Pediatric Hypothermia: An Ambiguous Issue

Abstract

Hypothermia in pediatrics is mainly about small body size. The key thermal factor here is the large surface-to-volume ratio. Although small mammals, including human infants and children, are adapted to higher heat losses through their elevated metabolic rate and thermogenic capacity, they are still at risk of hypothermia because of a small regulatory range and an impending metabolic exhaustion. However, some small mammalian species (hibernators) use reduced metabolic rates and lowered body temperatures as adaptations to impaired energy supply. Similar to nature, hypothermia has contradictory effects in clinical pediatrics as well: In neonates, it is a serious risk factor affecting respiratory adaptation in term and developmental outcome in preterm infants. On the other hand, it is an important self-protective response to neonatal hypoxia and an evidence-based treatment option for asphyxiated babies. In children, hypothermia first enabled the surgical repair of congenital heart defects and promotes favorable outcome after ice water drowning. Yet, it is also a major threat in various prehospital and clinical settings and has no proven therapeutic benefit in pediatric critical care. All in all, pediatric hypothermia is an ambiguous issue whose harmful or beneficial effects strongly depend on the particular circumstances.

Keywords: adaptation; body size; children; drowning; hibernation; hypothermia; hypoxia; infants; neonates; thermoregulation.

Figure 1

Tolerance to metabolic reduction as a function of body size. Following a general biological rule (“mouse-to elephant curve”), the specific (i.e., weight-related) basal metabolic rate of mammals (in watts per kg) decreases with increasing body mass [10,11,12]. Hibernating species (e.g., hazel mouse, ground squirrel, alpine marmot), while overwintering at near-zero body temperatures, reduce their metabolic rate to a fairly uniform minimal level that equals the specific basal metabolic rate achieved by the very largest mammals (elephant, blue whale) by body size alone [13,14,15]. Black bears fit into this overall hibernation pattern even though their body temperature remains higher (approximately 33 °C) and the relative amount of metabolic reduction is smaller due to their lower size-related basal metabolic rate [16,17]. Although there is no strict correlation between the (endogenous) metabolic reduction and the (concomitant) temperature decline in hibernators (both in hazel mice and in black bears, the magnitude of metabolic reduction exceeds the pure temperature effect), this relationship suggests an increasing hypothermia tolerance with decreasing body mass among mammals. This may also explain why human infants (HI) and small children tend to tolerate a higher degree of cold-induced metabolic reduction than human adults (HA), owing to their larger metabolic “drop height” [5,6].

Figure 2

Vicious cycle of hypothermia-triggered hypoxia in neonates. The increase in O₂ consumption rate caused by non-shivering thermogenesis in brown adipose tissue conflicts with the limited O₂ supply by the just aerated lungs, especially under conditions of cold-induced peripheral vasoconstriction. The metabolic (lactate) acidosis resulting from higher anaerobic metabolism leads to an increase in pulmonary vascular resistance, which further impairs O₂ uptake in the lungs and thus aggravates the mismatch between O₂ demand and supply (adapted from [3]).

Figure 3

Self-protective role of hypothermia in hypoxic neonates. As hypoxia and acidosis exert a suppressive effect on non-shivering thermogenesis in the brown adipose tissue, the usual thermoregulatory increase in O₂ consumption rate with decreasing ambient temperature is replaced by a direct cold-induced metabolic reduction, similar to induced hypothermia. Moreover, a spontaneous decrease in O₂ consumption rate (arrow) in response to hypoxia may already occur at thermoneutral temperatures (shaded area), suggesting that neonates have a “hibernation-like” ability to reduce their metabolic rate (hypoxic hypometabolism) before any drop in body temperature (adapted from rat study data by Mortola [46]).

Figure 4

Factors affecting favorable outcome in drowning accidents of young children in ice-cold waters. Both the abrupt overwhelming of the cold defense reaction and the marked diving response attenuate the resulting metabolic disturbances, thereby enhancing the intrinsic resistance of the child’s heart to ventricular fibrillation (VF) or other types of cold-induced cardiac arrest. The sustained slow heart beat provides central brain cooling in addition to continuing washout of waste products from the tissues and thus prevents hypoxia/ischemia from assuming a critical level before a protective degree of hypothermia has been reached (adapted from [74]).