Abstract
Heart rate responses to three different procedures (amyl nitrite inhalation, standing up, and isoproterenol infusion), were studied before and during acute adrenergic beta blockade with intravenous propranolol in three normal and six hypertensive subjects. Propranolol decreased, but did not completely abolish, the heart rate increase produced by amyl nitrite and standing, probably because of vagal participation (with-drawal) in heart rate increase produced by baroreceptor hypotension (amyl nitrite) and on assuming the upright posture. Heart rate responses to amyl nitrite varied greatly from patient to patient (from 27 to 97 per cent), but the drug proved to be the most potent stimulus for heart rate increase as a result of its marked hypotensive effect. However, this vasodilator-induced acute hypotension was well tolerated, and without deleterious hemodynamic consequences, despite the presence of beta blockade. Different degrees of correlation in heart rate increase were observed with the three procedures, reflecting probably the varying sympathetic-parasympathetic participation in reflex heart rate control. It is concluded that from the clinical stand point, neither amyl nitrite administration nor standing up can be used as a test to assess acurately the degree of beta blockade, because both procedures activate vagal withdrawal which increases heart rate regardless of the degree of beta blockade.
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