Vascular capacitance responses to hypercapnia of the vascularly isolated head

Hypercapnic stimulation of the brain may account for some of the decrease in vascular capacitance (venoconstriction) seen with whole-body hypercapnia. Six mongrel dogs were anesthetized with alpha-chloralose and paralyzed with pancuronium bromide. The vagi were cut and the carotid bodies and sinuses were denervated. The head circulation was isolated and perfused with normoxic [arterial partial pressure of O2 (Pao2) = 112 mmHg], normocapnic (PaCO2 = 40 mmHg) blood, or one of three levels of normoxic, hypercapnic (PaCO2 = 56, 68, or 84 mmHg) blood. A membrane oxygenator was used to change gas tensions in the perfusate blood. The systemic circulation received normoxic, normocapnic blood (Pao2 = 107 mmHg; PaCO2 = 32 mmHg). Systemic arterial pressure increased from 111 to 134 mmHg, and heart rate decreased from 174 to 150 beats/min with a head blood PaCO2 of 84 mmHg. Central blood volume was not affected by head hypercapnia. Cardiac output significantly decreased only with a head blood PaCO2 of 84 mmHg. Mean circulatory filling pressure increased by 0.014 mmHg/1 mmHg increase in head PaCO2. The sensitivity of the total peripheral resistance to cephalic blood hypercapnia was 0.88%/mmHg, whereas that for the mean circulatory filling pressure was only 0.19%/mmHg. We conclude that stimulation of the brain, via perfusion of the head with hypercapnic blood, causes a small but significant increase in mean circulatory filling pressure, due to systemic venoconstriction.