4.11. Electrophysiological study

Rats were reanaesthetized and mechanically ventilated as described above. The protocol used for the programmed electrical stimulation was performed as previously reported.54, 61 After monitoring the surface ECG, a programmed stimulation protocol was performed using electrodes implanted on the epicardial surface of the LV. Programmed electrical stimulation was performed to induce VAs as follows: eight paced beats at a cycle length of 120 ms (S1) were applied, followed by one to three extrastimuli (S2, S3 and S4) at shorter coupling intervals. The pacing protocols were interrupted if ventricular tachycardia was induced. Ventricular tachyarrhythmias, including ventricular tachycardia and ventricular fibrillation, were considered nonsustained when they lasted <15 beats and sustained when they lasted >15 beats. The VA scores were determined by the inducibility quotient of the ventricular tachyarrhythmias as follows: 0, noninducible; 1, nonsustained tachyarrhythmias induced with three extrastimuli; 2, sustained tachyarrhythmias induced with three extrastimuli; 3, nonsustained tachyarrhythmias induced with two extrastimuli; 4, sustained tachyarrhythmias induced with two extrastimuli; 5, nonsustained tachyarrhythmias induced with one extrastimulus; 6, sustained tachyarrhythmias induced with one extrastimulus; 7, tachyarrhythmias induced during a train of eight stimuli (8 × S1) at a basic cycle length of 120 ms; and 8, heart stopped before programmed electrical stimulation. When multiple forms of arrhythmias occurred in one heart, the highest score was used.62 The experimental protocols were typically completed within 10 minutes.