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°C) and 10 hypothermic (29.3°C) pigs that had been anesthetized and surgically implanted with instruments. Cardiac output ( \(\dot Q\) t), \(\dot V\) o ₂ [oxygen consumption, or \(\dot Q\) t × \(C(a - \bar v)O_2 \) , where \(C(a - \bar v)O_2 \) 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 ( \(P\bar vO_2 \) ) 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 \(P\bar vO_2 \) and critical total oxygen transport (TOT = \(\dot Q\) t × 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 \(\dot V\) o ₂ and blood lactate levels becamePo ₂ or TOT dependent. Again as predicted, the critical \(P\bar vO_2 \) was lower in the hypothermic animals (15.5 ± 1.0 mm Hg at 29.3°C compared with 22.0 ± 1.4 mm Hg at 36.8°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 \(P\bar vO_2 \) and TOT (the difference between the normoxic and the critical values).