Central and cerebral hemodynamics and metabolism of the healthy man during head-down tilting

Experiments on six healthy test volunteers, who underwent simultaneous catheterization of different cardiovascular compartments, were carried out to study the effect of head-down tilting at an angle of -20 degrees on the central and cerebral hemodynamics and metabolism. By the third hour of exposure, the test subjects showed a decrease in the systolic arterial pressure and increases in the systolic pressure of the right ventricle, heart rate, cardiac output, and oxygen intake. They also displayed a trend for an increase in the content of adenosine triphosphate, a decrease in the content of pyruvic acid, and essentially no changes in the concentration of lactic acid and activity of lactate dehydrogenase in the arterial blood and in the blood flowing out from the brain. The oxygen arteriovenous difference for the systemic and cerebral circulation decreased by 24% and 13%, respectively. The above circulatory and metabolic changes seem to reflect processes of adaptation of the human body to functioning under conditions of cephalad fluid shifts.