13.2 Thermoregulation

13.2 Thermoregulation

Quick Answer: Neonates lose heat by convection, conduction, radiation, and evaporation. Wet, extremely preterm skin makes evaporation the delivery-room killer; a polyethylene wrap or bag (face free) is the specific countermeasure. In the NICU, a neutral thermal environment (NTE) lets the infant keep a normal core temperature with the least oxygen and calorie cost. Servo (skin) mode heats to a probe target; air mode holds a set air temperature. Admission hypothermia is associated with higher mortality. Fever is infection until proven otherwise. Therapeutic hypothermia for HIE is chapter 11—do not treat this section as a cooling-protocol dump.

Thermoregulation is Multisystem problem MU-17. It is easy to dismiss as “keep the baby warm,” which is how infants die in the first hour and how exam items separate people who can name a mechanism from people who only remember a hat. OpenExamPrep independent teaching covers this patient problem as it appears on AACN Certification Corporation's current Neonatal CCRN Test Plan. Thermal care is also developmental care and Golden Hour practice, not a housekeeping footnote.

A neonate is a high-surface-area, low-insulation organism. Term infants have some brown fat and a flexed posture. Preterm infants have little subcutaneous fat, an extended posture, wet immature skin, and a brain and viscera that dump heat. Heat production means oxygen consumption. Cold stress is not a comfort issue; it is a metabolic, pulmonary, and neurologic insult.

Four paths of heat loss

Name the path, then name the fix. Exam items love a window, a wet towel, a cold scale, and an unheated gas blender in the same vignette.

Radiation is the trap. Teams say “convection” whenever the room feels cold. Radiation is loss to the surface of a cold object across space. Convection is air movement. If the air is still and the infant is next to a cold window, the named path is radiation.

Evaporation dominates the first minutes of life and the first days of ELBW life. A wet 500 g infant on an open warmer is a swamp cooler. You cannot outrun that physics with a hat alone.

Neutral thermal environment

The NTE is the range of environmental temperature in which a clothed or naked infant (specify which) maintains a normal core temperature with minimal metabolic work. Too cold: oxygen consumption, norepinephrine, brown-fat metabolism (if present), pulmonary vasoconstriction, hypoglycemia, acidosis, and apnea. Too hot: increased insensible loss, tachycardia, and in the extreme, heat illness. The clinical target for axillary or skin temperature is commonly discussed near 36.5–37.5°C. World Health Organization and Neonatal Resuscitation Program materials use that band for a normal temperature after birth. AACN does not publish a unique official degree cutoff. Follow the probe site your unit defined.

Core-periphery gap (central warm, feet cold) is a perfusion and thermal clue. A single skin probe on the abdomen can lie if it is off, wet, or lying on a heat source. Check probe adhesion whenever the incubator “will not come up” or is overheating.

Servo versus air mode

Servo-control (skin mode) uses a skin-temperature probe. The heater output rises if the skin is below the set point and falls if the skin is above it. Advantages: it tracks the infant when you open portholes or start phototherapy. Dangers: a detached probe reads the air as “cold infant” and can overheat; a probe lying on a heat mattress can under-heat. Always look at the infant, the probe, and the air temperature, not only the set point.

Air mode holds a chosen air temperature. Advantages: stable when you do not trust a probe, during some procedures, and when weaning toward an open crib. Dangers: the infant can drift hypothermic or febrile if you do not retitrate after a bath, a line attempt, or phototherapy.

Weaning to a crib is a thermal and calorie test, not a calendar test. An infant who maintains temperature in a light wrap, feeds well, and does not spike oxygen need is ready by physiology. An infant who is still “servo 36.8 and heater at 80%” is not ready because someone wrote day-of-life 14 on the board.

