Ventricular and atrial electrophysiological effects of a IC antiarrhythmic drug, cibenzoline, in the innervated dog heart. Role of sodium and calcium channels

Ischaemia-induced loss or reversal of the effects of the class I antiarrhythmic drugs on vulnerability to fibrillation

1. In the last decade, a number of clinical observations have questioned the efficacy of certain class I antiarrhythmic drugs against ischaemia-induced ventricular fibrillation. The effects of three drugs of this class, disopyramide (Ia), lignocaine (Ib) and flecainide (Ic) on the vulnerability to fibrillation during experimental ischaemia were investigated. 2. The study was carried out in anaesthetized, open-chest pigs (n = 8 for each of the drugs, in addition to the control group, n = 6). Vulnerability to fibrillation was evaluated by measuring electrical fibrillation threshold (EFT) by means of stepwise increased intensity of wide (100 ms) diastolic impulses applied to the ischaemic tissue at a 180 beats min-1 rate. Monophasic action potential (MAP) duration and conduction time in the ischaemic region were also measured. 3. EFT determinations were performed before and during periods of ischaemia induced by complete occlusion of the left anterior descending coronary artery near its origin. Ischaemic periods of increasing duration (30, 60, 90, 120, 150 s) were induced to determine the electrophysiological changes, of EFT especially, leading to fibrillation. 4. In the absence of ischaemia, all three drugs, administered by intravenous route (1 mg kg-1 plus 0.04 mg kg-1 min-1) increased EFT to a similar extent (from approximately 7 to 10 mA), despite a 25% prolongation of conduction time. 5. During ischaemia, none of the drugs prevented the fall in EFT towards 0 mA, resulting in spontaneous fibrillation. After 30 s of ischaemia, they no longer had any capacity for raising EFT and, after 60, 90 and 120 s of ischaemia, the decrease in EFT was exacerbated. This accelerated reduction in EFT shortened the time to onset of fibrillation (after 120 s of ischaemia, 62.5% of fibrillations with flecainide instead of 12.5 under control conditions, 75% instead of 25 with lignocaine and 50% instead of 25 with disopyramide). The reduction in MAP duration due to ischaemia was also significantly accelerated (at 60 s, 178 +/- 5 ms instead of 192 +/- 4 with flecainide, 175 +/- 3 ms instead of 194 +/- 5 with lignocaine and 180 +/- 5 ms instead of 196 +/- 3 with disopyramide) and the slowing of conduction was made worse (prolongation of conduction time by 70% instead of 50). 6. In conclusion, the antifibrillatory properties normally manifested by these drugs are first suppressed, then inverted by ischaemia, depending on oxygen debt varying with severity and duration of ischaemia.

Change of a beneficial effect into an untoward effect by ischaemia: effect of quinidine-like drugs on vulnerability to ventricular fibrillation

The effects of three quinidine-like drugs, disopyramide, lidocaine and flecainide were investigated in anaesthetized, open-chest pigs on vulnerability to ventricular fibrillation under normal conditions and under myocardial ischaemia conditions. Vulnerability to fibrillation was evaluated by electrical ventricular fibrillation threshold (VFT), measured with 100 ms duration diastolic impulses the intensity of which was increased by steps of 1.0 or 0.5 mA. Impulses were delivered at the rate of 180 beats · min(-1). The ventricles were subjected to pacing at the same rate before the VFT determination, particularly throughout periods of ischaemia of increasing duration (30, 60, 90, 120, 150 s), separated by appropriate intervals for reproducibility of the results. Monophasic action potential (MAP) duration and conduction time were monitored in the ischaemic area under pacing. Ischaemia was obtained by complete occlusion of the left anterior descending coronary artery near its origin. The three drugs were i.v. administered in clinical dose range (1.00 mg · kg(-1) plus 0.04 mg · kg(-1) · min(-1)). In the absence of ischaemia, they increased almost equally VFT (from about 7 to 10 mA), despite 25% prolongation of conduction time. But, none of them was able to impede the increasingly marked fall of VFT caused by ischaemia: at 30 s, they had already lost any capacity for raising VFT and, beyond this time, they even aggravated its fall which led to spontaneous fibrillation when VFT approached 0 mA. The faster fall of VFT shortened time to onset of fibrillation (20 24 fibrillations for the three drugs at 150 s as against 12 24 in control period), the ischaemia-induced reduction of MAP duration (by 20%) being also hastened and slowing of conduction enhanced, given the addition of the depressant effects of ischaemia and drugs on conduction. Consequently, the antifibrillatory properties normally manifested by the studied drugs are first suppressed, then inverted by ischaemia.

Enhancement by ischemia of the risk of cardiac disorders, especially fibrillation, in regional anesthesia with bupivacaine

The impairment of intraventricular conduction by bupivacaine may result in reentrant arrhythmias including ventricular fibrillation. The concentrations responsible for serious accidents are high (5.0 to 8.0 micrograms/ml), but likely to be lowered by myocardial ischemia which gives rise to similar disorders. Therefore we did an electrophysiological study of bupivacaine's effects in an ischemic area of the myocardium. Monophasic action potential (MAP) of the ventricular myocardium was recorded in 30 anesthetized, open-chest pigs. Conduction time and effective refractory period were also measured. Data were obtained during short periods (10-15 s) of pacing at 180 beats/min, but ventricular beats remained governed by the sinus node in the intervals. Ischemia was produced by occluding the left anterior descending coronary artery completely but transiently (up to 8 min), not far from its origin. Comparison was made between the effects of bupivacaine i.v. (n = 10), ischemia (n = 10) and both factors (n = 10). Two min after injection of bupivacaine 2.0 mg/kg (plasma levels 2.0-3.0 micrograms/ml), the duration of MAP was only slightly (7.5-15%) prolonged and its ischemia-induced shortening only slightly attenuated by bupivacaine. At the same time, conduction time was considerably (75-150%) lengthened and its ischemia-induced lengthening enhanced, so that ventricular fibrillation induced by coronary occlusion occurred sooner (about 100 instead of 300 s) in the presence of bupivacaine. Consequently, bupivacaine should be used only with caution in individuals whose myocardium is ischemic or liable to ischemia episodes.

