Low-energy intracardiac shocks during atrial fibrillation: effects on cardiac rhythm

Complete atrioventricular block following internal electrical cardioversion during atrial fibrillation ablation

BACKGROUND

Complete atrioventricular block (C-AVB) following internal electrical cardioversion (IEC) during atrial fibrillation (AF) ablation has not been fully investigated. We aimed to determine the prevalence and predictors of C-AVB following IEC during AF ablation.

METHODS

C-AVB (non-conducted sinus impulse after IEC) and ventricular pause (VP) (the interval between IEC and the QRS complex) following the first attempt of IEC, and baseline electrocardiographic parameters were investigated in patients who underwent first-time AF ablation.

RESULTS

We investigated the first attempt of IEC in 124 patients (mean age:70 ± 11 years, 81 men, 99 non-paroxysmal AF). AF was terminated in 109/124 (88%) patients, with a VP of 1590 [1014-2208] (maximum, 8780) ms. Transient C-AVB following IEC occurred in 14/109 (13%) patients. The VP was longer in patients with transient C-AVB than in those without transient C-AVB (2418 [1693-4425] vs. 1530 [876-2083] ms, p = 0.002). In multivariate analysis, the left atrial diameter (Odds ratio [OR]:1.21; 95% confidence interval (95%CI):1.06-1.39; p = 0.005) and preexisting intraventricular conduction abnormality (OR:9.22; 95%CI:1.60-53.3; p = 0.013) were predictors of transient C-AVB following IEC.

CONCLUSION

Left atrial diameter and preexisting intraventricular conduction abnormalities were predictors of transient C-AVB following IEC during AF ablation.

Transient complete atrioventricular block following transvenous electrical cardioversion of atrial fibrillation in a horse

Transvenous electrical cardioversion was attempted in a horse with drug refractory atrial fibrillation. A temporary pacing catheter and two defibrillation catheters were inserted transvenously into the right ventricular apex, the right atrium and the pulmonary artery, respectively. Under general anaesthesia 100, 200, 300 and 360 J monophasic shocks were delivered between both defibrillation catheters but sinus rhythm could not be restored. Immediately after the 200, 300 and 360 J shock, transient third-degree atrioventricular block occurred for a period of, respectively, 15, 40 and 55 s. These periods of profound bradycardia were corrected by temporary right ventricular pacing until spontaneous conduction resumed. It is concluded that temporary right ventricular pacing should be available during electrical cardioversion of atrial fibrillation in horses.

Marked reduction in atrial defibrillation thresholds with repeated internal cardioversion

OBJECTIVES

This study was performed to assess the atrial defibrillation threshold in patients with recurrent atrial fibrillation (AF) using repeated internal cardioversion.

BACKGROUND

Previous studies in patients with chronic AF undergoing internal cardioversion have shown this method to be effective and safe. However, current energy requirements might preclude patients with longer-lasting AF from being eligible for an implantable atrial defibrillator.

METHODS

Internal shocks were delivered via defibrillation electrodes placed in the right atrium (cathode) and the coronary sinus (anode) or the right atrium (cathode) and the left pulmonary artery. After cardioversion, patients were orally treated with sotalol (mean 189 +/- 63 mg/day). Eighty consecutive patients with chronic AF (mean duration 291 +/- 237 days) underwent internal cardioversion, and sinus rhythm was restored in 74 patients. Eighteen patients underwent repeated internal cardioversion using the same electrode position and shock configuration after recurrence of AF (mean duration 34 +/- 25 days).

RESULTS

In these 18 patients, the overall mean defibrillation threshold was 6.67 +/- 3.09 J for the first cardioversion and 3.83 +/- 2.62 J for the second (p = 0.003). Mean lead impedance was 55.6 +/- 5.1 ohms and 57.1 +/- 3.7 ohms, respectively (not significant). For sedation, 6.7 +/- 2.9 mg and 3.9 +/- 2.2 mg midazolam were administered intravenously (p = 0.003), and the pain score (0 = not felt, 10 = intolerable) was 5.1 +/- 1.9 and 2.7 +/- 1.8 (p = 0.001). Uni- and multivariate analyses revealed only the duration of AF before cardioversion to be of relevance, lasting 175 +/- 113 days before the first and 34 +/- 25 days before the second cardioversion in these 18 patients (p = 0.002).

