Changes in interstitial pressure during and after blood volume expansion in rats

Interstitial fluid pressure was measured via a chronically implanted capsule before, during and after acute isotonic, iso-oncotic blood volume expansion in normal or in 48-h dehydrated rats. At the same time, the patterns of body fluid distribution, of selected renal responses and of mean arterial and mean central venous pressure responses were studied. Dry tissue weight (DTW) was subsequently determined by freeze drying of the shaved carcass. Dehydration decreased plasma volume and interstitial fluid volume significantly below normal values. The initial intracapsular pressure in dehydrated animals (-3.7 +/- 0.6 mm Hg) was not significantly different from that in normal rats (-2.5 +/- 0.5), but dehydrated rats showed initially a very significantly lower effective interstitial compliance (0.0005 ml/mm Hg per gram DTW) than did the normal group (0.0704). In the course of the renal response to the volume load, effective interstitial compliance increased to 0.0350 in dehydrated rats but showed no change in normal rats. Neither group completely corrected its elevated blood volume; both returned their central venous pressures to pre-infusion levels; both decreased their interstitial fluid volumes below pre-infusion levels and both decreased their intracapsular fluid pressures 1 mm Hg below the level prevailing in non-infused animals at that time. It is concluded that a reduction in interstitial hydrostatic pressure can be a functionally important influence in the apparent control of central venous pressure following acute blood volume expansion.