Decreased critical mixed venous oxygen tension and critical oxygen transport during induced hypothermia in pigs.

The effects of hypothermia on oxygen delivery and tolerance to hypoxia were studied in 8 normothermic (36.8 degrees C) and 10 hypothermic (29.3 degrees C) pigs that had been anesthetized and surgically implanted with instruments. Cardiac output (QT), VO2 [oxygen consumption, or QT X C(a-v)O2, where C(a-v)O2 is arteriovenous oxygen content difference], arterial and mixed venous blood gas values, and lactate concentrations were measured as the animals were made progressively hypoxic. Under control, normoxic conditions, mixed venous oxygen tension (PvO2) was 41.4 +/- 2.1 mm Hg (mean +/- SE) in the normothermic animals and 26.1 +/- 1.6 mm Hg in the hypothermic animals; these values are close to those predicted in our previous theoretical analysis. To study tolerance to hypoxia during hypothermia, critical PvO2 and critical total oxygen transport (TOT = QT X CaO2, where CaO2 is oxygen content of arterial blood) were determined by decreasing the inspired oxygen concentration (FIO2) in steps and measuring the point where VO2 and blood lactate levels became PO2 or TOT dependent. Again as predicted, the critical PVO2 was lower in the hypothermic animals (15.5 +/- 1.0 mm Hg at 29.3 degrees C compared with 22.0 +/- 1.4 mm Hg at 36.8 degrees C), but critical venous oxyhemoglobin saturation values were not statistically different at the two temperatures. Critical TOT was also decreased during hypothermia, as was the margin of reserve in both PVO2 and TOT (the difference between the normoxic and the critical values).