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OBJECTIVE: To determine the contribution of fast and slow inward channels to the electrocardiogram (ECG) of ventricular fibrillation. METHODS: Ventricular fibrillation was induced by endocardial electrical stimulation in pigs anaesthetised with pentobarbitone sodium (30 mg/kg intravenously). 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 antiarrhythmic 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), and 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-treatment 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 fibrillation than saline and widened the bandwidth of frequencies around the dominant frequency. CONCLUSIONS: These results indicate that voltage-dependent sodium channel currents contribute to the rapid frequencies of ventricular fibrillation. Blockade of L-type inward calcium channel activity increases the fibrillation frequency and fractionates the frequencies of the fibrillation wavefronts.