Atlas of Procedures in Neonatology, 4th Edition
Temperature measurement is an important part of normal newborn care. Accurate measurement is important to detect deviations from normal and also for optimal incubator and radiant warmer function.
The purpose of monitoring temperature is to maintain the infant in a thermoneutral environment zone. This is defined as the narrow range of environmental temperature in which the infant maintains a normal body temperature without increasing metabolic rate and hence oxygen consumption.
Temperature monitoring may be done intermittently or continuously. The site of measurement may be core (rectum, esophagus, or tympanic) or surface (skin, axilla). Currently, the axillary route is preferred, especially for preterm neonates. The various methods are discussed further in this chapter.
Intermittent Temperature Monitoring
- Mercury-in-glass thermometer
- Benchmark standard
- Determination time >3 minutes.
- Risk of breakage/mercury poisoning from vaporized mercury (1).
- American Academy of Pediatrics (AAP) recommendation is that mercury thermometers be removed from medical offices, medical facilities, and homes (1).
- No longer used in neonatal units
- Electronic digital thermometers (Fig. 6.1)
- Most widely used
- Thermometer is a small hand-held device.
- Temperature sensor may be a thermistor or thermocouple.
- Temperature is sensed by the probe; the signal is then processed electronically and displayed digitally. There is an audible signal at the end of the determination time window.
- Determination time is <45 seconds.
- Resolution is 0.1°C.
- Probe-type electronic thermometers are designed to be used with disposable probe covers.
- Infrared electronic thermometry
- Sensitive infrared sensor detects infrared energy radiation from the tympanic membrane.
- Sensor converts the infrared signal to an electrical signal.
- Electrical signal is then processed and displayed digitally as temperature.
- Determination time is <2 seconds.
- Designed to be used with disposable sensor head covers.
- Cost-effective (2)
- Rectal measurement in very low-birthweight infants
- Infrared thermometers have less correlation with glass thermometer measurements than digital disposal or electronic thermometers (2).
- Studies have failed to show adequate correlation between infrared thermometer readings and rectal or axillary measurements in newborns (3,4).
- Readings can vary from axillary site depending on environment (radiant warmer, open crib, or incubator) (5).
- Probe-type electronic thermometers
- Always use disposable probe cover.
- Do not force probe, as perforation can occur, e.g., in rectal measurement.
- Infrared thermometers
- Always use disposable sensor head covers.
- Do not force sensor head into the ear canal.
- Do not use in infants with middle ear disease.
- Do not use in very low-birthweight infants because of inappropriate speculum size. Sensor head may not be small enough for low-birthweight infants weighing less than 1,000 g.
- Erroneous readings may result from:
- Not having the probe lined up with the tympanic membrane
- The presence of heavy cerumen
- The presence of serous otitis media (6)
FIG. 6.1. Electronic thermometers: Probe thermometer. (Courtesy of Welch Allyn, New York, USA.)
- Probe-type electronic thermometers
- Apply disposable probe cover to the probe.
- For core temperature, insert probe into the rectum (2 to 3 cm).
- For noninvasive approximation of core temperature, place the probe in the axilla (Fig. 6.2) (7,8 and 9).
- Hold the probe in place and wait for an audible beep before removing the probe.
- Read temperature and return the probe to its compartment to deactivate the unit.
- Infrared thermometers
- Apply disposable cover to the sensor head.
- Gently insert tapered end into the ear canal.
- While holding the unit steady, depress trigger.
- Remove from the ear canal and read temperature.
- Remove used disposable cover.
- Inaccurate reading (10,11,12 and 13)
- Tissue trauma
- Rectal or colonic perforation (10,14,15)
- Pneumoperitoneum (16)
FIG. 6.2. Axillary temperature being taken with electronic probe thermometer. The probe is held perpendicular to the patient, and the arm is held securely against the side of the chest.
- Risk of trauma to the tympanic membrane
- Thermometer housing unit can transmit infection; disinfect after each use.
Continuous Temperature Monitoring
- Provides reliable continuous monitoring of neonatal body temperature.
- Provides trend of temperature over time.
- Provides automated environmental control (Fig. 6.3).
- Sites used may be surface, e.g., skin over the abdomen, or core, e.g., rectum, esophagus.
