46 Section II ■ Physiologic Monitoring

2. Thermistor probes (most widely used)

a. The thermistor is a resistive device, having a high

negative temperature coefficient of resistance, so

that its resistance decreases proportionately as the

temperature increases.

b. As the resistance of the thermistor changes, the electrical current flowing through the probe changes

proportionally.

c. The level of current detected by the electronic monitor is converted to thermal units.

3. Thermocouple probes

a. The thermocouple probe is a very small bead made

up of the junction of two dissimilar metals.

b. The bead generates a very small voltage proportional to temperature.

c. The voltage generated by the bead is measured by

the monitor and converted to thermal units.

d. The thermocouple and the thermistor are not interchangeable.

e. Battery-powered interface devices are available that

allow the use of thermocouple probes with

thermistor-compatible monitors

f. Thermocouple probes are less expensive than

thermistors.

C. Indications

1. Continuous temperature monitoring and servocontrol

for whole-body cooling (Fig. 7.4)

2. Automatic control of heater output of radiant warmer

or incubator

D. Contraindications

1. Rectal route in tiny infants

E. Equipment Specifications:

Hardware and Consumables

1. Continuous temperature monitoring may be a component of the bedside monitor, free-standing, or incorporated into a radiant warmer or incubator.

2. Capabilities of the neonatal temperature monitor

should include:

a. Resolution to 0.1°C

b. Temperature display in both Fahrenheit and

Centigrade

3. Most free-standing temperature monitors are batterypowered.

4. The monitors employ a thermistor or thermocouple.

5. Monitors using thermistors are identified as Yellow

Springs Instrument Co.(YSI) 400- or YSI 700-compatible.

a. YSI 400-compatible probes are single-element

devices.

b. YSI 700-compatible probes are dual-element

devices.

c. YSI 400 and YSI 700 probes are physically identical

and are available in the same configurations but are

electrically different and will not work interchangeably.

6. Monitors using thermocouple probes are identified as

such, and the probe connection is different from the

thermistor type.

Fig. 7.3. Oxygen consumption as a function of temperature

gradient between skin and environment. (From Adamsons K Jr,

Gandy GM, James LS. The influence of thermal factors upon

oxygen consumption of the newborn human infant. J Pediatr.

1965;66:495, with permission.)

FIG. 7.4. Chest x-ray showing esophageal temperature probe,

used for servocontrolling of cooling blanket in whole-body cooling

protocol.