Role of sodium channels in ventricular fibrillation: a study in nonischemic isolated hearts

Because the role of sodium channels in the initiation and maintenance of VF is not fully elucidated, we studied the significance of sodium channel activity in VF using sodium channel blockers. In nonischemic isolated feline hearts, the following electrophysiologic parameters were measured before and after application of tetrodotoxin (5 x 10(-7) M, n = 6) or lidocaine (1 x 10(-5) M, n = 8): (a) during pacing, epicardial conduction time; refractoriness; the fastest rate for 1:1 pacing/response capture, and all tissue resistivity, indirectly reflecting intercellular electrical resistance; (b) during 8 min of electrically induced tachyarrhythmias, all tissue resistivity; peak frequency (to measure average frequency based on fast-Fourier transformation analysis); and normalized entropy (to measure the degree of arrhythmia organization). In nonischemic isolated rabbit hearts (n = 4), three-dimensional mapping was performed before and after application of lidocaine (1 x 10(-5) M). In feline hearts, lidocaine and tetrodotoxin application resulted in: (a) more spontaneous arrhythmia termination (63-67%) than in nontreated hearts (7%); (b) transformation from mainly VF into ventricular tachycardia with increased organization; and (c) prolongation of conduction time (155-248%) (p < 0.01 for all parameters). The ventricular refractory period was slightly prolonged by tetrodotoxin in the right ventricle and exhibited rate-dependent shortening in control and with lidocaine. Tetrodotoxin and lidocaine reduced the pacing rate for 1:1 pacing/response capture, and all tissue resistivity was not significantly affected. Peak frequency was decreased by tetrodotoxin and lidocaine mainly in the left ventricle (p < 0.01). In nontreated left ventricles, peak frequency was increased over time but was attenuated by lidocaine. In isolated rabbit hearts, several simultaneous wave fronts were detected during VF in nontreated hearts and were reduced to only one or two major wavefronts after application of lidocaine. Suppression of sodium channel activity that primarily slowed conduction time and had little or no effect on ventricular refractory period and all tissue resistivity resulted in less stable and more organized arrhythmias and reduced tachyarrhythmia rate compared with nontreated hearts. These results suggest an active role for sodium channels in the maintenance of ventricular fibrillation.