High voltage shock induced cellular electrophysiological effects: transient refractoriness and bimodal changes in action potential duration

Reversal of Electrical Remodeling After Cardioversion of Persistent Atrial Fibrillation

INTRODUCTION

In animals, atrial fibrillation results in reversible atrial electrical remodeling manifested as shortening of the atrial effective refractory period, slowing of intra-atrial conduction, and prolongation of sinus node recovery time. There is limited information on changes in these parameters after cardioversion in patients with persistent atrial fibrillation.

METHODS AND RESULTS

Thirty-eight patients who had been in atrial fibrillation for 1 to 12 months underwent electrophysiologic testing 10 minutes and 1 hour after cardioversion. At 1 week, 19 patients still in sinus rhythm returned for repeat testing. Reverse remodeling of the effective refractory period was not uniform across the three atrial sites tested. At the lateral right atrium, there was a highly significant increase in the effective refractory period between 10 minutes and 1 hour after cardioversion (drive cycle length 400 ms: 204 +/- 17 ms vs 211 +/- 20 ms, drive cycle length 550 ms: 213 +/- 18 ms vs 219 +/- 23 ms, P < 0.001). The effective refractory period at the coronary sinus and distal coronary sinus did not change in the first hour but had increased by 1 week. The corrected sinus node recovery time did not change in the first hour but was shorter at 1 week (606 +/- 311 ms vs 408 +/- 160 ms, P = 0.009). P wave duration also was shorter at 1 week (135 +/- 18 ms vs 129 +/- 13 ms, P = 0.04) consistent with increasing atrial conduction velocity.

CONCLUSION

The atrial effective refractory period increases, sinus node function improves, and atrial conduction velocity goes up in the first week after cardioversion of long-standing atrial fibrillation in humans. Reverse electrical remodeling of the effective refractory period occurs at different rates in different regions of the atrium.

Electrophysiologic predictors of the recurrence of persistent atrial fibrillation within 30 days of cardioversion

Patients with recurrence of persistent atrial fibrillation within 30 days of cardioversion had slower atrial conduction, a slower sinus rate, no difference in the absolute value of the effective refractory period, greater early reverse remodeling of the effective refractory period, and more premature atrial contractions than those who did not. These findings highlight the role of slow conduction and premature atrial contractions in the pathophysiology of atrial fibrillation and suggest a possible proarrhythmic effect of reverse remodeling.

Aortic input impedance and ventriculoarterial coupling following cardioversion/defibrillation

Defibrillation shocks are commonly used after cardiac surgery or during defibrillator implantation. The hemodynamic consequences of countershocks on circulatory dynamics are not completely understood, and there is a lack of information concerning the effects on ventriculoarterial interaction. The study presented here was performed to assess the influence of defibrillation shocks on arterial hemodynamics and ventriculoarterial coupling. Eight mongrel dogs (weight 15-18 kg) were anesthetized and median thoracotomy was performed. Pressure in the ascending aorta and the left ventricle and flow in the ascending aorta were continuously monitored. After induction of atrial or ventricular fibrillation, termination was achieved by epicardial low energy shocks (atrium, 3J; ventricle, 51). In an additional attempt ventricular fibrillation was terminated by a high energy shock (34J). Aortic input impedance was calculated by fast-Fourier-transformation of aortic flow and pressure signals, while ventriculoarterial coupling was expressed by the ratio of aortic and ventricular end systolic elastance (Ea/Ees). Defibrillation by low energy shocks of atrial or ventricular fibrillation did not result in changes of the aortic impedance spectrum, and ventriculoarterial coupling remained unaltered compared to control conditions. High energy defibrillation, however, resulted in a marked rise in total peripheral resistance (P < 0.03). The ratio of Ea/Ees increased significantly (P < 0.005). These effects were reversible within 15 minutes. Low energy defibrillation does not alter arterial hemodynamics and ventriculoarterial coupling in this experimental setup. High energy defibrillation, however, results in a temporary increase of ventricular load. This finding may be of particular interest in patients with poor left ventricular function.

Defibrillation depresses heart sarcoplasmic reticulum calcium pump: a mechanism of postshock dysfunction

Presently, the only therapy for ventricular fibrillation is delivery of high-voltage shocks. Despite "successful defibrillation," patients may have poor cardiac contractility, the mechanisms of which are unknown. Intracellular Ca2+ handling by the sarcoplasmic reticulum (SR) plays a major role in contractility. We tested the hypothesis that defibrillation shocks interfere with Ca2+ transport function of cardiac SR. Rats anesthetized with pentobarbital sodium had bilateral electrodes implanted subcutaneously for transthoracic shocks. A series of 10 shocks, 10 s apart, at 0-250 V was delivered from a trapezoidal defibrilator. The hearts were rapidly removed, SR-enriched membrane vesicles were isolated, and ATP-dependent Ca2+ uptake and Ca(2+)-stimulated ATP hydrolysis were determined. There was a marked, shock-related decline in Ca2+ uptake, whereas adenosinetriphosphatase activity remained unaltered. The polypeptide compositions were similar in control and shocked SR. In Langendorff hearts, shocks also decreased contractility and slowed relaxation. These data indicate that shocks with current densities similar to defibrillation depress Ca(2+)-pumping function of cardiac SR because of uncoupling of ATP hydrolysis and Ca2+ transport. Shock-induced impairment of Ca2+ pump function may underlie postshock myocardial dysfunction.

Local electrogram changes in response to a high-voltage intracardiac shock in humans

INTRODUCTION

Transvenous defibrillators may have difficulty sensing ventricular fibrillation following an unsuccessful shock. This study was undertaken to characterize the changes that occur in intracardiac electrograms following a defibrillator shock that may contribute to the failure to redetect arrhythmias.

METHODS AND RESULTS

Unipolar and bipolar electrogram recordings were made during sinus rhythm before and following monophasic and biphasic test shocks delivered in random order through a single lead defibrillator system in 15 patients. An additional 14 patients received only a biphasic shock. Electrogram amplitude, dV/dt, and activation recovery time were determined. Following biphasic shocks, unipolar electrogram amplitude decreased by a mean of 19.6% and the bipolar amplitude by a mean of 20.8% (P = NS). The mean dV/dt in the unipolar recordings decreased by 30.7% and in the bipolar recordings decreased by 33.0% (P = NS). Time constants for recovery were approximately 60 seconds. Reduction in dV/dt was greater when the shocking coil was closer to the distal sensing electrode. There was no significant influence of shock waveform (monophasic vs biphasic), recording type (unipolar vs bipolar), or clinical variables on the change in electrogram characteristics.

CONCLUSION

Changes in intracardiac electrograms following defibrillator shocks through transvenous leads are seen with shocks delivered in sinus rhythm and may be related to the proximity of the shocking coil to the distal sensing electrode.