1 atmosphere (or
approximately 15 psi) by only allowing the gas portion of the syringe contents to compress to half of
its original volume, or 5 mL. This makes use of Boyle’s law, which states that pressure times volume
must be constant. A pressure <15 psi is probably a safe threshold for initiating injection during
peripheral nerve blocks.
Another option is disposable pressure manometers specifically manufactured for this purpose.
These devices bridge the syringe and needle tubing, and via a spring-loaded piston, allow the
clinician to gauge the pressure in the system continuously. On the shaft of the piston are markings
delineating three different pressure thresholds: <15 psi, 15 to 20 psi, and >20 psi (Figure 5-4). An
advantage of this method is the ease with which an untrained assistant who is performing the injection
can read and communicate the pressures. In addition, the syringe does not have to be held upright, as
in the compressed air technique.
FIGURE 5-4. Inline pressure manometer with graded markings on the side (B-smart, Concert
Medical, Needham, MA).
Pressure monitoring may be a useful safety monitor for other aspects of peripheral nerve blocks. In
a study of patients receiving lumbar plexus blocks randomized to low (<15 psi) versus high (>20 psi)
pressures, Gadsden et al demonstrated that 60% of patients in the high-pressure group experienced a
bilateral epidural block.30 Furthermore, 50% in the same group reported an epidural block in the
thoracic distribution. No patient in the low-pressure group experienced bilateral or epidural
blockade. This has now become an important adjunct to lumbar plexus blockade in our institution, to
avoid this potentially dangerous complication.
Summary
Regional anesthesia has been making a transition from art to science as more rigorous and precise
means of locating nerves are developed. The same process should be expected for monitoring
peripheral blockade. The use of neurostimulation, ultrasonography, and injection pressure monitoring
together provides a complementary package of objective data that can guide clinicians to perform the
safest blocks possible. The flowchart in Figure 5-5 outlines how these monitors can be combined.
FIGURE 5-5. Flowchart depicting the order of correctly documenting nerve block procedures:
combining ultrasound (US), nerve stimulation (NS), and injection pressure monitoring.
SECTION II: DOCUMENTATION
Block Procedure Notes
Documentation of nerve block procedures has, by and large, lagged behind the documentation of
general anesthesia, and it is often relegated to a few scribbled lines in the corner of the anesthetic
record. Increasing pressure from legal, billing, and regulatory sources has provoked an effort to
improve the documentation for peripheral nerve blocks. A sample of a peripheral nerve block
documentation form that incorporates all of the monitoring elements discussed in this book
(ultrasound, nerve stimulation, and injection pressure monitoring) and can be adopted to individual
practice is shown in Figure 5-6. This form has a number of features that should be considered by
institutions attempting to formulate their own procedure note. These include the following:
FIGURE 5-6. Example of a procedure note.
Another useful aspect of peripheral nerve block documentation is the recording of an ultrasound
image or video clip, to be stored either as a hard copy in the patient’s chart or as an electronic copy
in a database. This is not only good practice from a medicolegal point of view but is a required step
that must be taken if the clinician wishes to bill for the use of ultrasound guidance. Any hard copies
should have a patient identification sticker attached, the date recorded, and any pertinent findings
highlighted with a marker, such as local anesthetic spreading around nerve circumferentially (Figure
5-6).
Informed Consent
Documentation of informed consent is another issue that is of importance to regional
anesthesiologists. Practice patterns vary widely on this issue, and specific written consent for nerve
block procedures is often not obtained. However, the written documentation of this process can be
important for a number of reasons:
Patients are often distracted and anxious on the day of surgery (when many consents are obtained), and
they may not remember the details of a discussion with their anesthesiologist. Studies have shown that
a written record of the informed consent process improves patient recall of risks and benefits.31
A written consent establishes that a discussion of risks and benefits occurred between the patient and
the physician.
A specific document for regional anesthesia can be tailored to include all the common and serious
risks. This allows the physician to explain them to the patient as a matter of routine and reduce the
chance of omitting important risks.
The following tips can be used to maximize the consent process:
A specific regional anesthesia consent form should be included as well. This will not be
applicable to all institutions but can be modified to suit the needs of each individual department.
REFERENCES
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and without acute beta-adrenergic blockade. Anesthesiology. 1990;73:386-392
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sedated patients. Anesth Analg. 2001;93:1612-1617.
