Range of critical temperatures in sick and premature newborn babies.

The human baby, even if premature, shows evidence of homeothermy (Bruck, 1961; Day, Curtis, and Kelly, 1943). When subjected to a cool environment, his metabolic rate increases in an attempt to keep pace with increased heat loss. When subjected to an excessively warm environment, he becomes restless and active with hurried panting respiration so that again metabolic rate is increased. Between these two extremes is a small range of thermal environment known as the neutral range where metabolic rate is minimal. Since the thermal environment depends on ambient air temperature, air flow rate, relative humidity, and radiant heat exchange, it is inadequate to express such an environment in terms of a single temperature-the 'environmental temperature', unless other factors are controlled or eliminated. In this study, conductive heat loss was limited by lying the baby on a napkin, air flow rate was constant at about 1 m./ min., ambient vapour pressure (not relative humidity) was constant, and the walls of the baby chamber were opaque to radiant heat (for further details, see Scopes and Ahmed, 1966a). Under these conditions and provided, (1) the baby is capable of a metabolic response to cold, and (2) the baby's colonic temperature remains essentially unchanged, one can define the neutral temperature in terms of environmental temperature by measuring 02 consumption rates at various environmental temperatures. The upper end of the range is inconstant because some infants in a very warm environment (36°-37° C.) remain sleeping with no immediate rise in rate Of 02 consumption, despite a rapidly rising colonic temperature. The lower end of the range is known as the critical temperature. It varies with size, age, and clinical condition. In the course ofa study of rates Of02 consumption in babies (Scopes, 1965), critical temperatures were frequently assessed. Since a knowledge of the critical temperature of an individual baby is useful