Relative effects of propranolol and timolol on ventricular fibrillation threshold and hemodynamics in the dog

Abstract

The relationship between the antifibrillatory and hemodynamic effects of propranolol and timolol was tested in 32 dogs divided into three groups. In closed-chest animals (group 1), the maximal increase of the ventricular fibrillation threshold (VFT) from the control state was 67.3 ± 22.6% ( n = 5, p < 0.01) for propranolol and 95.8 ± 18.8% ( n = 9, p < 0.005) for timolol. These effects were paralleled by a decrease in the cardiac index of 34.7 ± 10.2% ( p < 0.05) and 32.5 ± 9.4% ( p < 0.02) for propranolol and timolol, respectively. The mean systemic arterial pressure (SAP) also decreased by 25.0 ± 8.8% ( p < 0.05) and 19.2 ± 5.1% ( p < 0.01). In open-chest animals (group 2), timolol increased the VFT by 86.6 ± 27% (n = 5, p < 0.05) and decreased the cardiac index by 57.3 ± 6.3% ( p < 0.005) and the mean SAP by 28.5 ± 2.9% ( p < 0.02). In open-chest animals with stabilized peripheral hemodynamics (group 3), VFT increased by 354 ± 130% ( p < 0.05) and 437 ± 144% ( p < 0.05) after the maximal administered doses of propranolol ( n = 6) and timolol ( n = 7), respectively. These results suggest that the electrophysiologic and hemodynamic effects of beta blockade are parallel and interdependent, with the hemodynamic deterioration markedly blunting the beneficial electrophysiologic effects.