. Karmakar MK, Ho AM, Law BK, Wong AS, Shafer SL, Gin T. Arterial and venous pharmacokinetics
of ropivacaine with and without epinephrine after thoracic paravertebral block. Anesthesiology.
2005;103:704-711.
. Van Obbergh LJ, Roelants FA, Veyckemans F, Verbeeck RK. In children, the addition of epinephrine
modifies the pharmacokinetics of ropivacaine injected caudally. Can J Anaesth. 2003;50:593-598.
. Neal JM. Effects of epinephrine in local anesthetics on the central and peripheral nervous systems:
neurotoxicity and neural blood flow. Reg Anesth Pain Med. 2003;28:124-134.
. Perlas A, Niazi A, McCartney C, Chan V, Xu D, Abbas S. The sensitivity of motor response to nerve
stimulation and paresthesia for nerve localization as evaluated by ultrasound. Reg Anesth Pain Med.
2006;31:445-450.
. Chan VW, Brull R, McCartney CJ, Xu D, Abbas S, Shannon P. An ultrasonographic and histological
study of intraneural injection and electrical stimulation in pigs. Anesth Analg. 2007;104: 1281-1284.
. Voelckel WG, Klima G, Krismer AC, et al. Signs of inflammation after sciatic nerve block in pigs.
Anesth Analg. 2005;101: 1844-1846.
0. Tsai TP, Vuckovic I, Dilberovic F, et al. Intensity of the stimulating current may not be a reliable
indicator of intraneural needle placement. Reg Anesth Pain Med. 2008;33:207-210.
1. Bigeleisen PE, Moayeri N, Groen GJ. Extraneural versus intraneural stimulation thresholds during
ultrasound-guided supraclavicular block. Anesthesiology. 2009;110:1235-1243.
2. Hogan QH. Pathophysiology of peripheral nerve injury during regional anesthesia. Reg Anesth Pain
Med. 2008;33:435-441.
3. Sala-Blanch X, Ribalta T, Rivas E, et al. Structural injury to the human sciatic nerve after
intraneural needle insertion. Reg Anesth Pain Med. 2009;34:201-205.
4. Hadžić A, Dilberovic F, Shah S, et al. Combination of intraneural injection and high injection
pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med.
2004;29:417-423.
5. Koscielniak-Nielsen ZJ, Rasmussen H, Hesselbjerg L. Pneumothorax after an ultrasound-guided
lateral sagittal infraclavicular block. Acta Anaesthesiol Scand. 2008;52:1176-1177.
6. Bryan NA, Swenson JD, Greis PE, Burks RT. Indwelling interscalene catheter use in an outpatient
setting for shoulder surgery: technique, efficacy, and complications. J Shoulder Elbow Surg.
2007;16:388-395.
7. Russon K, Blanco R. Accidental intraneural injection into the musculocutaneous nerve visualized
with ultrasound. Anesth Analg. 2007;105:1504-1505.
8. Schafhalter-Zoppoth I, Zeitz ID, Gray AT. Inadvertent femoral nerve impalement and intraneural
injection visualized by ultrasound. Anesth Analg. 2004;99:627-628.
19. Loubert C, Williams SR, Helie F, Arcand G. Complication during ultrasound-guided regional
block: accidental intravascular injection of local anesthetic. Anesthesiology. 2008;108:759-760.
20. VadeBoncouer TR, Weinberg GL, Oswald S, Angelov F. Early detection of intravascular injection
during ultrasound-guided supraclavicular brachial plexus block. Reg Anesth Pain Med. 2008;33:278-279.
21. Martinez Navas A, DE LA Tabla González RO. Ultrasound-guided technique allowed early
detection of intravascular injection during an infraclavicular brachial plexus block. Acta AnaesthesiolScand. 2009;53:968-970.
22. Casati A, Baciarello M, Di Cianni S, et al. Effects of ultrasound guidance on the minimum effective
anaesthetic volume required to block the femoral nerve. Br J Anaesth. 2007;98:823-827.
23. Sandhu NS, Bahniwal CS, Capan LM. Feasibility of an infraclavicular block with a reduced volumeof lidocaine with sonographic guidance. J Ultrasound Med. 2006;25:51-56
24. Riazi S, Carmichael N, Awad I, Holtby RM, McCartney CJ. Effect of local anaesthetic volume (20
vs 5 ml) on the efficacy and respiratory consequences of ultrasound-guided interscalene brachial
plexus block. Br J Anaesth. 2008;101:549-556.
