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

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 less than 0.01) for propranolol and 95.8 +/- 18.8% (n = 9, p less than 0.005) for timolol. These effects were paralleled by a decrease in the cardiac index of 34.7 +/- 10.2% (p less than 0.05) and 32.5 +/- 9.4% (p less than 0.02) for propranolol and timolol, respectively. The mean systemic arterial pressure (SAP) also decreased by 25.0 +/- 8.8% (p less than 0.05) and 19.2 +/- 5.1% (p less than 0.01). In open-chest animals (group 2), timolol increased the VFT by 86.6 +/- 27% (n = 5, p less than 0.05) and decreased the cardiac index by 57.3 +/- 6.3% (p less than 0.005) and the mean SAP by 28.5 +/- 2.9% (p less than 0.02). In open-chest animals with stabilized peripheral hemodynamics (group 3), VFT increased by 354 +/- 130% (p less than 0.05) and 437 +/- 144% (p less than 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.