23

Maintenance of Thermal

Homeostasis

3

Dora C. Rioja-Mazza

A. Definitions

1. Homeostasis: Fundamental mechanism whereby living

things regulate their internal environment within tolerable limits, thus keeping a dynamic equilibrium and

maintaining a stable, constant condition. From the

Greek homeo (same, like) and stasis (stable state) (1).

2. Normal body temperature: The core body temperature

is maintained by the term infant within the range of

36.5°C to 37.5°C, and the skin temperature, from

0.5°C to 1°C lower (2).

3. Neutral thermal environment: The range of ambient

temperature required for the infant (for each gestational age and weight) to keep a normal body temperature and a minimal basal metabolic rate (2–4).

4. Thermoregulation: Mechanisms by which the infant

tries to balance heat production and heat loss to accommodate the thermal environment (5–7).

5. Cold stress: The infant senses heat loss as a stress and

responds with increased heat production and peripheral vasoconstriction, with centralization of circulation,

in an effort to maintain the core temperature (8).

6. Hypothermia: Heat losses exceed heat production,

dropping the infant’s temperature below the normal

range of 36.5°C to 37.5°C (97.7°F to 99.5°F) (9).

a. Mild hypothermia (cold stress): 36°C to 36.4°C

(96.8°F to 97.5°F)

b. Moderate hypothermia: 32°C to 35.9°C (89.6°F to

96.6°F)

c. Severe hypothermia: Below 32°C (89.6°F)

7. Hyperthermia: An increase in the infant’s temperature

to above 37.5°C (99.5°F), due to a warm environment.

Hyperthermia is less common than hypothermia but is

equally dangerous. Clinically, it may be difficult to distinguish hyperthermia from fever (infectious origin);

therefore, always consider both causes in any increase

in temperature (9).

B. Background

1. Effects of hypothermia:

a. Hypothermia may have severe consequences in

newborn infants and may even lead to arrhythmias

and death (10,11).

b. Peripheral vasoconstriction: Acrocyanosis, paleness,

and coldness to touch

c. Respiratory distress, apnea, and bradycardia (12,13)

d. Depletion of caloric reserves and hypoglycemia,

causing a shift to anaerobic metabolism and lactic

acid production (14,15)

e. Increased oxygen consumption and metabolic

demands result in metabolic acidosis—a strong pulmonary vasoconstrictor inducing hypoxemia and

central cyanosis (16–18).

f. Mobilization of norepinephrine, TSH, T4 and free

fatty acids: Norepinephrine release promotes pulmonary hypertension and pulmonary ventilation–

perfusion mismatch (19).

g. Decreased cardiac output, increased systemic vascular resistance, and decreased intestinal and cerebral

blood flow (11)

h. Decreased number, activation, and aggregation of

platelets (11)

i. Impaired neutrophil release and function (11)

j. Risk of kernicterus at low levels of serum bilirubin

(20)

k. Poor weight gain with chronic hypothermia (21)

l. Controlled hypothermia may have a neuroprotective effect in term and near-term infants with moderate to severe hypoxic ischemic encephalopathy

(22,23).

2. Effects of hyperthermia or overheating (9)

a. Peripheral vasodilatation: The skin is hot, the extremities are red, and the face is flushed. Diaphoresis present in full-term infants. Skin temperature is higher

than core temperature.

b. Apnea, tachypnea

c. Tachycardia and hypotension

d. The infant assumes a spread-eagle posture.

e. Hyperactivity and irritability: The infant becomes

restless and cries, then feeds poorly, with lethargy

and hypotonia.

f. If hyperthermia is severe, shock, seizures, and coma

may occur.

g. If the increase in temperature is due to hypermetabolism (infection), paleness, vasoconstriction,

cool extremities, and a core temperature higher

than skin temperature may be noted.

