The efficacy of lidocaine and verapamil alone and in combination on spontaneously occurring automatic ventricular tachycardia in conscious dogs one day after right coronary artery occlusion

Zatebradine slows ectopic ventricular rhythms in canine heart 24 hours after coronary artery ligation

Arrhythmias occur 24 h after occlusion of the left anterior descending (LAD) coronary artery in the canine heart and have been attributed to the abnormal spontaneous activity in subendocardial Purkinje fibers, which are markedly depolarized. The major current underlying normal automaticity in these fibers is i(f). Although the i(f) activation range is generally considered to be more negative than the diastolic membrane potential in these depolarized fibers in infarcts, this activation range has been shown to shift in a positive direction in response to hormonal influences. Thus i(f) could still mediate automaticity in these fibers in infarcts. Furthermore, recent reports indicate that a depolarizing diastolic current, probably i(f), also can be measured in ventricular muscle during abnormal experimental conditions, which may occur during ischemia. To test whether there is a role of i(f) currents in sustaining ventricular ectopy, we administered the selective i(f) channel blocker, zatebradine, 24 h after LAD ligation in canine hearts. We report that intravenous injections of zatebradine (0.25 or 1.0 mg/kg) significantly slow ventricular rhythms (with average reductions of 19 or 26%, respectively). Moreover, because zatebradine also slows sinus nodal rate, it can lead to an increased incidence of ectopic beats. However, during right atrial pacing, when sinus slowing has no effect on ventricular rhythms, capture of ventricular rhythms occurs at lower rates in the presence of zatebradine. The reduction of capture threshold is comparable to the reduction in the rate of the ectopic rhythm. Thus zatebradine eliminated the arrhythmia when the right atrium was paced at the original sinus rate.

Mechanisms of triggered activity induction at the border zone of normal and abnormal cardiac tissue

Ionic mechanisms that may be involved in inducing triggered activations at the border zone (BZTAs) of normal and abnormal Purkinje fiber segments were investigated. In a two-chamber bath, fibers were divided into a normal segment and segment treated with ethylenediaminetetraacetic acid to stimulate electrophysiologic alterations 24 hours after infarct. Interventions to normal segments included 1.8 mM lidocaine (n = 10), 3 x 10(-4) mM tetrodotoxin (n = 5), 10(-3) mM aconitine (n = 4), 3 mM cesium chloride (n = 7), 10(-2) mM verapamil (n = 4), and 6-8 mM (n = 7) of K+. Ethylenediaminetetraacetic acid (3.3 mM) prolonged action potentials and induced low diastolic potentials in the normal segment border zone. Tetrodotoxin, lidocaine, and high K+ levels suppressed BZTAs; cesium chloride and aconitine increased BZTAs; and verapamil did not reduce BZTAs. The finding that BZTAs were not abolished by verapamil suggests that abnormal automaticity is not a mechanism. Apparently, BZTAs depend on the Na+ inward current activated by depolarization of the membrane secondary to depolarization of adjacent cells.

Effects of gallopamil, a Ca2+ channel blocker in models of ventricular arrhythmia in dogs

The antiarrhythmic effects of gallopamil on adrenaline-, digitalis- and two-stage coronary ligation-induced arrhythmias and on adrenaline-induced triggered arrhythmia were investigated. Gallopamil suppressed adrenaline-induced and adrenaline-induced triggered arrhythmias, and these antiarrhythmic effects of gallopamil were similar to those of verapamil. Gallopamil also showed some antiarrhythmic effect on the 48-h coronary ligation-induced arrhythmia. The plasma concentration of gallopamil which decreased the arrhythmic ratio for adrenaline-induced arrhythmia by 50% (IC50) was 32 ng/ml. These results indicate that gallopamil may be a clinically useful antiarrhythmic drug.

Programmed electrical stimulation and drugs identify two subgroups of ventricular tachycardias occurring 16-24 hours after occlusion of the left anterior descending artery

BACKGROUND

Spontaneous sustained ventricular tachycardia (VT) occurring 16-24 hours after left anterior descending (LAD) coronary artery occlusion in the canine heart is most likely based on abnormal automaticity. In vitro, it has been demonstrated that the rate of the arrhythmia and the effect of overdrive pacing depends on the maximal diastolic potential (MDP). The MDP is also of importance in understanding the effect of antiarrhythmic drugs. To study 1) the possible presence of different responses to overdrive pacing and 2) the relation between the response to overdrive pacing and the effect of different antiarrhythmic drugs in the intact heart, we investigated the effect of 1) (prolonged) pacing and 2) lidocaine (3 mg/kg), verapamil (0.4-1.0 mg/kg), or flunarizine (2 mg/kg) during VT.

