Effects of ambient temperature on oxygen consumption and the circulation in newborn lambs at rest and during hypoxemia.

To assess the effects of environmental temperature on responses to hypoxemia, we studied five unsedated lambs in the first week after birth. We catheterized the carotid artery and pulmonary artery (via the jugular vein). After recovery of at least 1 day, we measured pH, blood gases, arterial and mixed venous blood O2 content, oxygen consumption (VO2), heart rate, carotid and pulmonary arterial pressures, and cardiac output in both warm (25 degrees C) and cool (17.4 +/- 1.1 degree C) environments. In the cool environment, with no shivering, VO2 increased 40% (14.9 to 20.8 ml/kg/min). There were also increases of arteriovenous blood O2 content difference of 19%, cardiac output of 18%, and heart rate of 14%. In four lambs, we studied the same variables during hypoxemia (FiO2 = 0.09 for 1 h) at both temperatures. In the cool environment, hypoxemia produced a greater fall of VO2 (26% versus 6%) and arteriovenous oxygen differences (30% versus 19%) and a smaller increase of cardiac output (8% versus 14%) and heart rate (26% versus 43%). Also in the cool environment, core temperature decreased more (2.1 versus 0.4 degree C), but base deficit was the same (-6 versus -5 mEq/liter). Despite the greater fall in VO2 during hypoxemia in the cool environment, the lowest value achieved was still higher than the level during normoxemia in the warm environment. Similarly, cardiac output during hypoxemia was greater in the cool than in the warm environment. These findings may explain the variability in reported normal resting values and responses to hypoxemia. Contrary to previous reports, they also indicate that during severe hypoxemia neonates have a decreased reserve of metabolic and cardiovascular responses in a cool compared with a warm environment.