Interaction of metoprolol and lidocaine on the ventricular fibrillation threshold in the anesthetized dog

In the present study, we evaluated the antiarrhythmic interaction (s) of metoprolol and lidocaine in 16 dogs using the ventricular fibrillation threshold(VFT) method. The right ventricle was stimulated with a 100 Hz train of 12, 4 ms pulses delivered after every eighth atrial paced beat at a basic cycle length of 300 ms. Lidocaine dosage was 2 mg/kg followed by a 70 μg/kg/min infusion and metoprolol dosage was a 75 μg/kg bolus. In Group 1. lidocaine was followed by metoprolol; in Group 2. first lidocaine then metoprolol and again lidocaine were given: And in Group 3, dogs received first metoprolol. then lidocaine. and subsequently metoprolol. Drug dose intervals were 45 min. In Group I. lidocaine elevated the VFT to 149% ± 20% and metoprolol to 204% ± 30% of control. (p < 0.01). In group 2. the VFT remained elevated after the second lidocaine administration (p < 0.05 vs. Group 1). In Group 3. the VFT was increased by metoprolol to 227%; ± 30% of control (p < 0.01). Interestingly, defibrillation induced by the combination of metoprolol and lidocaine occurred after 3.2 ± 0.5 s in four out of 16 animals (p < 0.05). This chemical'" defibrillation never occurred when only metoprolol or lidocaine alone was administered. Fibrillation was often more organized in the presence of the combination of metoprolol plus lidocaine, which might relate to the observed defibrillation associated with metoprolol plus lidocaine. In conclusion. the combination of metoprolol and lidocaine has no proarrhythmic effects and may enhance the electrical stability of the ventricles as measured by the VFT method