Response to cooling temperature in infants born at an altitude of 4,330 meters

The metabolic response to reduction in ambient temperature was studied in healthy, full-term, 1-d-old infants in Lima (50 m altitude, n = 20) and Cerro de Pasco (4,330 m, barometric pressure ~ 450 mm Hg, n = 20), Peru. Oxygen consumption (V̇O₂) and carbon dioxide production (V̇CO₂) were measured with an open-flow system as each infant rested quietly in a cylindrical humicrib, at wall temperatures of 35°C (warm) and 26°C (cool). The infants were exposed for 20 min to both temperatures, with the higher temperature followed by the lower, and oxygen consumption (V̇O₂) and carbon dioxide production (V̇CO₂) were measured over the last 8 min of each exposure. Average birth weight in Cerro de Pasco (2,933 ± 77 g [mean ± SE]) was less than in Lima (3,457 ± 73 g). In warm conditions, infants born at high altitude had slightly yet significantly lower body and skin temperatures than did those born at low altitude, with similar values of V̇O₂ and heart rate (HR). Neither body nor skin temperature changed in either group during cooling. At low altitude, cooling increased V̇O₂ (~ 34%), whereas no significant increase occurred in the high-altitude group. A similar response occurred for HR. Among several possibilities, the most likely interpretation of the results would be that of a decreased thermogenic capacity in the high- altitude infants because of the correspondingly lower oxygen availability during gestation.