Thermal Regulation of Newborn and Hypothermia

1. DEFINITIONS

• Neonatal Hypothermia (WHO): Axillary temperature $<36.5^{\circ }C$ ($97.7^{\circ }F$).
• Thermoneutral Environment (TNE): Narrow temperature range where the neonate maintains normal body temperature with minimal basal metabolic rate (BMR) and oxygen consumption.
• Normal Range: $36.5^{\circ }C-37.5^{\circ }C$.

2. THERMOREGULATION PECULIARITIES IN NEWBORN

Newborns are homeothermic but their ability to maintain temperature is easily overwhelmed.

• Physical Vulnerabilities:
  • High Surface Area to Mass Ratio: Predisposes to rapid heat loss.
  • Limited Insulation: Decreased subcutaneous fat.
  • Immature Skin: High permeability increases transepidermal water loss (evaporative loss), especially in preterms.
• Physiological Limitations:
  • Non-Shivering Thermogenesis (NST): Primary mechanism of heat production via metabolism of Brown Adipose Tissue (BAT).
  • Poor Shivering: Neonates have very limited ability to generate heat via shivering.
  • Vasomotor Control: Poor peripheral vasoconstriction in extreme preterms limits heat conservation.
  • Limited Stores: Low glycogen and fat stores limit metabolic response to cold.
• Mechanisms of Heat Loss:
  • Radiation: Loss to colder surrounding objects (e.g., cold incubator walls).
  • Convection: Loss to air currents (e.g., drafts, fans).
  • Evaporation: Conversion of water to gas (e.g., wet skin at birth); major loss in preterms.
  • Conduction: Loss to cold surfaces in direct contact (e.g., cold mattress).

3. PHYSIOLOGICAL AND BIOCHEMICAL CONSEQUENCES

Hypothermia causes serious homeostatic disturbances and is an independent predictor of mortality.

A. Physiological Consequences

• General: Peripheral vasoconstriction (pallor, acrocyanosis, cool extremities), irritability.
• CNS: Lethargy, poor suck/cry, hypotonia, apnea, intraventricular hemorrhage (IVH).
• Respiratory: Respiratory distress, increased pulmonary vascular resistance (PPHN), pulmonary hemorrhage.
• Cardiovascular: Bradycardia, hypotension, decreased cardiac output.
• Gastrointestinal: Abdominal distension, emesis, feed intolerance, necrotizing enterocolitis (NEC).
• Growth: Chronic cold stress leads to poor weight gain due to caloric diversion for thermogenesis.

B. Biochemical Consequences

• Hypoglycemia: Due to increased metabolic rate and glycogen depletion.
• Metabolic Acidosis: Due to tissue hypoxia and anaerobic metabolism (lactic acidosis).
• Hypoxia: Increased oxygen consumption leads to hypoxemia.
• Coagulation Failure: Disseminated intravascular coagulation (DIC) may occur.
• Electrolytes/Renal: Hyperkalemia, azotemia, oliguria.

4. PREVENTION: THE WARM CHAIN

A set of 10 interlinked steps to minimize heat loss.

1. Warm Delivery Room: Temperature $25-28^{\circ }C$, draft-free.
2. Warm Resuscitation: Use pre-warmed radiant warmer, warm linen/supplies.
3. Immediate Drying: Dry with warm towel, discard wet linen, cover head with cap.
4. Skin-to-Skin Contact (STS): Immediate STS for stable neonates; effectively maintains temperature and promotes breastfeeding.
5. Breastfeeding: Start within 1 hour to provide energy substrate.
6. Postpone Bathing: Delay until stable/24 hours; sponge bath or swaddle bath preferred.
7. Appropriate Clothing: 1-2 layers more than adults, cap, socks, mittens.
8. Mother and Baby Together: Rooming-in/bedding-in for warmth.
9. Warm Transportation: Use incubator or STS (Kangaroo Mother Care) during transport.
10. Training: Education of healthcare personnel.

Specifics for Preterm Neonates (<32 weeks):

• Occlusive Wraps: Use food-grade polyethylene bag/wrap immediately at birth without drying to reduce evaporative loss.
• Equipment: Use radiant warmer or incubator (double-wall preferred).
• Humidification: 80% humidity in incubators for <28 weeks gestation to reduce insensible water loss.

5. MANAGEMENT OF HYPOTHERMIA

Management depends on the severity (axillary temperature).

A. Classification

• Cold Stress: $36.0^{\circ }C-36.4^{\circ }C$.
• Moderate Hypothermia: $32.0^{\circ }C-35.9^{\circ }C$.
• Severe Hypothermia: $<32.0^{\circ }C$.

B. Action Plan

1. Cold Stress ($36.0-36.4^{\circ }C$):

• Immediate Action: Cover adequately, remove wet/cold clothes.
• Rewarming: Skin-to-skin contact with mother.
• Environment: Warm the room/bed; use heater if needed.
• Feeding: Initiate breastfeeding.

2. Moderate Hypothermia ($32.0-35.9^{\circ }C$):

• Rewarming: Use radiant warmer or incubator.
• Alternative: STS contact if equipment unavailable.
• Reduce Loss: Apply warm towels or phase-changing mattress; cover head.

3. Severe Hypothermia ($<32.0^{\circ }C$):

• Admission: Admit to NICU immediately.
• Rapid Rewarming: Use radiant warmer/incubator. Rapid rewarming until $34^{\circ }C$, followed by slow rewarming to $36.5^{\circ }C$ to avoid apnea/hypotension at rate of $0.5^{\circ }C$ per hour.
• Supportive Care:
  • Start IV fluids (10% Dextrose, 60-80 mL/kg) to prevent hypoglycemia.
  • Monitor blood glucose, oxygen saturation, and blood pressure.
  • Administer Vitamin K injection.
  • Sepsis screen and antibiotics if infection suspected (hypothermia is a sign of sepsis).