Venous pressure in man during weightlessness.

To determine whether the body fluid shift from the lower limbs toward the head that occurs during spaceflight leads to lasting increases of venous pressure in the upper body, venous pressure and hematocrit measurements were made on four astronauts before flight and 1 and 12 hours after recovery and compared with measurements in space. During the mission the hematocrit was elevated and the venous pressure lowered by 1 to 8 centimeters of water as compared with the preflight data. One hour after landing the hematocrit decreased, indicating a hemodilution, venous pressures were unexpectedly high, and a body weight loss of 4 to 5 percent was observed. Twelve hours later the venous pressures were the lowest recorded during the study. The fluid shift apparently takes place during the first several hours of spaceflight. Thereafter, the pressure in the peripheral veins and the central circulation is lower than that measured before flight.

The effect of graded immersion on heart volume, central venous pressure, pulmonary blood distribution, and heart rate in man.

The present experiments have been conducted to study the immediate effects of graded immersion on the central circulation. When taking heart volume as an indicator, it was found that immersion to the diaphragm of a standing subject produces the same changes as assumption of the supine posture. Heart volume increases by approximately 130 ml. When the water level is raised to the neck, an extra pressure corresponding to a water column extending from the diaphragm to the surface of the water of approximately 25 cm H2O forces blood into the thorax. The heart becomes distended by an additional 120 ml. Correspondingly the central venous pressure at the height of the right atrium increases from 2.5 to 12.8 mm HG when the water level rises from the diaphragm to the neck. The greater filling of the pulmonary circulation is accompanied by a decrease in vital capacity and visualized by scintigrams. The preferential increase in blood volume of the apical regions is striking. When raising the water level from the symphysis to the xiphoid heart rate falls by about 15%.

Time course of cardiac distension with rapid immersion in a thermo-neutral bath.

13 subjects standing in a narrow upright tank were exposed to rapid water immersion up to the neck. The tank was filled from an elevated reservoir in less than 4 s. In 8 subjects heart size was measured by Roentgen-cinematography combined with video tape recording. Planimetry of the diastolic postero-anterior area of the heart showed an average increase in heart size of 30% within 6 s. In 5 subjects central venous pressure and the height of hydrostatic pressure in the tank were recorded. The two pressures rose and fell simultaneously without delay when the hydrostatic pressure exceeded the level of the diaphragm. Rapid immersion caused a fall in heart rate of approximately 20%. The findings speak in favor of the concept that the right heart does not constitute a resistance and that the pulmonary circulation and the systemic capacitance vessels form a functional unit.

Effective compliance of the circulation in the upright sitting posture.

The effective compliance is defined as the relation of change in blood volume to change in central venous pressure. It was measured in 8 upright sitting male subjects and amounted to 3.3 ml/(mm Hg X kg BW). It is, therefore, by about 50% greater than the effective compliance in the supine subject which amounts to 2.3 ml/(mm Hg X kg BW). . This difference is probably due to the posture dependent blood volume distribution in the low pressure system whose "upper" and "lower" sections have nonlinear pressure-volume characteristics. Immersion to the neck reduces the effective complicance to about half the control value (1.9 ml/(mm Hg X kg BW) which probably constitutes the effective compliance of the intrathoracic circulatory compartment.

Recent advances in the physiology of whole body immersion.

Recent investigations have furnished a complete analysis of the hemodynamic events accompanying whole-body immersion. About 700 ml of blood are translocated into the intrathoracic circulation, and heart volume increases by 180 +/- 62 ml. These changes are followed by an increase in stroke volume and cardiac output of over 30%. At the same time a reflex reduction of total peripheral resistance and venous tone occurs. Renin and aldosterone activity are reduced while the 17-hydroxycorticosteroid is not affected. Treatment of the subject with DOCA attenuates but does not extinguish the excess sodium excretion of immersion. This finding strengthens the arguments in favor of an unknown factor enhancing sodium excretion. Finally, the relative activation of the three factors that serve volume control, the excretory function of the kidney, capillary filtration pressure, and the thirst mechanism, is discussed.