Role of calcium in the rate-dependent depression of atrioventricular nodal conduction in the dog heart under vagal influence

Opposite change with ischaemia in the antifibrillatory effects of class I and class IV antiarrhythmic drugs resulting from the alteration in ion transmembrane exchanges related to depolarization

It is known that class I antiarrhythmic drugs lose their antifibrillatory activity with severe ischaemia, whereas class IV antiarrhythmic drugs acquire such activity. Tachycardia, which is also a depolarizing factor, has recently been shown to give rise to an alteration of ion transmembrane exchanges which is particularly marked in the case of calcium. This leads one to wonder if the change in antifibrillatory activity of antiarrhythmic drugs caused by ischaemia depends on the same process. The change in antifibrillatory activity was studied in normal conditions ranging to those of severe ischaemia with a class I antiarrhythmic drug, flecainide (1.00 mg·kg-1 plus 0.04 mg·kg-1·min-1), a sodium channel blocker, and a class IV antiarrhythmic drug, verapamil (50 µg·kg-1 plus 2 µg·kg-1·min-1), a calcium channel blocker. The experiments were performed in anaesthetized, open-chest pigs. The resulting blockade of each of these channels was assessed at the end of ischaemic periods of increasing duration (30, 60, 120, 180, 300, and 420 s) by determining the ventricular fibrillation threshold (VFT). VFT was determined by means of trains of diastolic stimuli of 100 ms duration delivered by a subepicardial electrode introduced into the myocardium (heart rate 180 beats per min). Ischaemia was induced by completely occluding the left anterior descending coronary artery. The monophasic action potential was recorded concurrently for the measurement of ventricular conduction time (VCT). The monophasic action potential duration (MAPD) varied with membrane polarization of the fibres. The blockade of sodium channels by flecainide, which normally raises VFT (7.0 ± 0.4 to 13.8 ± 0.8 mA, p < 0.001) and lengthens VCT (28 ± 3 to 44 ± 5 ms, p < 0.001), lost its effects in the course of ischaemia. This resulted in decreased counteraction of the ischaemia-induced fall of VFT and decreased aggravation of the ischaemia-induced lengthening of VCT. The blockade of calcium channels, which normally does not alter VFT (between 7.2 ± 0.6 and 8.4 ± 0.7 mA, n.s.) or VCT (between 30 ± 2 and 34 ± 3 ms, n.s.), slowed the ischaemia-induced fall of VFT. VFT required more time to reach 0 mA, thus delaying the onset of fibrillation. Membrane depolarization itself was opposed as the shortening of MAPD and the lengthening of VCT were also delayed. Consequently there is a progressive decrease in the role played by sodium channels during ischaemia in the rhythmic systolic depolarization of the ventricular fibres. This reduces or suppresses the ability of sodium channel blockers to act on excitability or conduction, and increases the role of calcium channel blockers in attenuating ischaemia-induced disorders.Key words: pigs, ion transmembrane exchanges, myocardial ischaemia, sodium channel, calcium channel.

Magnesium--a profibrillatory or antifibrillatory drug depending on plasma concentration, heart rate and myocardial perfusion

BACKGROUND

The opinions on the efficacy of magnesium as an antiarrhythmic drug vary considerably. The action of magnesium on vulnerability to fibrillation was therefore investigated in anaesthetized, open-chest pigs under different conditions as regards plasma concentration, heart rate and myocardial perfusion.

METHODS

Vulnerability to fibrillation was assessed by electrical fibrillation threshold (EFT), measured with 100-ms duration diastolic impulses. These stimuli were delivered to the heart normally perfused, at a usual (90 and 120 beats/min) or accelerated (180 beats/min) rate. Vulnerability to fibrillation was also assessed at the high rate (180 beats/min) in the heart made ischaemic by complete occlusion of the left anterior descending coronary artery near its origin. EFT was then measured at the end of occlusion periods which were of increasing duration (30, 60, 90, 120 s). Monophasic action potential (MAP) duration, intraventricular conduction time, left ventricular dP/dt max (LVdP/dt max) and mean blood pressure were concurrently measured.

RESULTS

In the absence of ischaemia, 5 mumol.kg-1.min-1 magnesium i.v. infusion, which raised plasma concentration to 1.78 +/- 0.14 mmol/L, lowered EFT, measured at the rate of 116 beats/min, from 14.0 +/- 1.1 to 6.8 +/- 1.0 mA (P < 0.001), without significant variation of the other parameters. Administered as previously or in a markedly higher dose (400 mumol.kg-1 loading dose and 10 mumol.kg-1.min-1 infusion) which raised plasma concentration up to 4.84 +/- 0.52 mmol/L, magnesium significantly influenced neither EFT nor MAP duration, reduced by the high rate (180 beats/min) to 6.2-6.7 mA and 212-220 ms respectively. Under the same conditions, at the same 180 beats/min rate, ischaemia brings about a fall of EFT, from 6.9 down to nearly 0 mA, with occurrence of fibrillation, in approximately 120 s. Magnesium failed to slow this fall and to delay the onset of fibrillation. In contrast, within the minutes following the end of occlusion, magnesium increased EFT to a great extent (from 7.1 +/- 0.4 to 13.5 +/- 0.7 mA, P < 0.001), with a significant prolongation of MAP duration (212 +/- 6 to 234 +/- 8 ms, P < 0.01).

