The Role of Propranolol’s Negative Chronotropic Effect on Protection of the Ischemic Myocardium

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Abstract
Propranolol exerts a salutary effect on the ischemic myocardium. This beneficial influence is believed due mainly to a reduction in myocardial oxygen requirements, which, in turn, is caused by a decrease in both heart rate (HR) and contractility. The present study was performed to assess the salutary effect of propranolol on the ischemic myocardium, both when HR was allowed to decrease as well as when it was maintained constant by electrical pacing of the atria, then to compare the magnitude of its effect under these conditions. In 30 open-chest, anesthetized dogs, 10-min coronary artery occlusions (CAO) were performed 45 min apart, and intramural carbon dioxide tension (PmCO2) in the ischemic area was measured continuously with a mass spectrometer. 7 dogs received no intervention during any of the occlusions and, therefore, served as controls; 7 received propranolol, 2 mg/kg i.v. 10 min before the last CAO; 7 received the same dose of propranolol and, in addition, were paced to a HR identical to that during the previous CAO. In another 9 animals, atrial pacing was performed during the last CAO at a HR of 35–40 beats/min greater than during the previous CAO.