PATHOPHYSIOLOGY AND NATURAL HISTORY HYPERTENSION Arterial baroreflex sensitivity, plasma catecholamines,

Arterial baroreflex sensitivity, plasma norepinephrine (NE) and epinephrine (E), and pressor and depressor responses were assessed in 25 patients with essential hypertension and 29 normotensive control subjects. Sensitivity of the cardiac limb of the baroreflex was determined by blood pressure and interbeat interval responses associated with the Valsalva maneuver, externally applied neck suction and pressure, and injection of phenylephrine and nitroglycerin. By all these techniques, patients with essential hypertension had significantly decreased baroreflex sensitivity, even after adjustment for age mismatching between the hypertensive and normotensive groups. Hypertensive patients also had significantly higher mean levels of plasma NE and E in both brachial arterial and antecubital venous blood (246 vs 154 pg/ml arterial NE, 286 vs 184 pg/ml venous NE, 99 vs 55 pg/ml arterial E, and 65 vs 35 pg/ml venous E) and significantly larger pressor responses to injected phenylephrine (30.9 mm Hg/100 ,)g vs 16.7 mm Hg/100 ,ug). When baroreflex-cardiac sensitivity values measured by the various techniques were averaged, there was a significant inverse relationship between sensitivity and venous NE and between sensitivity and pressor responsiveness. The results indicate that decreased baroreflex-cardiac sensitivity, increased sympathetic outflow, and pressor hyperresponsiveness tend to occur together in some patients with essential hypertension. Decreased arterial distensibility and altered central neural integration can account for these findings. Circulation 68, No. 2, 234-240, 1983. THE ARTERIAL baroreceptor reflex system constitutes one of the most powerful and rapidly acting homeostatic mechanisms for controlling blood pressure, and as such has attracted the interest of hypertension researchers for many years. Baroreflex sensitivity measured by the change in interbeat interval per unit change in systolic pressure after injection of a vasoconstrictor and after the Valsalva maneuver is significantly decreased in patients with essential hypertension. 1-6 However, in investigations in which externally applied neck suction has been used to increase carotid transmural pressure and thereby stimulate carotid baroreceptors, decreased baroreflex sensitivity in hypertension has not consistently been detected.7-9 When directly compared, results obtained with these techniques have not agreed very well,'0 most likely because the techniques measure different aspects of baroreflex function. In this study, several different measurement techniques were used in each subject to determine ifand with what consistency across techniques patients with essential hypertension have decreased arterial baroreflex sensitivity. It was reasoned that averaging baroreflex sensitivities measured by all or several of the techniques might provide a more valid index of arterial baroreflex gain than relying on one technique alone. When baroreceptor afferents are stimulated, their reflexive circulatory effects result from vagal stimulation and sympathetic inhibition. It might therefore be predicted that circulating plasma norepinephrine (NE) levels, which have been used as an index of sympathetic neural activity,'2 might vary inversely with baroreflex activity among individuals. 'I In this study, plasma NE and plasma epinephrine (E) were measured and plasma NE was compared with averaged baroreflex sensitivity in a setting in which confounding environmental factors were restricted as much as possible. Since the injection technique for measuring baroreflex sensitivity involves administration of a vasoconstrictor and a vasodilator, pressor responsiveness was assessed and the extent of its relationship to baroreflex sensitivity and to resting plasma catecholamines was