Plastic wrap, humidity, and the Golden Hour

For ELBW and very preterm infants, NRP-style thermal care includes a preheated radiant warmer, a warm delivery room when feasible, delayed drying in favor of occlusive polyethylene wrap or a bag from the neck down, a hat, and transfer into a pre-warmed, preferably humidified, incubator. The face stays free for airway management. You do not leave the infant in a puddle of amniotic fluid under a blanket and call it thermoregulation. You do not unwrap the 24-week infant for a “full assessment” in a cold room.

Once in the unit, high incubator humidity cuts ongoing TEWL (section 13.1 and chapter 9). Wean humidity over days as skin keratinizes; abrupt dehumidification is a hypernatremia machine. Heated, humidified inspired gas reduces respiratory evaporative loss, which matters on a dry blender more than teams admit.

Admission hypothermia as a mortality risk

Admission temperature is a quality metric because it is a survival metric. Observational NICU literature has repeatedly associated admission hypothermia—often discussed as a temperature below 36.5°C, with worse gradients as temperature falls further—with higher mortality and with IVH, RDS, hypoglycemia, acidosis, and late-onset sepsis risk. This is not an AACN-published handbook constant; it is why Golden Hour bundles exist. Classify severity the way your unit and WHO-style teaching do (mild, moderate, severe), but do not argue taxonomy while the infant is 34°C on the scale.

Cold stress physiology to carry into items:

• Increased oxygen consumption → hypoxemia and pulmonary hypertension risk
• Norepinephrine → glycogenolysis then hypoglycemia
• Anaerobic metabolism → metabolic acidosis (section 13.1)
• Surfactant dysfunction and increased RDS work
• Pressure-passive cerebral circulation plus hemodynamic swings → IVH risk in the preterm infant (chapter 11)

Rewarm a hypothermic preterm infant gradually in a controlled environment. Aggressive external overheating, especially of a cold acidotic infant, can cause overshoot, vasodilation, and hypotension. Do not plunge an ELBW infant into a hot bath. Do not park a cold infant on an unmonitored heating pad.

Worked scenario: a 25-week infant arrives with an axillary temperature of 35.2°C, glucose 28 mg/dL, and pH 7.19 with a high lactate. The thermal failure is already a glucose and acid-base problem. Wrap, humidity, and a controlled NTE sit beside D10W and ventilation. Treating only the gas without closing the warmer is incomplete.

Fever is infection until you have a better reason

A true fever in a neonate is infection until cultures, a physical exam, and a unit sepsis pathway say otherwise. Other causes exist: overbundling, a servo probe failure that overheats, maternal fever and epidural effects in the first hours, dehydration, drug withdrawal, and central dysregulation after severe brain injury. You still evaluate sepsis. You do not document “overdressed” and skip a workup in a 9-day-old who is 38.5°C, mottled, and feeding poorly. Temperature instability—hypothermia or hyperthermia—is a classic late-onset sepsis clue (chapter 14).

What this section is not: therapeutic hypothermia

Therapeutic hypothermia for hypoxic-ischemic encephalopathy (typically whole-body cooling near 33.5°C for 72 hours with controlled rewarming) is taught in chapter 11 as neuroprotection and listed again among advanced therapies in chapter 16. This MU-17 section is homeostatic thermal care: preventing accidental hypothermia and recognizing fever. Do not cool a well preterm infant who is 36.0°C on admission “because cooling is good.” Do not withhold Golden Hour warming because you heard that some asphyxiated term infants are cooled. Those are different patients, different indications, and a different protocol with inclusion criteria, amplitude-integrated EEG, and sedation/pain considerations.

Exam traps: calling radiation “convection”; unwrapping an ELBW infant to “let them air-dry”; servo mode with a dangling probe; treating admission hypothermia as cosmetic; skipping a sepsis evaluation for fever; and recycling the 33.5°C HIE protocol as the answer to every temperature question. Pediatric thermal teaching at /study-guides/ccrn-pediatric still uses the four paths; neonatal ELBW wrap-and-humidity is the extra layer. Practice items: /practice/ccrn-neonatal.