a. An oversized cuff will yield lower BP values; an

undersized cuff will produce higher BP values.

2. Patient must be still during measurements.

3. For optimal infection control, cuffs should be for

single-patient use only.

4. Oscillometric BP measurement may lose accuracy in

very hypotensive states; this needs to be taken into consideration in such patients (16,17).

G. Technique

1. Become familiar with the monitor and the equipment

to be used. Be aware of the normal BP changes with

gestational and postnatal age (18).

2. Measure the circumference of the extremity where the

cuff is to be applied. Select the appropriately sized cuff

for the limb (Fig. 9.1).

3. Apply the cuff snugly to the limb. The cuff can be

applied over a thin layer of clothing if necessary; however, a bare limb is recommended.

4. Attach the monitor air hoses to the cuff. The limb from

which pressure is to be measured should be level with

the heart.

5. Turn the monitor on and ensure that it passes the

power-on self-test before proceeding.

6. Press the appropriate button to start a blood pressure

determination cycle.

7. If the values obtained from the initial cycle are questionable, repeat the measurement.

 Multiple readings with similar values yield the optimal assurance of accuracy.

8. If readings are still questionable after repeating the

cycle, reposition the cuff and repeat the measurement.

9. Periodic inspection of the cuff and extremity is critical

to avoid problems such as cuff detachment or shift in

extremity position.

10. Most NIBP systems can be programmed by the user to

measure BP automatically at user-determined intervals.

The interval between measurements should be long

Fig. 9.3. Oscillometric BP monitor. (Courtesy of GE Healthcare.)

60 Section II ■ Physiologic Monitoring

enough to ensure adequate circulation and minimize

trauma to the limb and skin distal to the cuff.

11. In infants with suspected congenital heart disease,

BP should be measured in all four extremities or at

least the right arm and a leg for pre- and postductal

comparison.

H. Complications

1. Perfusion in the limb may be compromised if the cuff is

not completely deflated.

2. Repeated continuous cycling may cause ischemia, purpura, and/or neuropathy in the extremity.

3. Cuff inflation will interfere with pulse oximetry measurement and IV infusion in the same limb.

4. Nosocomial infection may arise from using the same

cuff for more than one patient.

Continuous Blood Pressure Monitoring

(Invasive)

A. Purpose

Intra-arterial direct continuous BP monitoring is considered

to be the “gold standard” for measuring BP. It has the added

advantage of permitting access for repeated arterial blood

sampling in critically ill neonates.

B. Background

1. BP measurement is obtained from the vascular system

via a catheter that has been introduced into an artery,

either the umbilical artery in the neonate or a peripheral artery (Chapters 29 and 31).

2. The pressure waveform of the arterial pulse is transmitted by a column of fluid to a pressure transducer where

it is converted to an electrical signal, which is transformed into a visual display by a microprocessor.

3. A BP transducer is a device that converts mechanical

forces (pressure) to electrical signals. There are two

major types of transducers

a. Strain gauge pressure transducer: Composed of

metal strands or foil that is either stretched or

released by the applied pressure on the diaphragm

(1) Applied pressure causes a proportional and linear change in electrical resistance.

(2) Problems associated with strain gauges include

drift due to temperature changes (departure

from the real signal value), fragility, and cost.

b. Solid-state pressure transducer (semiconductor):

Composed of a silicon chip that undergoes electrical

resistance changes because of the applied pressure

(1) Lower cost, accurate, and disposables

(2) Because of the miniature integration on the

silicone chip, the circuitry necessary to minimize temperature drift is incorporated in the

device.

4. Miniature transducer-tipped catheters are available that

do not depend on fluid-filled lines for the transmission

of pressure. Microtransducer catheters in general have

better fidelity characteristics, but at a much higher cost

than conventional fluid-filled systems and are currently

not generally available for neonatal use

5. The standard medical BP transducer output rating is

5 μV/V/mm Hg. The pressure monitor processes the

electrical signal generated by the transducer and converts it to BP units in either millimeters of mercury or

kilopascals, including generation of systolic, diastolic,

and mean values. The monitor provides a user-friendly

numerical and graphical display allowing beat-to-beat

measurement of pressure and also allows analysis of the

waveform. Analysis can be clinical (e.g., morphology,

determining the position of the dicrotic notch or

“swing” that can give information regarding filling status and cardiac output) or computerized.

C. Indications

To continuously monitor intravascular pressure

1. In very small or unstable infants, particularly those

with severe hypotension, on inotropic support

2. During major procedures that could cause or exacerbate intravascular instability

3. To monitor infants on ventilator support or extracorporeal membrane oxygenation

4. Allow frequent arterial blood sampling.

D. Contraindications

None absolute, except for those specific to catheter placement

E. Limitations

1. The pulse pressure waveform measured in the periphery is narrower and taller than that in the proximal

aorta. Thus, systolic BP in the peripheral arteries can

be higher than that in proximal aorta. This amplification is greater in patients with increased vascular tone

or on inotropic therapy.

2. Very small-diameter catheters may result in underreading of systolic BP.

F. Equipment

There are five components of the intra-arterial BP monitoring system (Figs. 9.4 and 9.5). Commercial pressure monitoring kits have most components integrated.

1. Intra-arterial catheter: May be an umbilical arterial

catheter (Chapter 29) or peripheral arterial catheter

(Chapter 31).

2. Pressure monitoring tube: Fluid-filled tubing to couple

arterial cannula to the pressure transducer. This tubing

should be short (not exceed 100 to 120 cm from the

transducer to the patient connection) and stiff (low