Why wrapping premature neonates to prevent hypothermia can predispose to overheating

Abstract

Wrapping low-birth-weight neonates in a plastic bag prevents body heat loss. A bonnet can also be used, since large amounts of heat can be lost from the head region, but may provide too much thermal insulation, thus increasing the risk of overheating. We assessed the time required to reach warning body temperature ( t38°C), heat stroke ( t40°C), or extreme value ( t43°C) in a mathematical model that involved calculating various local body heat losses. Simulated heat exchanges were based on body surface temperature distribution measured in preterm neonates exposed to 33°C air temperature (relative air humidity: 35%; air velocity: <0.1 m/s) and covered (torso and limbs) or not with a transparent plastic bag. We also compared metabolic heat production with body heat losses when a bonnet (2 or 3.5 mm thick) covered 10%, 40%, or 100% of the head. Wrapping neonates in a bag (combined or not with a bonnet) does not induce a critical situation as long as metabolic heat production does not increase. When endogenous heat production rises, t38°C ranged between 75 and 287, t40°C between 185 and 549, and t43°C between 287 and 702 min. When this increase was accompanied by a fall in skin temperature, overheating risk was accentuated (37 ≤ t38°C ≤ 45; 99 ≤ t40°C ≤117; 169 ≤ t43°C ≤ 194 min). Thus plastic bag and bonnet may result in hyperthermia but only when metabolic heat production rises while skin temperature falls (impeding body heat losses), as can sometimes happen with fever.