Hypothermia in Infants:
The Important Factors

Al Hammadi T., Sinha T.,Ahmed O.K., Hamouda S.B.
Department of Anesthesia, Hamad Medical Corporation
Doha, Qatar

Abstract:

A prospective consecutive observational study over a period of six months was conducted in the pediatric surgical theatre of Hamad General Hospital, Qatar, on infants aged one day to one year who required general anesthesia with or without regional anesthesia. It was found that neonates (aged 1-29 days) had difficulty in maintaining a stable body temperature despite various methods of promoting warmth. Infants (aged 1-12 months) undergoing major surgery were more exposed to hypothermia than those in minor surgery. Causes of hypothermia were multifactorial.

Introduction:

The ability to establish and maintain core body temperature is one of the many physiological adaptations that must occur for humans to survive. The significance of thermal regulation for neonates was appreciated as early as the 1900s when Budin(1) noted a significant difference in neonatal mortality among infants with different body temperatures. Hypothermia is a frequent occurrence in infants and children undergoing surgery and anesthesia.

Exposure of body cavities to low environmental temperature and humidity, infusion of cold fluids and ventilation with dry gases all coupled with the infant’s physical characteristics of a large surface area to body weight ratio, deficient subcutaneous fat and narrow thermoregulatory range increases the potential for an infant to become hypothermic during anesthesia(2).

A fall in temperature is inevitable after general anesthesia but should be avoided in the pediatric age group due to their limited physiological reserve and hypothermia-related postoperative complications. Very little literature is available on factors affecting temperature in infants and less so on how neonates and infants differ in their behavior towards temperature control. This study was designed to see whether neonates differ from infants in their temperature regulation under anesthesia, what factors affect it and what measures can prevent it.

Materials and Methods:

1. Circulating water mattress (Normo-temp, Cincinnati Sub-zero Products, U.S.A.) set at 40oC.

2. Humidifiers for breathing circuits (Datex-Ohmeda, Instrumentarium Corp, USA).

3. Room temperature maintained at 22oC.

4. Intravenous fluids infused at room temperature and blood products, if used, were warmed.

If the temperature of the infant fell below 35.5oC, a forced warm air circulating blanket was added (Bair Hugger, Augustine Medical, Eden Prairee, MN, USA).

For the purpose of the study hypothermia was defined as a fall of body temperature to <36oC. If the body temperature fell below 36oC, infants were considered positive for hypothermia otherwise as negative for hypothermia. Infants who were operated upon in the open incubator with ceiling warmer and those who arrived at the operating room with a temperature < 36oC were excluded from the study.

Statistical analysis using Chi-square (X2)test was done to determine the significance of the difference between these groups and a p value < 0.05 was considered significant.

Results:

During the six months 109 infants were recruited into the study; 53 (48.6%) to group A and 56 (51.4%) to group B. The mean weight was 3.5 Kg and the weight group categorized as < 3.5 Kg and > 3.5Kg. The mean duration of operation was 60 minutes and the surgical duration groups were categorized as < 60 minutes and > 60 minutes. The distribution of patients in the various groups is given in Tables 1 and 2 which show that in the neonates group, 44 (83.0 %) are positive for hypothermia, higher than the 21(37.5%) in the other age group. Nine (17.0%) are negative for hypothermia in the neonate group, lower than the 35(62.5%) in the other negative hypothermic infant group; A highly significant difference between the groups (p= 0.0001).
 

Table 1:  Frequency Distribution of variables under study

Table 2:  Hypothermia (<36 degrees C by Age Group)

Table 3 gives the distribution of the hypothermic infants in group A and Group B depending on their weight, duration and type of operation. In group A a significantly higher number of neonates had major operations (32; 72.7%) of duration > 60minutes (26; 59.1%) and weighed < 3.5 kg (27; 61.4%) suggesting that neonates are more liable to hypothermia than other age groups and the factors influencing this are a major operation of long duration and light weight.

Table 3:  Age Group by Type of Operation and Weight of Hypothermic Patients

In group B sixteen (76.2%) had major operations but 15 (71%) of the infants who became hypothermic had a surgical duration of less than 60 minutes. This was because 11 of the 15 had major operations, an important factor in heat loss. Also considering the weights, 15 of 21 (71.4%) patients with a weight greater than 3.5 Kg became hypothermic; again the type of operation was a factor as nine (60%) had major surgery. It seems that as the infant grows the type of operation is a more important factor than the duration of the operation and the infant’s weight.

Discussion:

Perioperative measurement of body temperature is necessary in infants to detect both hypothermia and hyperthermia. Central temperature monitoring is the preferred method of determining the severity of heat loss. Esophageal, nasopharyngeal and rectal routes are central sites that do not differ significantly(3), and two should be used simultaneously whenever possible.

Hypothermia (< 36°C) that is considered mild according to Okamura4 has many complications such as vasoconstriction increasing systemic vascular resistance leading to increased arterial blood pressure, after load and myocardial oxygen consumption. Mild postoperative hypothermia, though a common occurrence after surgery, may impede immune responses to perioperative wound infection(5). Prevention of hypothermia in infants is important and various warming devices and techniques have been recommended such as the accepted techniques used in this study, maintaining operating room temperature6, using airway humidifiers(7), using circulating hot water mattress or blankets(8) and forced warm air convection blankets(9).

In this study 83% of the neonates became hypothermic irrespective of the warming techniques used. Exposing the body and scrubbing with antiseptic solution is enough to cause a fall in body temperature. The water mattress was not adequate as it warmed only the posterior body surface where the loss of heat was minimal and due to its bulk and weight its use over the body was difficult. The Bair Hugger airflow warmer is more efficient in preventing heat loss and warming the infant. Its efficacy and safety has been described(10). The plastic tube is light, can be secured easily over and around the body of the infant and allows heat to be regained quickly.

In the infant age group (1– 12 months) the maturation of the thermoregulatory centre helps to stimulate heat production and replace heat loss. As the infant grows other factors such as major surgery become more important as causes of hypothermia and weight and the duration of an operation are less significant.

Preterm infants, or those small for gestational age, lose proportionately more heat than their full term counter parts(11) but the difference in their behavior under anesthesia has not been studied. In this study there was no statistically significant difference between preterm and full term neonates probably because larger cohorts were needed.

Conclusion:

Neonates (aged 1-29 days) had difficulty in maintaining a stable body temperature despite various methods of promoting warmth. Infants (aged 1-12 months) undergoing major surgery were more exposed to hypothermia than those in minor surgery. Causes of hypothermia in this group were multifactorial.
 

References:

11. Jahnukainen T, van Ravenswaaij-Arts C, Jalonen J. Dynamics of vasomotor thermoregulation of the skin in term and preterm neonates. Early Human Dev 1993; 33: 133-143.