CONCLUSION

Magnesium may develop profibrillatory or antifibrillatory effects depending on plasma concentration, heart rate and myocardial perfusion.

Attenuation of the ischaemia-induced fall of electrical ventricular fibrillation threshold by a calcium antagonist, diltiazem

Calcium antagonists have been reported to decrease the incidence of sudden death in postinfarction management and vulnerability to fibrillation secondary to experimental coronary occlusion. In order to confirm such beneficial results regarding ischaemic ventricular fibrillation, the threshold intensity for fibrillation electrically induced with impulses of 100 ms and 180 beats.min-1 was measured during the course of ischaemias obtained by total occlusion of the left anterior descending coronary artery near its origin in open-chest pigs. The variations of electrical fibrillation threshold with ischaemia duration (30, 60, 120, 180, 240, 360 s) were compared under control conditions and after i.v. diltiazem (0.50 mg.kg-1 plus 0.02 mg.kg-1.min-1 over 25 min). Electrical fibrillation threshold was not influenced by diltiazem before, but raised during ischaemia, particularly from the 60th s (1.7 to 4.0 mA), with delay in the triggering of fibrillation which occurs when the fibrillation threshold falls down to the pacing threshold (0.2 to 0.3 mA). In 6 pigs out of 8, fibrillation was even avoided in the longest of the ischaemic periods considered (360 s), for fibrillation threshold ceased falling before reaching the critical level. These experimental results obtained with diltiazem are consistent with the clinical effectiveness of calcium antagonists recently observed in the prevention of postinfarction sudden death, provided that myocardial contractility is not too much adversely affected. But, left ventricular dP/dtmax was not reduced by more than 6.8% in the present experiments.

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.

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

The effects of cibenzoline, rightly known as a sodium channel inhibitor (class IC antiarrhythmic drug), were investigated in anaesthetized, closed-chest dogs, on conduction in the contractile fibres, ventricular and atrial, the His-Purkinje system and the atrioventricular node. In ventricular muscle, conduction time was measured between base and apex by two endocavitary electrodes. The other conduction times were obtained from the recording of the His bundle potentials. In addition, effective refractory period was determined by the extrastimulus method in ventricular and atrial muscle and in the atrioventricular node, and sinus rate monitored in the intervals of pacing periods. In the absence of vagal tone, cibenzoline in 4 mg.kg-1 dose prolonged conduction times in the ventricular contractile tissue, His-Purkinje system and atrial contractile tissue to a large extent, but decreasingly from the former to the latter. This prolongation was antagonized by hypernatremia (174 mmol.l-1). In contrast, conduction time in the atrioventricular node, effective refractory periods and sinus rate were very little influenced. In the presence of vagal tone, the prolongation of conduction times in the ventricular contractile tissue. His-Purkinje system and atrial contractile tissue did not differ substantially from previously. It was the same for ventricular effective refractory period. But atrial effective refractory period was then considerably lengthened, while conduction time and effective refractory period in the atrioventricular node were greatly shortened and sinus rate notably accelerated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of isoproterenol and propranolol on pharmacologically induced depression of intraventricular conduction

The cardiac adrenergic system is known to have practically no influence on conduction velocity in the ventricles under normal conditions. The effects of isoproterenol and propranolol were investigated on depression of intraventricular conduction induced by a class IC antiarrhythmic drug, cibenzoline, in anaesthetized, closed-chest dogs. In addition to electrocardiogram for measurement of QRS duration in sinus rhythm, conduction time was measured in the ventricular contractile tissue between an electrode advanced to the apex and a pacing electrode near the base, at 400- and 200-ms pacing periods. Effective refractory period (ERP) was measured concurrently according to the extrastimulus method. After intraventricular conduction had been slowed down by cibenzoline IV administered (loading dose of 3 mg/kg plus infusion of 0.2 mg/kg/min over 15 min), isoproterenol was infused or propranolol injected by the intravenous route also (0.5 mn/micrograms/kg/min over 5 min and 0.4 mg/kg, respectively). When conduction time has been raised by 75/150% (depending on the pacing rate), isoproterenol appears to attenuate and propranolol to aggravate substantially the impairment of conduction, whereas the reduction undergone by ERP does not differ from usual. Thus, reentrant arrhythmias might be prevented by isoproterenol and triggered by propranolol. Intraventricular conduction, when depressed, therefore, is sensitive to adrenergic drugs, probably because of the enhanced influence of polarization of the fibres in the presence of a sodium conductance impairment.