The Valsalva maneuver as an indirect, non-invasive indicator of central blood volume shift.

Our objective was to determine whether the Valsalva maneuver may be used as an indicator of fluid shift during spaceflights. The subjects, 21 healthy young men, conducted the maneuver against expiratory pressures of 20, 30, and 40 mm Hg, each lasting 30 s, at body positions of vertical, horizontal, and 6 degrees head down tilt (HDT). Heart rate was continuously recorded on a beat by beat basis together with the expiratory pressure. The increase in heart rate at equal intrathoracic pressures was maximal in the vertical position, significantly lower (p less than 0.001) in the horizontal position and lowest (p less than 0.05) in the 6 degrees HDT position. Obviously, the blood volume shift in the horizontal and 6 degrees HDT position partly compensated the impaired venous return during the Valsalva maneuver. This simple and non-invasive technique might be used to indicate the time course of blood volume shift during the initial stage of an orbital flight.