Efficacy of a beta-adrenergic receptor antagonist, propranolol, in preventing ischaemic ventricular fibrillation: dependence on heart rate and ischaemia duration.

OBJECTIVES: To investigate the prevention of ventricular fibrillation with a beta-adrenergic receptor (beta-AR) antagonist in anaesthetized, open-chest pigs in a model of ischaemia, intended to reproduce what happens either in anginal attack or in the first hour of infarction.
METHODS: Ventricular fibrillation threshold (VFT) was determined with trains of diastolic stimuli of 100 ms duration delivered by a subepicardial electrode inserted in the area subjected to ischaemia. Ischaemia was obtained by the complete occlusion of the left anterior descending coronary artery, either near its origin during brief but increasing periods (30, 60, 90, 120, 150, 180, 240, 300 s), or half-way from its origin for a much longer time (more than 60 min).
RESULTS: During transient proximal occlusion and isoprenaline infusion (0.25 microgram/kg/min), propranolol (50 micrograms/kg plus 2 micrograms/kg/min) attenuated both tachycardia and the fall in VFT to 0 mA. The shortening of MAP duration accompanying depolarization of the fibres was concurrently slowed down, and time to fibrillation prolonged (122 +/- 15 to 262 +/- 14 s, p < 0.001). In the absence of isoprenaline infusion, propranolol exerted similar effects, but to a lesser degree, in proportion to heart rate dependent on sympathetic activity. In contrast, it became unable to raise VFT before and during ischaemia, when heart rate was kept constant by pacing. After persistent midportion occlusion, significant differences in VFT were found only at the 5th min, depending on whether heart rate was accelerated by isoprenaline (0.8 +/- 0.2 mA), left normal (1.8 +/- 0.3 mA) or slowed down by propranolol (1.6 +/- 0.3 mA). Later on, especially after 15 and 25 min of ischaemia, VFT, which was below 1.0 mA, did not appear to be influenced by the activation or blockade of beta-ARs: spontaneous fibrillations were observed in the same number in this period with or without the administration of propranolol. Beyond 30 min after occlusion, the rise in VFT, subsequent to the first irreversible cell damage, also occurred in the same way.
CONCLUSIONS: The prevention of ischaemic ventricular fibrillation by a beta-AR antagonist, judged from VFT, is easily checked experimentally when ischaemia is only transitory, especially if sympathetic activity is high. The maintenance of VFT at a relatively high level is essentially related to the depressant effect on the sinus rate. The same animal model does not give support to an effective protection in the first hour of infarction. However, the control of heart rate may also be beneficial in these circumstances by attenuating systemic haemodynamic disorders.