24 Section I ■ Preparation and Support

3. Factors affecting heat loss

a. Infant

(1) Large surface area relative to body mass

(2) Relatively large head with highly vascular fontanelle

(3) Skin maturation/thickness, epidermal barrier functionally mature at 32 to 34 weeks. Transepidermal

water loss may be 10 to 15 times greater in preterm

infants of 25 weeks’ gestation (2).

(4) Decreased stores of subcutaneous fat and brown

adipose tissue in more premature infants (7)

(5) Inability to signal discomfort or trigger heat production (shivering) (7)

b. Environment (3,4)

(1) Physical contact with cold or warm objects

(conduction)

(2) Radiant heat loss or gain from proximity to hot

or cold objects (radiation)

(3) Wet or exposed body surfaces (evaporation)

(4) Air currents in nursery or in incubator fan (convection)

(5) Excessive or insufficient coverings or clothing

c. Other factors

(1) Metabolic demands of disease: Asphyxia, respiratory distress, sepsis (11)

(2) Pharmacologic agents (e.g., vasodilating drugs,

maternal analgesics, and unwarmed IV infusions, including blood products) (11)

(3) Medical stability of infant prior to procedure

(4) Thermogenic response matures with increase in

postconception age (4)

C. Indications

1. Maintenance of thermal homeostasis is necessary at all

times, but particular attention should be paid when the neonate is undergoing diagnostic or therapeutic procedures.

2. Avoids increase in insensible water loss (IWL) and

improves caloric utilization

D. Equipment, Techniques, and

Complications

1. Prevention of heat loss in the delivery room

a. Warm environment (Table 3.1), room temperature

>25°C; place infant on a radiant warmer, dry the

skin with a prewarmed towel, and then remove any

wet towels immediately (9,14,24).

b. Use occlusive plastic blankets/bags (2,10,25) (Fig. 3.1).

(1) Polyethylene bags (20 cm × 50 cm) prevent

evaporative heat loss in infants <29 weeks’ gestation. Their diathermancy allows transmission of

radiant heat to the infant. Immediately after

delivery, open the bag under the radiant warmer;

wrap the wet infant’s body from the shoulders

down, and dry only the head. Place hat on head.

Remove the wrap after the infant has been stable in the neonatal intensive care unit (NICU),

in a humidified environment, for 1 hour.

(2) Environment: Maintains temperature and

reduces IWL by 25% (24,25)

(3) Access: Allows neonatal resuscitation (secure airway, intubation, and chest compressions), but

vascular access is limited

(4) Asepsis: Limited by access

(5) Precautions: Record core temperature every 5 to

10 minutes until infant is stable.

(6) Complications: Hyperthermia, skin maceration,

risk of infection

c. Hats (2,10,26)

(1) Stockinette caps are not effective in reducing

heat loss in term infants in the delivery room;

there is insufficient evidence in preterm

infants.

(2) Woolen hats may reduce or prevent heat loss in

term infants in the delivery room.

2. Prevention of heat loss in the NICU

a. Rigid plastic heat shields (heat shielding)

(1) Environment: Reduces IWL by 25% (27)

(2) Access: Very limited

(3) Asepsis: Limited by access

(4) Precautions: Avoid direct skin contact.

(5) Complications: Hyperthermia, skin maceration,

risk of infection

b. Heat lamp: As an extra heat source (28)

(1) Environment: Increased IWL

(2) Access: Limited by other equipment used (open

incubator, bassinette walls)

(3) Asepsis: May be affected by limited access

(4) Precautions: Record temperature every 5 to

10 minutes or use a continuous monitor. To avoid

Table 3.1 Room Temperatures and Humidity by Gestational Age

and Birthweight

Gestational Age (wks) Birthweight (g) Delivery/Stabilization Room Humidity

≤26 ≤750 26–27°C (78–80°F) 50%

27–28 750–1,000

29–32 1,001–1,500 ≥22°C (≥72°F)

33–36 1,501–2,500 None

37–42 ≥2,501 ≥21°C (≥70°F)