Adverse effects of normovolemic polycythemia and hypoxia on hemodynamics in the dog.

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Abstract
Although polycythemia commonly occurs in hypoxic patients, most hemodynamic studies of polycythemia were done in the normoxemic state. The combined effects of polycythemia and hypoxia were studied. In 11 splenectomized, anesthetized dogs, hematocrit was increased from 42.6 .+-. 1.2 to 65.5 .+-. 0.6% by isovolumic exchange transfusions with fresh canine packed RBC [red blood cells]. Studies were conducted during normoxia (PaO2 [arterial O2 partial pressure] = 113.8 .+-. 4.2 mm Hg) and hypoxia (PaO2 = 40.5 .+-. 1.6 mm Hg). Polycythemia alone increased pulmonary vascular resistance by 112 .+-. 4.6% (P < 0.01) and hypoxia alone increased pulmonary vascular resistance by 141 .+-. 11.4% (P < 0.01). Combined hypoxia and polycythemia increased pulmonary vascular resistance by 308 .+-. 28.1% (P < 0.005), an effect significantly greater than that of hypoxia or polycythemia alone (P < 0.005). Systemic vascular resistance increased with polycythemia by 90 .+-. 8.8% (P < 0.01); hypoxia had no effect on systemic vascular resistance alone or when combined with polycythemia. Polycythemia decreased cardiac output by 50 .+-. 1.8% (P < 0.01); hypoxia had no significant effect alone or when combined with polycythemia. O2 transport was decreased by polycythemia by 29 .+-. 0.8% (P < 0.01) due to decreased cardiac output. Hypoxia decreased O2 transport by 28 .+-. 0.9% (P < 0.01) due to a decrease in arterial O2 content. Combined hypoxia and polycythemia decreased O2 transport by 50 .+-. 2.6% (P < 0.01), an effect greater than that of hypoxia or polycythemia alone. In 4 control dogs, exchange with whole blood produced no change in the variables studied. Synergistic effects of hypoxia and polycythemia on pulmonary vascular resistance reflect combined influences of increased blood viscosity and hypoxic pulmonary vasoconstriction. Such a combination may contribute to the occurrence of cor pulmonale in patients with hypoxia and secondary polycythemia.