Hemorheologic effects of vasodilation in essential hypertension

Arterial hypertension is often associated with plasma volume contraction and hemoconcentration, which negatively affect the vascular flow resistance and microcirculation. Since some antihypertensive drugs can affect blood rheology, the purpose of this study was to evaluate whether acute and long-term arterial vasodilation with cadralazine influences rheologic parameters in essential hypertension. Twelve patients with unsatisfactorily controlled essential hypertension were studied. In the acute double-blind phase of the study the patients were allocated to treatment with either cadralazine or placebo. Intraarterial blood pressure, cardiac output (dye dilution), and blood rheology (viscosity, hematocrit) were registered before and after the first dose of cadralazine. Then 9 patients (3 dropouts) were treated with cadralazine and placebo during two four-week crossover periods and continued with cadralazine medication during the eight-week open phase of the study. Blood pressure and blood hemoglobin were followed during long-term treatment. A single oral dose of cadralazine caused vasodilation (total peripheral resistance index decreased from 45 to 33 U x m2, P < 0.05), which was accompanied by hemodilution (hematocrit declined from 46.9% to 42.5%, P < 0.05) and a blood viscosity reduction of more than 10%. Viscosity of 45% suspension of erythrocytes in plasma was also reduced, suggesting a possible modification of the microrheologic factors. The changes in total peripheral resistance correlated negatively with the changes in hematocrit. An antihypertensive effect of cadralazine was still observed during the chronic phase of the study, which was not accompanied by hemodilution. It is concluded that arterial vasodilation with cadralazine reduces flow resistance in the circulatory system in hypertension and has acute rheologic effects that disappear during chronic medication.