EFFECTS OF BLOCKADE OF FAST AND SLOW INWARD CURRENT CHANNELS ON VENTRICULAR-FIBRILLATION IN THE PIG-HEART

Objective-To determine the contribution of fast and slow inward channe ls to the electrocardiogram (EGG) of ventricular fibrillation. Methods -ventricular fibrillation was induced by endocardial electrical stimul ation in pigs anaesthetised with pentobarbitone sodium (30 mg/kg intra venously). ECGs simultaneously recorded from the body surface (lead II ) and from the endocardium were studied by power spectrum analysis (0- 40 Hz). Results-The mean (SEM) dominant frequency of fibrillation (9.0 (1.1) Hz in lead II at 0-40 s) did not change significantly with time in pigs given intravenous saline. However, the dominant frequency was significantly reduced by intravenous pretreatment with the class I an tiarrhythmic drugs, lignocaine (3 mg/kg, 6.5 (0.5) Hz; 10 mg/kg, 4.2 ( 0.6) Hz), mexiletine (3 mg/kg, 6.2 (0.4)Hz; 10 mg/kg, 5.5 (0.4) Hz), a nd disopyramide (2.5 mg/kg, 5.4 (0.6) Hz). After flecainide (3 mg/kg, 6.9 (0.5) Hz) the reduction in frequency was not significant. Similar data were obtained with endocardial recordings. In contrast pre-treatm ent with verapamil (0.2 mg/kg, 11.7 (0.8) Hz; and 1.0 mg/kg, 12.9 (1.6 ) Hz) produced a significantly higher dominant frequency of fibrillati on than saline and widened the bandwidth of frequencies around the dom inant frequency. Conclusions These results indicate that voltage-depen dent sodium channel currents contribute to the rapid frequencies of ve ntricular fibrillation. Blockade of L-type inward calcium channel acti vity increases the fibrillation frequency and fractionates the frequen cies of the fibrillation wavefronts.