25. Bigeleisen PE. Nerve puncture and apparent intraneural injection during ultrasound-guided axillary
block does not invariably result in neurologic injury. Anesthesiology. 2006;105:779-783.
26. Selander D, Dhuner KG, Lundborg G. Peripheral nerve injury due to injection needles used for
regional anesthesia. An experimental study of the acute effects of needle point trauma. Acta
Anaesthesiol Scand. 1977;21:182-188.
27. Claudio R, Hadžić A, Shih H, et al. Injection pressures by anesthesiologists during simulated
peripheral nerve block. Reg Anesth Pain Med. 2004;29:201-205.
28. Theron PS, Mackay Z, Gonzalez JG, Donaldson N, Blanco R. An animal model of “syringe feel”
during peripheral nerve block. Reg Anesth Pain Med. 2009;34:330-332.
29. Tsui BC, Knezevich MP, Pillay JJ. Reduced injection pressures using a compressed air injection
technique (CAIT): an in vitro study. Reg Anesth Pain Med. 2008;33:168-173.
30. Gadsden JC, Lindenmuth DM, Hadži A, Xu D, Somasundarum L, Flisinski KA. Lumbar plexus
block using high-pressure injection leads to contralateral and epidural spread. Anesthesiology.
2008;109:683-688.
31. Gerancher JC, Grice SC, Dewan DM, Eisenach J. An evaluation of informed consent prior to
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6
Indications for Peripheral Nerve Blocks
Jeff Gadsden
Introduction
During the past 20 years, increasing knowledge in functional regional anesthesia anatomy, coupled
with new technologies for locating peripheral nerves, has resulted in expansion of regional anesthesia
techniques. This phenomenon served to provide the clinician with a wide variety of techniques from
which to choose. Nevertheless, many nerve block techniques are quite similar and result in a similar,
if not exact, distribution of anesthesia. The proper choice of the nerve block for a particular surgical
procedure and/or patient, however, is far more important than deliberation on the minutia of various
technical techniques. In this chapter, a rational selection of the nerve block techniques is approached
in three sections. In the first section, indications for common nerve blocks are listed with a short
summary of the advantages and disadvantages of each technique selected. In the second section,
specific protocols for intraoperative anesthesia and postoperative analgesia for the common surgical
procedures are suggested as practiced by anesthesiologists affiliated with the St. Luke’s and
Roosevelt Hospitals in New York. This cookbook approach was chosen to allow clinicians to
duplicate the results that we have found, via trial and error, to work best in our own practice. The last
section is a more comprehensive compendium of published medical literature on the indications for
peripheral nerve blocks.
Section I: Advantages and Disadvantages of Specific Nerve Blocks
Upper Limb Blocks
With the advent of ultrasound guidance for nerve blocks, the choice of which brachial plexus block to
perform has become less relevant because the block can be extended by needle repositioning into the
desired area. For example, the interscalene approach was not recommended in the past for
procedures on the hand or elbow because it was believed that local anesthetic would not sufficiently
cover the inferior trunk of the brachial plexus. However, this barrier can be overcome with the use of
a low-interscalene approach or by using sonographic guidance to target all three trunks. Multiple
injections at different levels of the brachial plexus through a single-needle insertion site can make the
interscalene brachial plexus applicable for most upper limb procedures. Regardless, the common
approaches to brachial plexus block are sufficiently different in their anesthetic coverage to deserve
knowledgeable consideration when making a decision about which block to use. In addition to the
anesthetic coverage, the block selection should also take into consideration other factors, such as
patient comfort, preexisting respiratory dysfunction, and practitioner experience. Table 6-1 lists
common nerve block procedures and their indications.
TABLE 6-1 Common Upper Limb Blocks
Lower Limb Blocks
Achieving quality anesthesia or analgesia of the lower limb is more challenging than with an upper
extremity. This is because its innervation stems from two major plexuses, the lumbar and the sacral.
