Effects of Hypocarbia and Normocarbia on Cardiovascular Dynamics and Regional Circulation in the Hypothermic Dog

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Abstract
The effects of CO2 on the cardiovascular system, cerebral, mesenteric and renal blood flows and total-body O2 consumption under surface-induced hypothermia to 24.degree. C were evaluated in 12 dogs. In Group I (6 dogs) PaCO2 (CO2 partial pressure) was allowed to decrease from 35-18 torr during cooling without the addition of CO2 to the inspired gas mixture. In Group II (6 dogs) CO2 was added to the inspired gases to maintain PaCO2 34-38 torr during cooling. Arterial blood pH increased in Group I (7.39-7.50) but decreased in Group II (7.35-7.27). Cardiac index decreased markedly with cooling in Group II from 3.37-1.18 l/min per m2; it showed an initial increase in Group I at 34.degree. C followed by a decrease to 1.62 l/min per m2 at 24.degree. C. Stroke index did not change significantly but heart rate decreased significantly in either group; Group II showed a greater decrease. Mean arterial pressure was significantly decreased in either group from about 120-80 torr; there were no significant differences in mean arterial pressures between groups at the same hypothermic temperatures. Mean pulmonary arterial and pulmonary capillary wedge pressures were essentially unchanged in both groups. Pulmonary vascular resistance showed significantly greater increases in Group II than in Group I. Internal carotid arterial blood flow was significantly greater in Group II than in Group I, there was no difference in renal or superior mesenteric arterial blood flows between the 2 groups. Total-body O2 consumption in either group decreased from about 127 ml/min per m2 at 37.degree. C to 41 at 24.degree. C; there was no significant difference between groups. Adding CO2 to the inspired gases to maintain normal PaCO2 during hypothermia may be desirable for cerebral perfusion but harmful to the cardiovascular system.

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