Distribution of saline following acute volume loading: postural effects

The influence of posture on the transvascular partitioning of an acute isotonic saline load was studied in a large number of functionally nephrectomized anaesthetized dogs. In 10 supine dogs given Ringer's-lactate (7% body weight) over 30 min, there was significant protein efflux from the vascular space during the infusion interval accompanied by a 67% loss of infusate into the interstitial space. At the end of three hours, only 10.9% of the infusate remained within the plasma, representing 42% of the anticipated recovery. When animals were placed in their usual quadruped position, the acute saline load was accompanied by protein loss as previously, but there was in addition continuing protein loss into the interstitial space during the post-infusional period. At 3 h, only 5.1% of the infusate had been retained. Volume loading with dogs placed in the 45 degrees head-up tilt position produced zero net recovery of infusate and loss of original plasma so that at 3 h post-infusion, plasma volume had declined by 11% from control values. Tilting by itself for this time period had caused protein efflux without fluid loss. The application of lower body positive counter-pressure to dogs tilted up at 45 degrees improved the partitioning, so that instead of net loss, 13% of the expected volume remained within the vascular space. The provision of iso-oncotic colloid instead of saline prevented volume losses in supine but not tilted dogs. We conclude that the rapid infusion of isotonic solution is associated with augmented loss of protein into the interstitial space leading to diminished vascular retention of infusate.