The lumbar plexus is formed by the roots of L1-L4 and gives rise to the femoral, obturator, and lateral
femoral cutaneous nerves, among others. The sacral plexus originates from L4-S3, and its principal
branch is the sciatic nerve. Most of the indications for lower limb blockade involve joint surgery on
either the hip or the knee. Because both joints are supplied by elements of each plexus, complete
anesthesia often requires at least two nerve blocks. Consequently, many clinicians choose to perform
just one block for the purpose of analgesia. Table 6-2 lists some common lower limb blocks and
their advantages and disadvantages.
TABLE 6-2 Common Lower Limb Blocks
Section II: Protocols
A variety of different methods are available to provide intraoperative and postoperative analgesia for
surgery on the extremity. Any anesthetic or analgesic plan is based on patient and surgical factors as
well as practical considerations such as the practitioner’s skill level, availability of a block room,
availability of skilled assistants, and departmental and hospital policies. The protocols for most
common major orthopedic procedures outlined in this section were refined through trial and error and
are the actual methods used in our daily practice.
The choice of the block combination for postoperative pain is based on several factors. The
orthopedic surgeons at St. Luke’s-Roosevelt Hospital prefer a regimen of twice-daily dosing of low
molecular weight heparin (LMWH) for thromboprophylaxis, which makes the use of an indwelling
epidural catheter for postoperative pain impractical or unsuitable. Similarly, although we recognize
there is some controversy regarding the use of lumbar plexus catheters in the same setting, by and
large, we treat them as neuraxial catheters and remove them before the first dose of LMWH. Other
perineural catheters are routinely placed and maintained even in patients who are treated with
anticoagulants.
In recent years, we have made an effort to minimize the use of parenteral opioids for postoperative
pain if possible. In particular, patients admitted to the ward with a perineural catheter and intravenous
patient-controlled opioid analgesia can find it confusing to have two buttons to press, and therefore
they do not use the catheter effectively, leading to inadequate analgesia. For this reason, we strive to
make use of a multimodal regimen instead, consisting of acetaminophen, a nonsteroidal antiinflammatory drug, and an oral opioid.
For lower limb surgery, such as total knee replacement, clinicians often debate whether the sciatic
nerve and/or obturator blocks should be routinely used in addition to the femoral (or lumbar plexus)
block. We do not routinely do this but rather assess the patient after the femoral/lumbar plexus
blockade is performed. In our practice, in the majority of patients, postoperative pain is adequately
managed (visual analog scale [VAS] ≤3) by continuous femoral nerve block alone. A small
proportion of patients (about 20%) may require a sciatic nerve block for adequate pain control.
Although often debated and taught in various regional courses, the usefulness of the obturator block in
our practice is questionable at best. Consequently, we do not use obturator blocks in patients having
knee arthroplasty.
The timing of block placement is institution dependent, and it relies on the presence of various
factors, such as availability of the designated block personnel, operating room flow, ancillary staff,
and a separate block area. Single-injection nerve blocks for surgery are performed either in the
holding area or operating room immediately prior to the surgical procedure. Catheters for upper limb
surgery are usually placed in a similar manner if the technique is used for surgical anesthesia as well.
In contrast, most of our lower limb nerve blocks or catheters are placed in the postanesthesia care
unit before the resolution of the neuraxial block. Although the practice of performing blocks in
anesthetized patients (in this case in the presence of spinal anesthesia), we believe that when modern
monitoring is used (combination of ultrasound, nerve stimulation, and injection pressure monitoring),
it is irrelevant whether the blocks are performed in anesthetized or nonanesthetized patients.
Finally, we do not routinely combine general anesthesia with regional anesthesia, although this is a
widely used practice elsewhere. Our regional anesthetics are often used as the primary anesthesia
modality, rather then solely for the purpose of postoperative analgesia. Instead of general anesthesia,
we typically use sedation with propofol and/or intravenous midazolam titrated to light sleep and
spontaneous breathing with supplemental oxygen via a facemask. Table 6-3 lists some common
surgical procedures, peripheral nerve blocks that are suitable for anesthesia and analgesia, as well as
other common analgesic options.
TABLE 6-3 Common Surgical Procedures and Analgesic Options
Section III: Compendium of the Literature
The previous two sections described some of the most common indications for peripheral nerve
blocks in our practice. However, the usefulness of peripheral nerve blocks is much greater than the
few common ones discussed here. For the sake of completeness, the compendium of indications for
peripheral nerve blocks reported in medical literature is listed in the accompanying chart. Readers
should use their own discretion when determining whether any indications would fit the realm of their
own clinical practice.
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