Atropine-induced cardioacceleration in patients on chronic propranolol therapy: comparison with the positive chronotropic effect of isometric exercise.

The hemodynamic effects of intravenous atropine administration were examined in 24 patients on chronic propranolol therapy. In the first 13 patients the safety of atropine administration was tested by giving the drug in small increments to a total dose of either 1.2 mg (five patients) or 1.7 mg (eight patients). The heart rate after atropine administration in these patients varied between 57 and 82 bpm and no adverse effects were noted. The other 11 patients received 1.2 mg atropine intravenously with hemodynamic measurements obtained prior to and 3 minutes after administration of the drug. Heart rate increased from 57.5 +/- 8.7 to 72.8 +/- 13.9 bpm, mean pulmonary arterial and left ventricular end-diastolic pressure declined, and cardiac index increased. Total systemic resistance decreased in most of the patients. Isometric exercise performed prior to atropine administration in the same 11 patients accelerated heart rate from 57.4 +/- 8.6 to 68.4 +/- 10.8 bpm. A close correlation, r = 0.909, was demonstrated between the postatropine heart rate and the rate during isometric exercise. It is concluded that atropine in a dose of 1.2 to 1.7 mg may be administered safely in patients on chronic propranolol therapy. Isometric exercise may be useful in unmasking vagal tone in beta-blocked patients.