olateral aspect of a

foot (Fig. 10.6) (1).

c. For infants weighing >3 kg, use the palm, thumb,

great toe, or index finger (1).

d. Align the LEDs (light source) and the detector so

they are directly opposite each other.

e. Reusable sensors should be applied with nonadhesive elastic wrap.

Fig. 10.5. Pulse oximeter. Vertical column indicates pulse.

(Courtesy of Nellcor, Pleasanton, California.)

Fig. 10.6. Disposable sensor applied to foot.

Chapter 10 ■ Continuous Blood Gas Monitoring 69

f. Tighten sensor snugly to the skin but not so as to

impede circulation. The probe should then be left

in place for several seconds until extremity movement stops and the signal is stable.

g. Secure the sensor to the site to prevent tugging or

movement of the sensor independent of the body part.

h. Cover the sensor to reduce the effect of intense light

levels, direct sunlight, or phototherapy.

3. Attach the sensor to the system interconnecting cable

and turn on the monitor. (Attaching the sensor to the

baby before connecting the cable to the monitor that is

already turned on will shorten the time taken for data

acquisition and display of SpO2 information.) (6)

4. Calibration of the system is not required (internal autocalibration).

5. After a short interval, if all connections are correct, the

monitor will display the pulse detected by the sensor. If

the pulse level is adequate, it will display SpO2 and

pulse rate. If the pulse indicator is not synchronous

with the patient’s pulse rate, reposition the probe. After

repositioning the sensor, if the pulse detector is still not

indicating properly, change the sensor site.

6. Once reliable operation is achieved, set the high and

low alarm limits.

a. Although pulse oximeters can detect hyperoxemia,

it is important that type-specific alarm limits are set

and a low specificity is accepted (16,17). Alarm limits are determined by gestational age, presence of

acute or chronic lung disease, cardiac disease, and

risk of retinopathy of prematurity (18).

b. The optimal alarm limit, defined as having a sensitivity of 95% or more, associated with maximal specificity, will differ depending on which particular

monitor is used.

 Note that SpO2 is a more sensitive indicator of

hypoxemia and decreased tissue oxygenation than is

PaO2. Lower alarm limits should be individualized

to alert the user when the oxygenation requirements

of the given patient are not satisfied.

H. Complications

1. Management based on erroneous readings caused by a

misapplied sensor or conditions affecting instrument

performance.

2. Limb ischemia if sensor applied too tightly, particularly

in an edematous limb.

Transcutaneous Blood Gas Monitoring

Transcutaneous measurements of oxygen and carbon dioxide are useful in the neonatal intensive care unit because

they provide continuous and relatively noninvasive estimation of these parameters to supplement arterial blood gas

measurements.

A. Definitions

1. Transcutaneous measurement of oxygen is referred to

as PtcO2.

2. Transcutaneous measurement of carbon dioxide is

referred to as PtcCO2.

B. Purpose

1. Noninvasive blood gas monitoring of PO2 and PCO2

2. Trending of PO2 and PCO2 over time

C. Background

1. Transcutaneous monitoring measures skin-surface PO2

and PCO2 to provide estimates of arterial partial pressure of oxygen and carbon dioxide. The devices increase

tissue perfusion by heating the skin and then electrochemically measuring the partial pressure of oxygen

and carbon dioxide.

2. Accomplished by two electrodes contained in a

heated block that maintains the electrodes and the

skin directly beneath it at a constant temperature (19)

(Fig. 10.7)

a. Arterialized capillary oxygen levels are more accurately measured by heating the skin to establish

hyperemia directly beneath the sensor.

b. The electrodes are covered with an electrolyte solution and sealed with a semipermeable plastic membrane.

3. A modified Clark electrode is used to measure oxygen.

a. It produces an electrical current that is proportional

to PO2.

b. Measured current is converted to PO2 and then corrected for temperature.

4. A Severinghaus electrode is used to measure CO2.

a. pH-sensitive glass electrode

b. CO2 diffuses from the skin surface through the

membrane. The CO2 changes the pH of the electrolyte solution bathing the electrode.

c. The measured pH is converted to PCO2 and then

corrected for temperature.

 Conversion of electric current and pH to PO2 and

PCO2, respectively, is based on conversion equations adjusted by a two-point calibration. This is part

of the setup and calibration process.

D. Indications

1. To approximate arterial PaO2 and PaCO2 for respiratory

management (19)

a. To monitor the effect of therapeutic ventilatory

maneuvers particularly in infants who have combined oxygenation and ventilation problems

b. For stabilization and monitoring during transport

2. To reduce the frequency of arterial blood gas analysis

(19,20)

70 Section II ■ Physiologic Monitoring

3. To determine by a noninvasive and continuous method

the regional arterial oxygen tension (19,20)

4. To infer regional arterial blood flow (e.g., in the lower

limbs of infants with duct-dependent coarctation of the

aorta) (19,20)

E. Contraindications

1. Skin disorders (e.g., epidermolysis bullosa, staphylococcal scalded skin syndrome)

2. Relative contraindications

a. The extremely low-birthweight infant (19,20)

b. Severe acidosis

c. Significant anemia

d. Decreased peripheral perfusion

e. PtcO2 may underestimate PaO2 (19,20)

F. Equipment—Specifications

1. Transcutaneous monitor components

a. Dual electrode

b. Electrode cleaning kit

c. Electrolyte and membrane kit

d. Contact solution

e. Double-sided adhesive rings

f. Calibration gas cylinders with delivery apparatus

2. Digital display shows values for PtcO2, PtcCO2, and site

of sensor (Fig. 10.8).

 Monitor with controls for both high and low alarm

limits, and for electrode temperature. The monitor may

also have a site placement timer that will alarm as an

indication to change the site of the electrode.

G. Precautions

1. Be aware that

a. Equilibration requires approximately 20 minutes

after the electrode is placed, with the response

time for PtcO2 being much faster than that for

PtcCO2. Therefore, management changes based

on transcutaneous values should be guided by values that have been consistent for at least 5 minutes.

b. Periodic correlation with PO2 from appropriate arterial sites is recommended (19,20)

c. PtcO2 may underestimate PaO2 in the infant with

hyperoxemia (PaO2 >100 mm Hg), with reliability

of PtcO2 measurement decreasing as PaO2 increases

(19,20)

d. PtcO2 may underestimate PaO2 in older infants with

bronchopulmonary dysplasia (21,22).

e. Pressure on the sensor (e.g., infant lying on sensor)

may restrict blood supply, resulting in falsely low

PtcO2 values.

f. Manufacturers’ parts are not interchangeable. Only

supplies of the same brand and designated for the

monitor should be used.

2. To avoid skin burns, change electrode location at least

every 4 hours.

3. PtcO2 may underestimate PaO2 in the presence of

a. Severe acidosis

b. Severe anemia

c. Decreased peripheral perfusion

H. Technique

1. Familiarize yourself with the system before proceeding.

A

C

B