ESTIMATIONS OF THE DECREASE IN EFFECTIVE BLOOD VOLUME WHEN PRESSURE BREATHING AT SEA LEVEL.

Studies conducted in this laboratory and reported to the OSRD (1) as early as August, 1944, drew attention to the value of counter-pressurization of the extremities in preventing collapse when breathing at intermittent pressures of 30 to 40 mm. Hg. Recent reviews of the physiology of pressure breathing (2, 3) comment on the tendency towards circulatory collapse during pressure breathing and suggest that the failure is probably associated with a decrease in effective circulating blood volume. This could be engendered by a pooling of blood in the congested veins of the extremities, by a loss of fluid due to filtration from the circulation under conditions of increased capillary hydrostatic pressure, or by a combination of both mechanisms. Fenn, Otis, and their collaborators (4) have measured the approximate amount of blood trapped in the leg veins during pressure breathing by a technique involving comparisons of a teeter-board and leg plethysmograph data. They conclude that during pressure breathing some 3 per cent of the total blood volume may be sequestered in the legs alone. Barach, et al. (3) have calculated the loss of fluid from the circulation during pressure breathing from measurements of changes in the oxygen capacity and the plasma protein concentrations of the blood. In men subjected to continuous pressure breathing at levels of 20 cm. water for periods of less than one hour, the oxygen capacity data indicate a hemoconcentration of about 5 per cent, while the plasma protein data give a calculated fluid loss of 6.5 per cent. Since both losses are associated with abnormal relations between the intravascular and tissue pressures in the extremities, it is theoretically possible