PATHOPHYSIOLOGICAL EFFECTS OF HEMODILUTION IN CHRONIC MOUNTAIN-SICKNESS IN RATS

We examined the effect of isovolemic hemodilution in a rat model of ch ronic mountain sickness (CMS). After 30 days at simulated high altitud e (5,500 m), Hilltop rats had developed evidence of CMS: severe hypoxe mia, polycythemia, and pulmonary arterial hypertension. Isovolemic hem odilution to a mean hematocrit of 46 +/- 5% was well tolerated by both the hypoxia-sensitive Hilltop rats and the companion Madison rat stra in that does not develop CMS. After hemodilution, we found no evidence of sustained improvements in ventilation or gas exchange in either st rain. Despite the fall in blood viscosity, cardiac output increased on ly marginally, and pulmonary arterial hypertension persisted in the Hi lltop rats. Vascular hindrance increased after hemodilution, preventin g a significant decline in pulmonary and systemic vascular resistances in the Hilltop rats. Blood O-2 content and the coefficient of O-2 del ivery fell after hemodilution, but O-2 consumption was sustained at a normal level after hemodilution by increasing the extraction fraction in the Hilltop strain. There was systemic hypotension through the firs t day of hemodilution, but this was the only apparent adverse effect o f hemodilution. We conclude that isovolemic hemodilution was well tole rated despite the reduction in tissue O-2 delivery. However, hemodilut ion failed to improve any of the respiratory and cardiovascular manife stations of CMS in Hilltop rats.