Increased blood oxygen affinity decreases canine brain oxygen consumption.

We have studied the effect of increased blood O2 affinity on O2 delivery to the isolated canine brain. After surgical isolation, the brain, enclosed in the calvarium, was perfused alternately from two pump-oxygenators with normal blood (P 50 [7.4] = 30 +/- 2 torr [S.D.]) and with blood whose P50 was reduced to 18 +/- 2 torr by carbamylation. [Hb], acid-base balance, blood gases, and flow rate were carefully matched in the two circuits. Although blood [Hb] was reduced to approximately 10 gm/dl, other perfusion variables such as CBF (65 +/- 6 ml/min/100 gm) and arterial blood oxygen saturation (96% to 99%) were normal for the dog. Under these conditions cerebral VO2 (Fick) averaged 3.87 +/- 0.73 ml/min/100 mg (S.D.) with control blood and 2.94 +/- 0.69 with low P50 blood (mean delta = 24%, n = 14, p less than 0.001), and PVO2 averaged 31 +/- 2 and 21 +/- 2 torr, respectively (p less than 0.001). The fall in VO2 during low P50 perfusion was associated with a decrease in [A-V]O2 difference and a rise in CVO2 of 1.2 ml/dl, which suggests that O2 extraction at PVO2 approximately 20 torr is curtailed. The EEG, previously shown to correlate with VO2 in this model, invariably deteriorated after 30 to 60 sec of low P50 perfusion and improved in 30 to 60 sec after reperfusion with normal blood. CBV increased by 0.9 ml/100 gm during low P50 perfusion, implying capillary recruitment. In a parallel series of experiments, four brains were alternately perfused with normal blood (pH 7.41, PCO2 38 torr, P50 [7.4] = 30 torr) and alkalotic blood (pH 7.98, PCO2 39 torr, P50 [7.98] = 17.3 torr). With flow rates equal for both normal and experimental blood, PVO2 averaged 31 +/- 4 (S.D.) and 21 +/- 3 torr (p less than 0.001), respectively, and VO2 averaged 4.33 +/- 0.52 ml/min/100 gm and 3.18 +/- 0.52 (p less than 0.001). With pH at 7.4 and 7.8, VO2 averaged 4.42 +/- 0.77 ml/min/100 gm and 3.66 +/- 0.99, respectively (p less than 0.01). The data indicate that a reduced P50 limits O2 diffusion to brain at a normal but fixed blood flow rate despite capillary recruitment.