Six cardiac transplant recipients underwent maximal exercise testing before and after the administration of intravenous propranolol to assess the effect of beta blockade on their exercise heart rate response and exercise capacity. Before propranolol the patients were capable of a mean of 6.8 minutes of exercise and heart rate increased from a resting value of 102 +/- 25 a minute to 138 +/- 34 at peak exercise--a mean increase of 35%. All tests were terminated because of tiredness or muscle weakness. After one hour's rest, intravenous propranolol (0.2 mg/kg over 10 minutes) was administered with a reduction in resting heart rate from 109 +/- 28 a minute to 83 +/- 16. During the repeat exercise test the patients were capable of a mean of 4.5 minutes of exercise and all tests were terminated by extreme exhaustion and/or unsteadiness requiring immediate cessation of the treadmill. Heart rate increased from a resting value of 83 +/- 16 a minute to 96 +/- 18 at peak exercise. The exercise capability of the denervated heart is conspicuously reduced by beta blockade, presumably because of its reliance on circulating catecholamines.
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