Comparison of the inotropic and chronotropic effects of metoprolol and propranolol.

The cardiovascular responses to intravenous doses of isoproterenol were measured in eight male volunteers before and during administration of 50 mg metoprolol, orally every 6 hours and 40 mg propranolol, orally every 6 hours for a total of five doses. The dose of isoproterenol required to produce an increase in heart rate of 25 beats/min (the ID25) was 2.0 +/- 1.4 microgram before beta blockade, 6.2 +/- 4.4 microgram during metoprolol, and 44.4 +/- 12.0 microgram during propranolol administration. Similar changes in diastolic blood pressure, QS2I, preejection period, and preejection period/left ventricular ejection time ratio occurred at the ID25 during treatment with both metoprolol and propranolol. In volunteers, propranolol produces a much more intense blockade of the inotropic and chronotropic effects of isoproterenol than does metoprolol.