Abstract
Baroreflex sensitivity was evaluated in 19 patients with essential hypertension (EH), 8 patients with borderline hypertension (BH) and 12 age-matched normal controls (N), by measuring the reflex-mediated changes in plasma norepinephrine (NE) and heart rate (RR interval) while a phenylephrine hydrochloride or a sodium nitroprusside solution was infused in graded doses for a total of 24 minutes. Changes in RR interval and plasma NE showed a significant linear correlation to those in mean arterial pressure (MAP) in every subject studied. The slopes of RR/MAP and %NE/MAP tended to be reduced in EH and BH patients during both pressor and depressor stimulations. There was a significant (p<0.01) inverse correlation between the basal MAP levels an RR/MAP or %NE/MAP except for %NE/MAP during pressor stimulation. Fifteen minutes after the pressor stimulation was stopped, MAP and RR interval in each group tended to be greater than their baselines. Plasma NE remained significantly (p<0.01) depressed in N and BH subjects while those in EH returned to their baselines. When pressor stimulations were repeated twice at intervals of 15 minutes in hypertensive subjects, the second response curves of MAP-RR interval tended to shift slightly to the right while the slopes of the second response curves of plasma NE were significantly (p<0.05) reduced compared with the first ones. These findings indicate that an inability of the baroreflex to produce a sustained suppression of sympathetic nerve activity associate with an efficient ability to reset at the higher pressure levels in EH patients results in a continued tendency for blood pressure to rise and contributes to the development of hypertension.
