Heat Losses in Newborn Infants of Different Body Size Measured by Direct Calorimetry in a Thermoneutral and a Cold Environment

Abstract

Small-for-date infants (less than the 10th weight percentile, group I), appropriate-for-date (between the 10th and the 90th percentile, group II), and large-for-date infants (above the 90th percentile, group III) were exposed to ambient temperatures (T_a) of 28 and 32 °C in a direct calorimeter. Total heat losses ( + Ċ + Ė) expressed per unit body surface were similar in the three groups, but when expressed per kg of body weight, ( + Ċ + Ė) were more elevated in group I than in groups II and III. At 28 °C T_a ( + Ċ + Ė) was 4.35 ± 0.15, 3.6 ± 0.1, and 3.04 ± 0.09 W/kg in groups I, II and III, respectively. Metabolic rate was higher at 28 than at 32 °C in the three groups. Heat storage (⋅) at 28 °C was negative in the three groups; at 32 °C, ⋅ was negative in group I, but thermal equilibrium was reached in groups II and III (⋅ = 0). Thermal body insulation between the three groups was not significantly different. In order to practically determine the dry heat losses of a newborn infant a cooling constant of 5.7·W·m^––²·°C ^––1 was calculated. Our results show that at low T_a, the surface-body mass ratio of small-sized infants plays a more important role in inducing a negative thermal balance, than a lack of stimulation of heat production or an inefficient thermal body insulation.