INCREASED BLOOD-OXYGEN AFFINITY DECREASES CANINE BRAIN OXYGEN-CONSUMPTION

  • 1 January 1982
    • journal article
    • research article
    • Vol. 100  (3) , 411-424
Abstract
The effect of increased blood O2 affinity on O2 delivery to the isolated canine brain was studied. After surgical isolation, the brain, enclosed in the calvarium, was perfused alternately from 2 pump-oxygenators with normal blood (P50 [blood O2 pressure at 50% O2 saturation] [7.4] = 30 .+-. 2 torr [SD]) and with blood whose P50 was reduced to 18 .+-. 2 torr by carbamylation. [Hb] [Hb concentration], acid-base balance, blood gases, and flow rate were carefully matched in the 2 circuits. Although blood [Hb] was reduced to .apprx. 10 g/dl, other perfusion variables such as CBF [cerebral blood flow] (65 .+-. 6 ml/min per 100 g) and arterial blood O2 saturation (96-99%) were normal for the dog. Under these conditions cerebral .ovrhdot.VO2 [O2 consumption] averaged 3.87 .+-. 0.73 ml/min per 100 g (SD) with control blood and 2.94 .+-. 0.69 with low P50 blood (mean .DELTA. = 24%, n = 14, P < 0.001), and PVO2 [venous blood O2 pressure] averaged 31 .+-. 2 and 21 .+-. 21 .+-. 2 torr, respectively (P < 0.001). The fall in .ovrhdot.VO2 during low P50 perfusion was associated with a decrease in [A-V]O2 [arteriovenous O2 content difference] difference and a rise in CVO2 [venous blood O2 content] of 1.2 ml/dl, which suggests that O2 extraction at PVO2 [venous blood O2 pressure] .apprx. 20 torr is curtailed. The EEG, previously shown to correlate with .ovrhdot.VO2 in this model, invariably deteriorated after 30-60 s of low P50 perfusion and improved in 30-60 s after reperfusion with normal blood. CBV [cerebral blood volume] increased by 0.9 ml/100 g during low P50 perfusion, implying capillary recruitment. In a parallel series of experiments, 4 brains were alternately perfused with normal blood (pH 7.41, PCO2 [partial pressure of CO2] 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 (SD) and 21 .+-. 3 torr (P < 0.001), respectively, and .ovrhdot.VO2 averaged 4.33 .+-. 0.52 ml/min per 100 g and 3.18 .+-. 0.52 (P < 0.001). With pH at 7.4 and 7.8, .ovrhdot.VO2 averaged 4.42 .+-. 0.77 ml/min per 100 g and 3.66 .+-. 0.99, respectively (P < 0.01). A reduced P50 limits O2 diffusion to brain at a normal but fixed blood flow rate despite capillary recruitment.