Arrhythmogenicity of antiarrhythmic drugs and intraventricular conduction disorders: possible aggravation by myocardial ischemia--study in the porcine in situ heart

The effects of three antiarrhythmic drugs were investigated in anesthetized, open-chest pigs, in a left ventricular area, under pacing at a constant high rate (180 beats/min), in the absence and presence of ischemia. Ischemia was produced by transient complete occlusion of the left anterior descending coronary artery near its origin. In addition to the surface electrocardiogram, conduction time and monophasic action potential were recorded in the contractile fibers. In the absence of ischemia, intravenous flecainide and propafenone 2.5 mg/kg, and intravenous cibenzoline 2.0 mg/kg considerably lengthened conduction time (by 50-90%) but had no significant effect on the monophasic action potential duration. Consequently, the cited antiarrhythmic drugs enhance the prolongation of conduction time by 60% but do not limit the 30% shortening of the monophasic action potential caused by ischemia. Contrary to what was expected, they largely reduced the time to onset of the fibrillation due to ischemia from about 120 to 25 seconds. Thus, they manifested profibrillatory properties (more pronounced than those of other antiarrhythmic drugs of class I), which might be explained by their potent action on depolarization.

Role of asynchronous activation of the ventricular fibres by an ectopic pacemaker in the accidents, especially fibrillation, caused by Ic antiarrhythmic drugs

Class Ic antiarrhythmic drugs, which are known to slow down conduction in the ventricular muscle, are likely to impair synchrony in activity of the ventricular fibres. Asynchronous activation was first investigated between an ischaemic and a normal area by the simultaneous recording in anaesthetized, open-chest pigs of two left ventricular monophasic action potentials (MAPs) under ventricular pacing at a high rate of 180 beats.min-1. Asynchronous activation was then investigated in the intact myocardium according to the distance separating the recording from the pacing electrode. Furthermore, mechanical effects of left ventricular systole were observed by recording dP/dt(max) and mean arterial blood pressure during the pacing periods. Ischaemia was produced by transient complete occlusion of the left anterior descending coronary artery near its origin; as a result, activation time reached 85 ms in the ischaemic area under flecainide administered iv in a 2.5 mg.kg-1 dose instead of approximately 60 ms in the normal area for fibres equi-distant from the pacing electrode. Similar delays in activation were observed in the intact myocardium, depending on whether the explored region was close to or far from the pacing electrode. In the latter case, the difference in activation time may become markedly greater if the distance or the dose of flecainide are increased. This difference, which possibly exceeds one-third of the MAP duration (practically unchanged by flecainide), may account for the occurrence of fibrillation or the sudden loss of systole mechanical efficacy.

Cibenzoline. A review of its pharmacological properties and therapeutic potential in arrhythmias

Cibenzoline is a class I antiarrhythmic drug with limited class III and IV activity which can be administered orally or intravenously. An elimination half-life of about 8 to 12 hours permits twice daily administration, although age and renal function must be considered when determining dosage. Cibenzoline has some activity in ventricular and supraventricular arrhythmias, including drug-refractory ventricular tachycardia or ventricular arrhythmias following recent acute myocardial infarction, although results in patients with sustained ventricular tachycardia are less promising. In comparative trials, cibenzoline has demonstrated efficacy similar to or better than that of a variety of other class I antiarrhythmic drugs and was at least as well tolerated, with a more convenient dosage schedule. However, further studies to clarify the proarrhythmic effects of cibenzoline and its use in patients with impaired left ventricular function are required, and the use of cibenzoline (and other class I antiarrhythmic agents) in patients with other than potentially lethal ventricular arrhythmias should be avoided following the results of the CAST studies. Thus, cibenzoline is an effective antiarrhythmic agent with a favourable pharmacokinetic profile that may be considered with other class I drugs in patients requiring therapy for high risk arrhythmias.

Class Ic antiarrhythmic drugs and myocardial ischaemia: study in the pig heart in situ

The effects of three Ic antiarrhythmic drugs, flecainide, propafenone and cibenzoline, were investigated in anaesthetized, open-chest pigs, in a left ventricular area, during pacing at a constant high rate (180 beats min-1), in the absence and the presence of ischaemia. Ischaemia was produced by transient complete occlusion of the left anterior descending coronary artery 1-1.5 cm from its origin. In addition to surface electrocardiogram, conduction time and monophasic action potential were recorded in the contractile fibres. Measurement of the effective refractory period was added in the absence of ischaemia. In this event, flecainide and propafenone, each in a dose of 2.5 mg kg-1 i.v. and cibenzoline, 2.0 mg kg-1, i.v., considerably lengthened (by 50-90%) conduction time, but did not affect or hardly affected the duration of the monophasic action potential or the effective refractory period. Thus, it seems that these Ic antiarrhythmic drugs enhance the prolongation of conduction time by 60% and do not prevent the 30% shortening of monophasic action potential caused by ischaemia: contrary to expectation, they produced a large reduction (from about 120 to 25 s) in the onset time of fibrillation due to ischaemia. Thus, they manifested profibrillatory properties (more pronounced than those of other class I antiarrhythmic drugs), which might be explained by their potent action on depolarization with almost total absence of action on repolarization.