AJP - Heart and Circulatory Physiology, Vol 249, Issue 6 1061-H1069, Copyright © 1985 by American Physiological Society
Vascular capacitance responses to severe systemic hypercapnia and hypoxia in dogs
C. F. Rothe, P. M. Stein, C. L. MacAnespie and M. L. Gaddis
The magnitude of vascular capacitance change induced by hypercapnia,
hypoxia, or hypoxic hypercapnia was estimated during the administration of
experimental gas mixtures to anesthetized dogs for 25 min. Mean circulatory
filling pressure (Pcf) was determined by fibrillating the heart and
equilibrating arterial and venous pressures with a pump. We assumed that
the total blood volume remained constant and that the magnitude of change
in peripheral venous volume equaled the sum of the changes in blood volume
in the cardiopulmonary and arterial beds. We further assumed that active
(reflex) peripheral venoconstriction occurred if the cardiopulmonary and
arterial bed blood volumes, as well as the Pcf, increased. Within 3 min,
severe hypercapnia and hypoxic hypercapnia induced a 5.2 and 7.3 ml/kg
reduction in systemic vascular capacity, and, by 19 min of experimental gas
presentation, increased Pcf by 5.5 and 7.0 mmHg, respectively. Severe
hypoxia had less effect (0.7 ml/ kg and 2.5 mmHg, respectively) at 19 min.
Severe hypercapnia also increased the central venous, systemic arterial,
and pulmonary arterial pressures and decreased heart rate. Hypoxic
hypercapnia additionally increased cardiac output. We conclude that severe
systemic hypercapnia, whether alone or in combination with hypoxia, causes
a significant active reduction in vascular capacitance, but severe hypoxia
is less effective.
