Programmed electrical stimulation after myocardial infarction and reperfusion in conscious dogs

Efficacy of Verapamil against Ventricular Arrhythmias Induced by Programmed Electrical Stimulation in the Late Myocardial Infarction Phase in Dogs

The aim of the present study was to investigate the antiarrhythmic potential of verapamil in the late myocardial infarction period in conscious dogs. Verapamil was administered in cumulative doses (0·3 + 0·3 mg kg−1). The drug significantly lowered systolic and diastolic blood pressure after both doses. ECG signals showed short-lasting significant decrease in RR and QT intervals together with an increase in QTc interval. The parameters of the atrioventricular conduction system (PQ interval, 2:1 AV-conduction point) were significantly prolonged over the entire observation period. Ventricular effective refractory periods remained unaltered. In contrast to results obtained during acute ischaemia and in the first week thereafter, the present study demonstrates that verapamil moderately increases intraventricular conduction time 14 days after acute myocardial infarction. Verapamil prevented the induction of arrhythmias by programmed electrical stimulation (PES) in only 11% of all induction attempts. The lack of lengthening of refractory periods in the presence of a prolongation of intraventricular conduction time may be responsible for the poor antiarrhythmic efficacy. We conclude that verapamil is only of negligible value for the management of PES-induced ventricular arrhythmias in the late myocardial infarction period.

Effect of ajmaline on sustained ventricular tachycardia induced by programmed electrical stimulation in conscious dogs after myocardial infarction

As yet the antiarrhythmic efficacy of ajmaline with regard to suppressing the induction of sustained ventricular tachycardia after myocardial infarction has not been determined. Therefore, programmed electrical stimulation was performed in 8 conscious, chronically instrumented mongrel dogs 8-20 days after a 4-hour occlusion of the left anterior descending coronary artery. At baseline all animals responded with sustained ventricular tachycardia. Thereafter, ajmaline was administered at two consecutive i.v. doses: a bolus of 0.7 mg kg-1 followed by infusion of 2 mg kg-1 h-1 and infusion of 4 mg kg-1 h-1. The induction of sustained ventricular tachycardia was prevented in 2/8 animals by 2 mg kg-1 h-1 ajmaline and in 1/8 animals by 4 mg kg-1 h-1 ajmaline. During sinus rhythm only 4 mg kg-1 h-1 ajmaline significantly increased QRS-duration and intraventricular activation times, but during rapid right ventricular pacing (cycle length = 330 ms) both doses of ajmaline increased QRS duration and intraventricular conduction times. 4 mg kg-1 h-1 ajmaline also increased the cycle length of induced sustained ventricular tachycardia. In 3 animals induction of sustained ventricular tachycardia during infusion of 4 mg kg-1 h-1 ajmaline was achieved by introduction of less extrastimuli than at baseline. Furthermore the coupling intervals of extrastimuli that induced sustained ventricular tachycardia were substantially prolonged by this dose. Inhomogeneity of conduction between left ventricular normal zone and left ventricular infarct zone was significantly increased by 4 mg kg-1 h-1 ajmaline during rapid right ventricular pacing, but not during sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Reproducibility of the model of induced ventricular tachycardia in conscious dogs with infarction

The canine model of ventricular tachycardias (VT) induced by programmed stimulation is used routinely in several laboratories to test antiarrhythmic drugs. The aim of the present study was to determine the rate of success and reproducibility of this model. We analyzed a group of 58 dogs that underwent a 2-hr occlusion and were submitted to programmed electrical stimulation at least 4 days after the surgery. Only 29 dogs (50%) were inducible and included in the study, as 22 dogs died following myocardial infarction, and seven dogs were never inducible. Out of 130 trials, 92 (70%) performed on inducible dogs were positive with 11% of nonsustained ventricular tachycardias, 63% of sustained monomorphic ventricular tachycardias, and 26% of ventricular fibrillation. Inducibility decreased over time in a subgroup of 19 dogs that was submitted to four trials during the first month after the infarction (68% of inducible dogs versus 46% in trials 1 and 4, respectively). Ventricular effective refractory period decreased significantly from 146 +/- 7 msec at trial 1 to 114 +/- 6 msec at trial 4, and the severity of the induced ventricular tachycardias increased. This variability should be considered when planning studies on antiarrhythmic drugs in this model.

Lidocaine has a narrow antiarrhythmic dose range against ventricular arrhythmias induced by programmed electrical stimulation in conscious postinfarction dogs

The aim of the present study was to investigate the dose-dependent antiarrhythmic efficacy of lidocaine against electrically induced tachycardias in conscious, chronically instrumented postinfarction dogs. Programmed electrical stimulation (PES) was performed in 16 dogs 8 to 21 days after a 4 h occlusion of the left anterior descending coronary artery (LAD). Infusion of saline in 8 control animals with sustained ventricular tachycardia (SVT) inducible at baseline did not affect subsequent inducibility. In the treatment group 7 of 8 animals responded with SVT and one exhibited ventricular fibrillation at baseline. After an initial bolus of 1 mg/kg lidocaine intravenously (i.v.), the drug was infused at infusion rates of 40, 80 and 120 micrograms/kg/min (i.v.). During 80 micrograms/kg/min lidocaine (mean plasma level 3.5 micrograms/ml) 7 out of 8 animals displayed an antiarrhythmic response; both the lower and the higher infusion rate were associated with a smaller antiarrhythmic efficacy (3 of 8 animals responded to 40 micrograms/kg/min and 4 of 8 to 120 micrograms/kg/min). Licocaine did not affect ventricular refractory periods, but induced an increase in intraventricular conduction time at all infusion rates, from 66.2 ms at baseline to 67.7 ms (p less than 0.05), 67.7 ms (p less than 0.05), 70.0 ms (p less than 0.01) respectively. In conclusion the present study demonstrates that lidocaine is of considerable value in the management of PES-induced ventricular arrhythmias in the postinfarction phase. However there is only a small optimal therapeutic plasma level range, where lidocaine exhibits its antiarrhythmic efficacy against this type of arrhythmia; this makes a carefully titration of the drug necessary both in the experimental and in the clinical setting.