Biventricular Pacing Has an Advantage over Left Ventricular Epicardial Pacing Alone to Minimize Proarrhythmic Perturbation of Repolarization

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Effect of cardiac resynchronization therapy on ventricular repolarization: a meta-analysis

Objective:

Cardiac resynchronization therapy (CRT) was thought to have a proarrhythmic effect on ventricular repolarization. But the results of previous studies were inconsistent. The aim of this study was to determine the effect of CRT on ventricular repolarization.

Methods:

A meta-analysis of studies focused on the effect of CRT on ventricular repolarization in patients undergoing CRT was conducted. Endpoints including QT interval (QT), JT interval (JT), QT dispersion(QTD) and interval between the peak to end of T wave (Tp-e).

Results:

A total of 14 studies were included in our meta-analysis. After pooling the data, no significant difference was observed in QT, JT and Tp-e between biventricular (BV) pacing and intrinsic ventricular rhythm. BV paced QTD was lower than intrinsic QTD, but the significance was ambiguous [mean difference (MD): -17.33, 95% CI -34.44 to -0.22, p=0.05]. Left ventricular (LV) paced Tp-e was significantly longer than intrinsic Tp-e (MD: 21.44, 95% CI 2.37 to 40.51, p=0.03). No significant difference was observed in QT, JT and QTD between LV pacing and intrinsic ventricular rhythm.

Conclusion:

In patients undergoing CRT, BV pacing has no deteriorating effect on ventricular repolarization, but LV pacing has a prolonging effect on Tp-e.

Global and regional ventricular repolarization study by body surface potential mapping in patients with left bundle-branch block and heart failure undergoing cardiac resynchronization therapy

BACKGROUND

The controversial effects promoted by cardiac resynchronization therapy (CRT) on the ventricular repolarization (VR) have motivated VR evaluation by body surface potential mapping (BSPM) in CRT patients.

METHODS

Fifty-two CRT patients, mean age 58.8 ± 12.3 years, 31 male, LVEF 27.5 ± 9.2, NYHA III-IV heart failure with QRS181.5 ± 14.2 ms, underwent 87-lead BSPM in sinus rhythm (BASELINE) and biventricular pacing (BIV). Measurements of mean and corrected QT intervals and dispersion, mean and corrected T peak end intervals and their dispersion, and JT intervals characterized global and regional (RV, Intermediate, and LV regions) ventricular repolarization response.

RESULTS

Global QTm (P < 0.001) and QTc(m) (P < 0.05) were decreased in BIV; QTm was similar across regions in both modes (P = ns); QTc(m) values were lower in RV/LV than in Intermediate region in BASELINE and BIV (P < 0.001); only RV/Septum showed a significant difference (P < 0.01) in the BIV mode. QTD values both of BASELINE (P < 0.01) and BIV (P < 0.001) were greater in the Intermediate than in the LV region. CRT effect significantly reduced global/regional QTm and QTc(m) values. QTD was globally decreased in RV/LV (Intermediate: P = ns). BIV mode significantly reduced global T peak end mean and corrected intervals and their dispersion. JT values were not significant.

CONCLUSIONS

Ventricular repolarization parameters QTm, QTc(m), and QTD global/regional values, as assessed by BSPM, were reduced in patients under CRT with severe HF and LBBB. Greater recovery impairment in the Intermediate region was detected by the smaller variation of its dispersion.

Optimization of repolarization during biventricular pacing: a new target in patients with biventricular devices?

BACKGROUND

Evaluation of repolarization during sequentional biventricular pacing.

METHODS

Patients with biventricular devices, and left ventricular leads placed to the basal part of lateral left ventricular wall were enrolled. QRS, QTc, JTc, and corrected Tpeak-Tend intervals were compared during sequentional biventricular, left ventricular, and right ventricular pacing.

RESULTS

Five patients with nonischemic and five with ischemic cardiomyopathy due to anterior myocardial infarction were enrolled. No correlation was observed between values of repolarization among patients. The optimal values of repolarization were significantly different from values of echocardiographically guided hemodynamic optimization. Two patients with biventricular pacing-induced ventricular fibrillation were successfully treated by reprogramming of V-V delay according to interventricular delay resulting in shorter Tpeak-Tend interval, although delayed effect of amiodarone in one of these patients cannot be ruled out.

CONCLUSIONS

Patients with biventricular devices may be prone to development of ventricular arrhythmias depending on programmed V-V interval. We suggest that optimization of repolarization may be performed in patients with biventricular pacemakers in the absence of backup ICD and those with frequent episodes of ventricular tachyarrhythmias, although this finding deserves further study.

Electrical remodeling in the failing heart

PURPOSE OF REVIEW

We focus on the molecular and cellular basis of excitability, conduction and electrical remodeling in heart failure with dyssynchronous left ventricular contraction (DHF) and its restoration by cardiac resynchronization therapy (CRT) using a canine tachy-pacing heart failure model.

