Effect of blood pressure on plasma volume loss in the rat under increased permeability

To evaluate the effects of change in blood pressure on plasma volume under increased permeability.

Prospective randomized laboratory study.

Sixty-one adult male Sprague-Dawley rats.

Permeability was increased via an anaphylactic reaction by injection of 0.5 ml dextran 70. One hour later, volume expansion with 15 ml/kg of 5% albumin was given for 15 min. Plasma volume was measured just before and 2.5 h after the albumin infusion (125 I-albumin tracer technique). The study included a control group, a noradrenalin group and a metoprolol/clonidine group (n = 10 in each group). The vasoactive treatment started after the albumin infusion and continued throughout the experiment. We also investigated the effect of noradrenalin on plasma volume under hypovolemia. Central venous pressure was measured to estimate the venous pressure effect of noradrenalin (n = 6). The results were compared with corresponding plasma volume effects of noradrenalin under normal permeability.

The remaining increase in plasma volume 2.5 h after the albumin infusion was 11.8+/-3.6 ml/kg in the control group, 0.5+/-6.3 ml/kg in the noradrenalin group (p < 0.01) and 12.6+/-4.9 ml/kg in the metoprolol/clonidine group (ns). The loss of plasma volume by noradrenalin under hypovolemia was 3.5+/-3.0 ml/kg. The remaining increase in plasma volume after the albumin and noradrenalin treatment under normal permeability was 13.7+/-3.4 ml/kg.

Increase in blood pressure by noradrenalin induces loss of plasma volume, which is much greater under increased than under normal permeability and less pronounced in hypovolemia. According to the two-pore theory of transvascular fluid exchange, the loss may be explained by increased hydrostatic capillary pressure.