THE ACUTE CIRCULATORY EFFECTS OF THE HEAD-DOWN POSITION (NEGATIVE G) IN NORMAL MAN, WITH A NOTE ON SOME MEASURES DESIGNED TO RELIEVE CRANIAL CONGESTION IN THIS POSITION 1

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Abstract
Using the Hamilton manometer, pressures were measured in the internal jugular, right atrium, and inferior vena cava following catheterization in normal human subjects during tilting into the head-down position from the horizontal and the upright position. Arterial pressures were detd. simultaneously in the femoral and brachial arteries. Immediately following inversion all pressures changed in relation to the hydrostatic shift. The "zero reference plane" or the point at which no change occurred in the arterial circuit lay in the aorta close to or slightly caudad to the diaphragm. As noted previously in the dog by Clarke et al., the inferior and superior venous chambers were separated dynamically in man by the action of the heart so that each behaved more or less independently and each possessed an individual zero reference plane detd. by the atrial pressure. This factor, which depends on the elasticity of the great vessels and the peripheral resistance, tended to diminish the hydrostatic elevation of pressure in cranial vessels under negative G. The cranial venous pressure rose relatively more than the intra-cranial arterial pressure. Within a few sec. after tilting the arterial pressure and pulse rate decreased sharply. Since the cardiac output (ballistocardiograph) appeared to increase, the fall in pressure was attributed to widespread vasodilatation. Measurements of renal blood flow and glomerular filtration rate failed to reveal any significant change in the kidney. Application of venous occlusion cuffs to the lower extremities during the tilt prevented movement of blood from the legs into the head and reduced slightly the venous pressure effects. The Muller maneuver was most effective in blocking or reversing the elevation in cranial venous pressure.