Newer thinking on the hemodynamics of hypertension.

For more than 30 years there has been general agreement that in the established phase of essential hypertension, the cardinal hemodynamic disturbance is an increased total peripheral resistance with cardiac output normal during rest, but subnormal during exercise. However, there are still different opinions about the hemodynamics in the starting phase of essential hypertension. Several studies from 1993--mainly performed in the offspring of hypertensive parents--reported high as well as low cardiac index values, depending on the patient selection and the methods used. Irrespective of whether the early phase of essential hypertension is caused primarily by increased cardiac output or increased total peripheral resistance, an increased sympathetic activity is usually held responsible for either hemodynamic disturbance. Simultaneous recordings of central and regional hemodynamics and sympathetic nerve traffic in young subjects with mild hypertension did show increased cardiac output, reduced calf blood flow, and increased nerve traffic as the characteristic features. However, recent studies in subjects with borderline hypertension have shown that if the subjects were aware of having hypertension, they showed higher blood pressure and higher reactivity to mental stress and cold-pressor tests than uninformed subjects with similar blood pressures. With respect to regional circulation, studies have shown that the coronary reserve is reduced in early hypertension, and also in subjects without left ventricular hypertrophy. The role of the endothelium in the regulation of blood flow has attracted great interest. Abnormalities in the production of vasoconstricting and vasodilating substances from the endothelial cells may be at least partially responsible for the increased vascular resistance characteristic of essential hypertension.