Change in plasma volume from a state of hyper-, normo- or hypovolemia with or without noradrenalin infusion in the rat

Fluid substitution is important in critically ill patients to maintain normovolemia, but there is always a risk that the treatment is too aggressive resulting in fluid overload, or is insufficient with maintenance of hypovolemia. The present study on the rat aims at evaluating the change in plasma volume after 2.5 h from a state of hyper- and hypovolemia. The analysis was made without and with noradrenalin infusion, based on the fact that noradrenalin infusion is a common drug to maintain an adequate arterial pressure, and noradrenalin may induce transcapillary filtration. Plasma volume was determined at baseline and at the end of the experiments with a 125I-albumin tracer technique. Arterial and central venous pressure, and urine output were recorded. We showed that induction of hypervolemia with a 5% albumin solution (15 ml/kg) resulted in successive loss of plasma volume, which was aggravated with noradrenalin infusion. Hypovolemia induced by hemorrhage (15 ml/kg) resulted in transcapillary absorption, an absorption almost abolished during noradrenalin infusion. There was no plasma volume loss in the sham group. Urine output was higher under hypervolemia than under normovolemia, which in turn was higher than under hypovolemia. We conclude that hypervolemia induces plasma volume loss, which is aggravated by noradrenalin infusion. The compensatory absorption effect after hemorrhage is counteracted by noradrenalin. The results can be explained by differences in hydrostatic capillary pressure via alterations in arterial and venous pressure, according to the 2-pore theory of transcapillary fluid exchange.