Circulating plasma volume changes in anesthetized dogs during positive pressure breathing

¹ Aeromedical and Physical Environment Laboratory, University of Illinois, Chicago, Illinois

Various circulatory functions were measured in anesthetized dogs subjected to 18.5 mm Hg positive pressure breathing. Immediately upon raising the intrapulmonary pressure there occurred a five- to sixfold increase in venous pressure as well as a decrease in mean arterial pressure. Accompanying these pressure changes was a progressive decrease in circulating plasma volume as measured by the T-1824 method. A 30% decrease in plasma volume was recorded after 160 minutes of increased intrapulmonary pressure. Hemoconcentration was also indicated by the increased hematocrits, although calculated fluid loss was only 13%. All circulatory changes returned to prepressure breathing levels upon release of pressure breathing. Other changes, such as oliguria, periods of apnea and an alkaline urine accompanied positive pressure breathing. The data suggest that the decrease in plasma volume is the result of venostasis caused by the rapid increase in venous pressure.