Pharmacological characterisation of a re-entry model for atrial tachycardia in conscious dogs

In vivo antiarrhythmic and cardiac electrophysiologic effects of a novel diphenylphosphine oxide IKur blocker (2-isopropyl-5-methylcyclohexyl) diphenylphosphine oxide

The antiarrhythmic efficacy of the novel ultrarapid delayed rectifier potassium current (IKur) blocker (2-isopropyl-5-methylcyclohexyl) diphenylphosphine oxide (DPO-1) was compared with efficacies of the standard class III rapidly activating component of delayed rectifier potassium current (IKr) blockers [+-N-[1'-(6-cyano-1,2,3,4-tetrahydro-2-napthalenyl)-3,4-dihydro-4-hydroxyspiro[2H-1-benzopyran-2,4'-piperidin]-6-yl] methanesulfonamide hydrochloride (MK499) and ibutilide and the class IC agent propafenone in a canine model of Y-shaped intracaval and right atrial free wall surgical lesions producing the substrate for reentrant atrial flutter. Electrocardiographic and cardiac electrophysiologic effects also were assessed at the effective antiarrhythmic doses of test agents. DPO-1 terminated atrial arrhythmia (six/six preparations; 5.5 +/- 2.0 mg/kg i.v.) while significantly increasing atrial relative and effective refractory periods (+15.7 and +15.2%, respectively) but having no significant effects on ventricular refractory periods or electrocardiogram (ECG) intervals. Effective antiarrhythmic doses of MK499 (five/five preparations; 0.004 +/- 0.002 mg/kg i.v.) and ibutilide (five/five preparations; 0.003 +/- 0.001 mg/kg i.v.) similarly increased atrial relative (+23.2 and +25.1%, respectively) and effective (+21.6 and +31.9%, respectively) refractory periods. However, antiarrhythmic doses of MK499 and ibutilide also consistently and significantly increased ventricular relative (+9.9 and +7.6%, respectively) and effective (+10.4 and +9.9%, respectively) refractory periods, rate-corrected ECG QTc (+6.7 and +7.8%, respectively), and paced QT (+7.3 and +8.5%, respectively) intervals. Doses of propafenone that terminated atrial arrhythmia (five/five preparations; 0.94 +/- 0.54 mg/kg i.v.) significantly increased ECG QRS interval (+11.1%). These findings support the approach of atrial selective modulation of refractoriness through block of IKur for the development of potentially safer and more effective atrial antiarrhythmic agents.

Endocardial mapping of reentry around an anatomical barrier in the canine right atrium: observations during the action of the Class IC agent, flecainide

INTRODUCTION

Flecainide is effective in terminating stable atrial flutter in the conscious dog with a Y-shaped right atrial lesion. In this model, flutter is due to circus movement of the impulse around a fixed anatomical barrier.

METHODS AND RESULTS

To investigate the mechanism of flecainide-induced termination of this type of reentry, we determined the pattern of endocardial activation of the right and left atria before and during administration of flecainide by recording simultaneously from 192 electrode pairs in the isolated blood perfused heart. At least five consecutive flutter beats were analyzed before and during flecainide for each of eight termination episodes in five hearts. In all, flecainide increased flutter cycle length (164 +/- 24 msec) by 89% to 309 +/- 77 msec (P < 0.05) before termination. Atrial refractory period and conduction time during paced beats were also increased by flecainide. In five episodes, termination was due to conduction block of the impulse at critical sites within the reentrant circuit (mode 1). Cycle length oscillations (+/- 30 msec) at sites proximal to site of block preceded termination in three of these episodes. In three other episodes, interruption of the original circuit occurred when there was failure of a lateral boundary, giving rise to an impulse that reset the original circuit (mode 2). In these episodes, long-short cycle length oscillations led to return reexcitation by the impulse within the primary path and subsequent termination.

CONCLUSION

In summary, similar to our previous findings with the Class III agent, d-sotalol, two different modes of termination of atrial reentry were observed with flecainide.

Activation mapping of reentry around an anatomical barrier in the canine atrium: observations during the action of the class III agent, d-sotalol

INTRODUCTION

In the chronically instrumented animal and the isolated blood perfused heart, atrial reentry via a fixed path around an anatomical obstacle has been described and is terminated by the Class III antiarrhythmic agent, d-sotalol. The precise mechanism by which d-sotalol terminates this arrhythmia is not known.

METHODS AND RESULTS

In the present study, right atrial (RA) activation sequences in the isolated, coronary artery perfused canine heart during episodes of sustained flutter (n = 7) and drug administration were determined. A fixed array of bipolar electrodes was used to record endocardial electrograms from 96 sites on the RA simultaneously. Maps of all control flutters showed that the rhythm was due to persistent circus movement of the impulse around the tricuspid valve ring. D-sotalol was effective in terminating atrial reentry in this model. In all episodes, block of the excitatory impulse in a specific region of the reentrant circuit accompanied these terminations. However, the events preceding the occurrence of block of the impulse were not similar. Two different modes of termination are described.

CONCLUSION

The Class III antiarrhythmic agent d-sotalol can terminate atrial reentry in several ways. In one mode, complete conduction block of the reentering impulse within the fixed path occurs to terminate the rhythm. In the other mode, interruption of the original reentrant circuit occurs when there is failure of a lateral boundary. Often in this latter case, interruption of the original circuit is by an extra impulse that is secondary to a change in the path of the impulse. In both modes cycle length oscillations are observed.

