Agreement among three different indices of sympathetic nervous system activity in essential hypertension

Although plasma norepinephrine concentration often is elevated in patients with essential hypertension, the relevance of this to the pathogenesis of the blood pressure elevation remains uncertain. If the high concentration of plasma norepinephrine truly indicates sympathetic nervous system overactivity, it should be accompanied by hemodynamic features suggestive of sympathetic cardiovascular stimulation. This proposition was tested, and in part confirmed, in a study performed on 21 patients who had mild essential hypertension and on 11 normal subjects. Among hypertensive patients, plasma norepinephrine concentration was related to cardiac function but not to the level of the blood pressure or to total peripheral vascular resistance. Plasma norepinephrine concentration correlated significantly with heart rate and cardiac preejection period index at rest and with changes in cardiac output, heart rate, and preejection period after cardiac beta-adrenergic blockade produced by propranolol, 0.2 mg/kg intravenously. Patients with an elevated plasma norepinephrine concentrations (9 of 21) showed features of sympathetic cardiac stimulation, manifested by an elevated mean cardiac index and heart rate at rest and by normalization of both after treatment with propranolol. Thus, substantial agreement existed among three commonly used indices of sympathetic nervous system activity--plasma norepinephrine concentration, general hemodynamics at rest, and the hemodynamic response to pharmacologic autonomic blockade.