- Thermistor probes (most widely used)
- The thermistor is a resistive device, having a high negative temperature coefficient of resistance, so that its resistance decreases proportionately as the temperature increases.
- As the resistance of the thermistor changes, the electrical current flowing through the probe changes proportionally.
- The level of current detected by the electronic monitor is converted to thermal units.
- Thermocouple probes
- The thermocouple probe is a very small bead made up of the junction of two dissimilar metals.
- The bead generates a very small voltage proportional to temperature.
- The voltage generated by the bead is measured by the monitor and converted to thermal units.
- The thermocouple and the thermistor are not interchangeable.
- Battery-powered interface devices are available that allow the use of thermocouple probes with thermistor-compatible monitors
- Thermocouple probes are less expensive than thermistors.
FIG. 6.3. 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
- Continuous temperature monitoring for whole-body cooling
- Automatic control of heater output of radiant warmer or incubator
- Rectal route in tiny infants
- Equipment Specifications: Hardware and Consumables
- Continuous temperature monitoring may be a component of the bedside monitor, free-standing, or incorporated into a radiant warmer or incubator.
- Capabilities of the neonatal temperature monitor should include:
- Resolution to 0.1°C
- Temperature display in both Fahrenheit and centigrade
- Most free-standing temperature monitors are battery-powered.
- The monitors employ a thermistor or thermocouple.
- Monitors using thermistors are identified as YSI 400- or YSI 700-compatible.
- YSI 400-compatible probes are single-element devices.
- YSI 700-compatible probes are dual-element devices.
- 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.
- Monitors using thermocouple probes are identified as such, and the probe connection is different from the thermistor type.
- Probes for both thermistors and thermocouples are available in different configurations for different sites. For example:
- Surface skin probe
- Tympanic membrane thermocouple probe
- Do not apply skin probes to broken or bruised skin.
- Do not apply skin probes over clear plastic dressings.
- Do not use fingernails to remove skin surface probes.
- Do not force core probes during insertion.
- Do not reuse disposable probes.
- Shield skin probe with reflective pad if used with radiant warmer or heat lamp.
- When using servocontrol mechanisms for environmental control, take intermittent temperatures at other sites to monitor effectiveness (17).
- Do not use core temperature to servoregulate the patient's environment (18).
- Skin surface probe (Table 6.1)
- Prepare the skin using an alcohol pad to ensure good adhesion to the skin.
- Cover probe with a reflective cover pad (foil-covered foam adhesive pad, incorporated in the disposable probe) (Fig. 6.4). Probe must be covered with an aluminum foil disk to reflect back added heat from devices such as radiant warmers, phototherapy lights, infrared warming lights, and any other external radiant heat-generating sources (19).
- Apply probe over the liver in the supine infant.
- Apply probe to the flank in the prone infant.
- Ensure that skin probe is free of contact with bed (Fig. 6.5).
TABLE 6.1 Site for Temperature Monitoring
- Application of core probe (Table 6.1)
- Choose probe size according to site, i.e., rectum or esophagus.
- Esophageal probe
- Does not need lubrication prior to placement
- Estimate the length of insertion, using the sum of the distance from the mouth to the tragus of the ear and the distance from the ear to the xiphoid.
- Insert probe through nostril until the desired length is reached.
FIG. 6.4. Skin probe properly placed on infant (note that probe has protective foil cover and lies flat on the skin surface).
- Rectal probe
- Lubricate probe before placing in rectum.
- Probe should be placed approximately 3 cm beyond anal sphincter; avoid further advancement to avoid risk of perforation.
- Do not force either probe.
- Connect the probe to the monitor.
- Monitor energy output changes.
- Reposition or replace the probe if temperature recorded does not correlate with that recorded using an electronic thermometer. Skin surface temperature will be cooler than core temperature.
FIG. 6.5. Newborn infant with skin probe free of contact from bed surface.
- Tissue trauma caused by core temperature probe
- Rectal or colonic perforation
- Unsafe environmental temperature control caused by unshielded skin probes or loosely adhered probe, when monitoring is used to servoregulate temperature (Table 6.2).
TABLE 6.2 Potential Pitfalls of Servocontrolled Heating Devices
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