Effects of antiarrhythmic drugs on canine atrial flutter due to reentry: role of prolongation of refractory period and depression of conduction to excitable gap

Antiarrhythmic drugs prolong the effective refractory period and depress conduction. To determine the exact role played by these two electrophysiologic effects in the termination of reentry, the effects of disopyramide, flecainide, propafenone and E-4031, a new class III drug, were examined in a canine model of atrial flutter (cycle length 120 +/- 4 to 131 +/- 3 ms) caused by reentry. Atrial flutter was induced in 32 anesthetized open chest dogs after placement of an intercaval crush. The excitable gap ranged from 9 +/- 2% to 11 +/- 4% of the basic flutter cycle length. The effective refractory period in the reentrant circuit during atrial flutter was estimated by subtracting the excitable gap from the basic flutter cycle length. Prolongation of flutter cycle length by the test drugs was proportional to the interatrial conduction time (r = 0.87, p less than 0.001). Atrial flutter was terminated by each test drug in all dogs except for flecainide and propafenone in one dog each. E-4031 prolonged the refractory period during atrial flutter to 129 +/- 6 ms, which did not differ significantly from the flutter cycle length immediately before termination (134 +/- 4 ms). The refractory period during atrial flutter after injection of the other drugs was shorter than the flutter cycle length before termination of atrial flutter (for example, flecainide 126 +/- 5 vs. 179 +/- 11 ms, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of N-acetylprocainamide and sotalol on ion currents in isolated guinea-pig ventricular myocytes

The effects of N-acetylprocainamide (NAPA) and sotalol on membrane current systems of guinea-pig ventricular myocytes were examined and compared with those of quinidine using patch-clamp techniques. All of the drugs prolonged the action potential duration (i.e. Class III effect) in isolated guinea-pig papillary muscles. In isolated ventricular cells. NAPA (300 microM) and sotalol (100 microM) produced a decrease in the delayed outward potassium current (IK) concomitantly with a slight decrease in the calcium current (ICa), which was similar to quinidine (10 microM). NAPA also slightly depressed the inward rectifier potassium current (IKrect). Thus, NAPA and sotalol both inhibited IK, and this action appears to be mainly responsible for their Class III effect.

A dual electrophysiologic test for atrial antireentry and ventricular antifibrillatory studies. Effects of bethanidine, procainamide, and WY-48986

We have developed a dual electrophysiologic test that allows measurement of both antireentry and antifibrillatory activities of potential antiarrhythmics in the same anesthetized dog. The reentry portion of the model was created surgically by a Y-shaped crushing around the tissue between the superior and inferior vena cava and tissue parallel to the AV groove. The pacing-induced tachycardia that results from circus movements around the tricuspid ring is very persistent in duration and regular in cycle length. The antifibrillatory activities were assessed by determination of the ventricular fibrillation threshold (VFT) using train-stimuli method. Control VFT was measured every 15-20 min in duplicate and followed by induction of atrial reentry. A drug was infused to intervene the atrial tachycardia. After the conversion of the arrhythmia (either by drug regimens or pacing), postdrug VFT was measured, again in duplicate. Bethanidine (20 mg/kg), procainamide (30 mg/kg), and WY-48986 (10 mg/kg), a Class III antiarrhythmic, were evaluated in this dual test. Bethanidine and procainamide prolonged the cycle length of atrial reentry to a greater extent than WY-48986. The atrial arrhythmias were consistently terminated by procainamide and WY-48986 whereas bethanidine converted the tachycardias in one of the five dogs studied. All three agents elevated VFT with bethanidine producing higher values than procainamide and WY-48986. In conclusion, the dual electrophysiologic testing system offers both economic and scientific advantages for the study of modes of action of antiarrhythmic agents.

A dual model for cardiac arrhythmias: coexistence of re-entry and abnormal automaticity and effects of antiarrhythmic agents

1. We have developed a dual model for arrhythmia anaesthetized dogs. The model consists of an inducible re-entrant atrial tachycardia and spontaneous ventricular ectopies in the same heart. 2. The model for re-entrant atrial tachycardia was created by crushing the right atrium longitudinally in the intercaval region and transversely in the front free wall parallel to the atrioventricular groove. Ventricular abnormal automaticity was produced by prior (20 approximately 24 h) left anterior descending coronary artery occlusion. The ventricular arrhythmia was partially suppressed during rapid pacing-induced atrial tachycardia and resumed after atrial re-entry was terminated. 3. Mapping experiments indicate that the atrial tachycardia was due to circus movement occurring in the tissue around the tricuspid ring. This re-entrant circuit was identical to that induced in the model created by the incision method. 4. Clofilium (0.75 mg kg-1, n = 5) increased the cycle length of atrial re-entry by 14 +/- 4% from 139 +/- 12 to 159 +/- 18 ms (P less than 0.05). Flecainide (1.8 +/- 0.9 mg kg-1, n = 5) prolonged the cycle length of the tachycardia by 114 +/- 57% from 158 +/- 11 to 332 +/- 66 ms (P less than 0.05). 5. Both drugs terminated the atrial arrhythmia, but re-entry could be reinduced only in flecainide-treated dogs. Flecainide reduced ventricular ectopies by 89 +/- 19%, whereas clofilium did not change ventricular abnormal automaticity or maximum pacing cycle length that is necessary to overdrive the ventricle fully. 6. These data indicate that the dual model provides coexisting arrhythmias of different mechanisms in the same heart and that Class I and Class III anti-arrhythmic drugs may be differentiated from each other by the distinct patterns of pharmacological activities produced in this test system.