CONCLUSIONS

If the duration of AF is reduced, a significant reduction in defibrillation energy requirements for internal cardioversion ensues. This might extend the group of patients eligible for an implantable atrial defibrillator despite relatively high initial defibrillation thresholds.

The atrial defibrillator: a stand-alone device or part of a combined dual-chamber system?

Atrial fibrillation (AF) is an extremely common arrhythmia seen in clinical practice. Because of the limited efficacy of traditional therapeutic strategies to restore and maintain normal sinus rhythm, several nonpharmacologic options have evolved. The promising results achieved with internal atrial defibrillation have facilitated the development of an implantable atrial defibrilator. Preliminary results obtained from an initial study on a small number of highly selected patients with refractory AF suggest that atrial defibrillation can be performed effectively and safely with adequate patient tolerance by using a stand-alone device. The extension of this therapy will depend on the results of well-designed prospective studies comparing this new therapeutic option with traditional methods. Several acute studies have shown that internal conversion of AF is feasible at low energies with current endocardial transvenous lead configurations primarily designed for ventricular defibrillation, but long-term efficacy has, to date, only been demonstrated with atrial implantable defibrillator lead systems. As AF is a frequent arrhythmia in implantable cardioverter defibrillator (ICD) recipients, it would seem desirable to incorporate the capability for atrial defibrillation into an ICD. Clinical studies have shown that an atrial defibrillator, as part of a combined dual-chamber ICD system, may not require a potentially complicated switching network for establishing different electrode configurations for atrial and ventricular tachyarrhythmia. The efficacy in atrial cardioversion of such a combined, less complex device seems to be as high as reported for a pure atrial defibrillator, but generally at somewhat higher energy requirements. The results of further investigations will show whether a dual-chamber cardioverter defibrillator would be of clinical relevance in patients with ventricular and supraventricular tachyarrhythmia.

Cardiac parasympathetic stimulation via QRS-synchronous low-energy shocks in humans

INTRODUCTION

In patients receiving test shocks to verify lead connections at implantation, we anecdotally have observed postshock delay. The purpose of this study was to determine whether QRS-synchronous low-energy shocks delivered by implantable defibrillators result in postshock cycle length prolongation, and to determine the mechanism of this phenomenon.

METHODS AND RESULTS

Twenty-five patients undergoing defibrillator testing were studied, three with epicardial patches and 22 with transvenous leads. Each patient received QRS-synchronous shocks of 0.2, 0.4, 0.6, and 2.0 J in random order. Patients were further randomized to receive either saline or 2.0 mg atropine intravenously, and then given a second sequence of shocks. At baseline, the postshock cycle length (1,035+/-245 msec) was significantly longer than the preshock cycle length (968+/-177 msec, P = 0.01). In patients with a coronary sinus (CS) or superior vena cava (SVC) lead, the mean prolongation was 91+/-160 msec, compared with 12+/-106 msec for patients without such a lead (P < 0.0001). All energy levels resulted in significant postshock prolongation compared with preshock cycle lengths (P < 0.05). Postshock prolongation before atropine was 76+/-162 msec, compared with -13+/-52 msec afterward (P < 0.00001). Biphasic shocks resulted in greater postshock prolongation than monophasic shocks of equal energy.

CONCLUSION

Low-energy shocks delivered during the QRS complex cause postshock cycle length prolongation in man. This effect required the presence of a CS or SVC lead. Atropine inhibited this effect, suggesting the phenomenon was mediated by direct cardiac parasympathetic nerve stimulation by the intracardiac shock.