Effects of Posture and Atropine on the Cardiac Output1

Abstract

The cardiac output response to tachycardia induced by the intravenous administration of atropine sulfate has been studied in 12 normal subjects in recumbency and tilted head-up at 60 degrees. In 4 instances the effects of antigravity suit compression on the response to atropine in tilted subjects were studied. In 5 subjects the effects of peripheral venous pooling of blood on the response to atropine were also observed. Significant increases in cardiac output, primarily the result of increased heart rate with little change in mean stroke volume, occurred in recumbent subjects. Only slight elevation of cardiac index, despite even greater tachycardia associated with a fall in mean stroke volume, occurred in the tilted group. Similar results following atropine were seen during peripheral pooling of blood. Sustained anti-gravity suit inflation restored, in part, the cardiac responsiveness to atropine in tilted subjects. These data support the thesis that the central venous reservoir is an important determinant of cardiac responsiveness.