1426 EFFECTS OF AMBIENT TEMPERATURE ON CIRCULATION AND O₂ CONSUMPTION (VO₂) IN NEWBORN LAMBS, AT REST AND DURING ACUTE HYPOXIA

Abstract

To assess the effects of environmental temperature on the response to hypoxia in neonates, we studied 5 unsedated lambs in the first week after birth. We catheterized the carotid artery (CA) and pulmonary artery (PA) (via the jugular vein). After at least one day, we measured pH, blood gases, arterial and mixed venous O₂ content, VO₂, heart rate (HR), CA and PA pressures, and cardiac output (CO) in both warm (W) (25°C) and cool (C) (17.4 ± 1.1°C) environments. In C, with no shivering, VO₂ increased 40% (14.9 to 20.8 ml/kg/min) and arteriovenous O₂ content difference (A-V) 19%, CO 18%, and HR 14%. In 4 lambs, we studied the same variables during hypoxia (FIO₂=0.09 for 1 hr) at both temperatures. In C, hypoxia produced a greater fall of VO₂ (26% vs. 6%) and of A-V (30% vs. 19%) and a smaller increase of CO (8% vs. 14%) and of HR (26% vs. 43%); in C, core temperature decreased more (2.1° vs. 0.4°), but base deficit was the same (-6 vs. -4.5 meq). Despite the greater fall in VO₂ during hypoxia in C, the lowest value achieved was still higher than that in W, even during normoxia. Similarly, CO during hypoxia was greater in C than in W. These findings may explain variabilities reported in normal resting values and responses to hypoxia. They also indicate that, contrary to previous reports, during severe hypoxia neonates have a decreased reserve of metabolic and cardiovascular responses in a cool compared with a warm environment. (Supported by NIH grant HL 23681.)

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