METHODS AND RESULTS

In 21 conscious dogs with chronic atrioventricular block, 60 sustained VTs were observed 1 day after LAD occlusion. During VT, pacing with interstimulus intervals of 400, 300, and 200 msec for 15, 60, and 120 seconds was done on 40 VTs. Based on their response to pacing, VTs were divided into a pacing-suppressible (PS group) and a pacing-nonsuppressible group (PNS group). The mean cycle length in the PS group was significantly longer (410 +/- 50 msec) than in the PNS group (360 +/- 35 msec, p less than or equal to 0.01). Suppression was directly related to the rate and duration of pacing. Spontaneous recurrence of VTs was observed after 26 +/- 45 seconds. Lidocaine and verapamil increased cycle length of the suppressible VTs and terminated them, whereas flunarizine had no effect. Except for verapamil, which increased cycle length of the VTs, no effects were seen in the PNS group.

CONCLUSIONS

In conscious dogs showing sustained VTs 16-24 hours after LAD occlusion, 1) the slower VTs can be suppressed by pacing, verapamil, and lidocaine but not by flunarizine, and 2) the faster VTs are not affected by pacing, lidocaine, and flunarizine, and are only slowed by verapamil. These findings are compatible with in vitro findings of abnormal automaticity, with the slower VTs originating from a higher MDP than the faster VTs.

Cellular electrophysiologic characteristics of surviving subendocardial fibers in chronically infarcted right ventricular myocardium susceptible to inducible sustained ventricular tachycardia

Permanent occlusion of the right coronary artery (RCA) is associated with inducible sustained ventricular tachyarrhythmias (VT) during days 3 to 10 post RCA occlusion period in the conscious dog; VT could no longer be induced beyond this post occlusion period. The aims of the present study were to determine if subendocardial (SE) fibers in the infarcted right ventricle (RVI) during both inducible and noninducible phases of VT remain viable, and if so, to characterize their transmembrane potential properties with the microelectrode and to assess their morphologic features. The RCA was occluded in 13 closed-chest anesthetized dogs with intracoronary balloon inflation. In one group (N = 7), the infarcted tissues were isolated during the VT inducible phase and in another group (N = 6) these tissues were isolated during the VT noninducible phase. Resting membrane potential, action potential amplitude, maximum upstroke velocity, and action potential duration of the surviving SE Purkinje fibers (PF) and ventricular muscle (VM) in the IZ (first layer) were not significantly different in the two groups. Conduction velocity for both basic and premature stimuli from the base to the apex were similar in the two groups. Rapid stimulation at cycle lengths of 300 to 200 msec failed to induce triggering of automatic activity in the two groups. Electron microscopy of SEPF in the IZ showed a drastic reduction in cytosolic lipid droplet accumulation when compared to 24-hour-old infarct. We conclude that: (1) SEPF and VM network in the infarct zone remain electrically viable during the chronic phase of RVI; (2) transmembrane potential properties of this fiber network remain constant and independent of temporal changes of VT inductibility; and (3) ultrastructural improvement of this fiber network suggests an evolution toward normalcy.

The efficacy of cibenzoline and propafenone against inducible sustained and nonsustained ventricular tachycardias in conscious dogs with isolated chronic right ventricular infarction: a comparative study with procainamide

The efficacy of intravenous cibenzoline (3 mg/kg), propafenone (4 mg/kg), and procainamide (20 mg/kg) against inducible sustained and nonsustained ventricular tachycardias (VT) was evaluated in 12 conscious dogs with chronic isolated right ventricular (RV) infarction. RV infarct was caused by permanent occlusion of the right coronary artery in the closed-chest dog by intracoronary balloon inflation. Three to 10 days following the occlusion period, programmed electrical stimulation reproducibly induced sustained and/or nonsustained VT, allowing evaluation of antiarrhythmic drug efficacy. Propafenone was effective in preventing the induction of sustained VT in only one out of six dogs tested, but caused a significant (p less than 0.05) slowing of VT rate (269 +/- 13 to 230 +/- 10 bpm). Procainamide had effects similar to those seen with propafenone. Propafenone and procainamide were ineffective against nonsustained VT, and on established sustained VT once induced. Cibenzoline was effective in preventing the induction of sustained VT in two out of seven dogs, an effect which was not significantly different from either propafenone or procainamide. However, cibenzoline was significantly (p less than 0.05) more effective than either procainamide or propafenone in terminating an established induced sustained VT (four out of six dogs). Furthermore, cibenzoline converted nonsustained to sustained VT in four out of seven dogs tested. Histopathologic studies have shown infarction of the basal two thirds of the RV (38.5 +/- 7.8% of the RV) with no left ventricular involvement. It is concluded that the isolated RV infarction model is highly suitable for serial drug testing against inducible VT in conscious dogs, and this model of VT appears to be fairly resistant to standard and newer antiarrhythmic drug therapy.