Effect of hemorrhage on cardiac output of sympathectomized dogs

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Abstract
Blood volume, cardiac output (C.O.), and arterial pressure were measured in unanesthetized, sympathectomized dogs before and after various degrees of hemorrhage. The posthemorrhage cardiac output, expressed as a percentage of the control, showed a linear relationship with the per cent reduction in blood volume (Vr), (C.O. = 107 – 2.51 Vr). After a moderate hemorrhage, the per cent decrease in the arterial pressure was less than that of the cardiac output, indicating a rise of peripheral resistance. This is attributed mainly to the distensibility of the vascular wall. After more severe hemorrhages, the peripheral resistance tended to decrease, due probably to the reduction in hematocrit and the accumulation of metabolic products. By analyzing the volume-flow-pressure relationship in sympathectomized dogs, one can quantify the various compensatory adjustments to hemorrhage. When the blood volume is reduced by less than 10%, the arterial pressure is maintained primarily by the nonneural factors. With a volume reduction between 10 and 25%, the sympathetic activity, the inhibition of vagi, and the nonneural factors are of about equal importance. When the reduction of blood volume is greater than 25%, the sympathetic activity becomes the most significant compensatory factor.