RECENT FINDINGS

The electrophysiological hallmark of cells and tissues isolated from failing hearts is prolongation of action potential duration (APD) and conduction slowing. In human studies and a number of animal models of heart failure, functional downregulation of K currents and alterations in depolarizing Na and Ca currents and transporters are demonstrated. Alterations in intercellular ion channels and extracellular matrix contribute to heterogeneity of APD and conduction slowing. The changes in cellular and tissue function are regionally heterogeneous, particularly in the DHF. Furthermore, beta-adrenergic signaling and modulation of ionic currents is blunted in heart failure. CRT partially reverses the DHF-induced downregulation of K current and improves Na channel gating. CRT significantly improves Ca homeostasis, especially in lateral myocytes, and restores the DHF-induced blunted beta-adrenergic receptor responsiveness. CRT abbreviates DHF-induced prolongation of APD in the lateral myocytes, reduces the left ventricular regional gradient of APD and suppresses development of early afterdepolarizations.

SUMMARY

CRT partially restores DHF-induced electrophysiological remodeling, abnormal Ca homeostasis, blunted beta-adrenergic responsiveness, and regional heterogeneity of APD, and thus may suppress ventricular arrhythmias and contribute to the mortality benefit of CRT as well as improving mechanical performance of the heart.

Functionally distinct sodium channels in ventricular epicardial and endocardial cells contribute to a greater sensitivity of the epicardium to electrical depression

A greater depression of the action potential (AP) of the ventricular epicardium (Epi) versus endocardium (Endo) is readily observed in experimental models of acute ischemia and Brugada syndrome. Endo and Epi differences in transient outward K(+) current and/or ATP-sensitive K(+) channel current are believed to contribute to the differential response. The present study tested the hypothesis that the greater sensitivity of Epi is due in part to its functionally distinct early fast Na(+) current (I(Na)). APs were recorded from isolated Epi and Endo tissue slices and coronary-perfused wedge preparations before and after exposures to elevated extracellular K(+) concentration ([K(+)](o); 6-12 mM). I(Na) was recorded from Epi and Endo myocytes using whole cell patch-clamp techniques. In tissue slices, increasing [K(+)](o) to 12 mM reduced V(max) to 51.1 +/- 5.3% and 26.8 +/- 9.6% of control in Endo (n = 9) and Epi (n = 14), respectively (P < 0.05). In wedge preparations (n = 12), the increase in [K(+)](o) caused selective depression of Epi APs and transmural conduction slowing and block. I(Na) density was not significantly different between Epi (n = 14) and Endo (n = 15) cells, but Epi cells displayed a more negative half-inactivation voltage [-83.6 +/- 0.1 and -75.5 +/- 0.3 mV for Epi (n = 16) and Endo (n = 16), respectively, P < 0.05]. Our data suggest that reduced I(Na) availability in ventricular Epi may contribute to its greater sensitivity to electrical depression and thus may contribute to the R-ST segment changes observed under a variety of clinical conditions including acute myocardial ischemia, severe hyperkalemia, and Brugada syndrome.

Biventricular Pacing and QT Interval Prolongation

INTRODUCTION

The purpose of this study was to examine BiV pacing-dependent changes in QT interval and the related potential for proarrhythmia. Biventricular (BiV) pacing has emerged as a promising therapy for patients with advanced congestive heart failure (CHF) and bundle branch block (BBB).

METHODS AND RESULTS

One hundred and seventy-six consecutive patients (123 men and 53 women; mean age 67 +/- 16 years) with ischemic (n = 128) or nonischemic (n = 48) cardiomyopathy in New York Heart Association Class II (8%) or III (92%) CHF (ejection fraction 24 +/- 9%) underwent atrial synchronous BiV pacing. The QRS, QT, and JT intervals were measured at 30 minutes after initiation of BiV pacing, at 24 hours, and at 1 month postimplant. QT interval was defined as the time interval between the initial deflection of the QRS complex and the point at which the T wave crossed the isoelectric line. At baseline, the average QRS duration was 178 +/- 10 ms, attributable to left BBB (n = 158) or intraventricular conduction delay (n = 18). BiV pacing resulted in a small but statistically significant reduction in QRS duration (148 +/- 9 ms during BiV pacing vs 178 +/- 10 ms at baseline [P < 0.0001]), yet the QT increased to 470 +/- 34 ms with BiV pacing versus 445 +/- 32 ms at baseline [P < 0.0001]). The JTc interval during BiV pacing was significantly shorter than during LV pacing (290 +/- 9 ms vs 320 +/- 20 ms, P < 0.0001). During a mean follow-up of 24 +/- 6 months, one patient developed recurrent torsade de pointes. That was eliminated once left ventricular pacing was discontinued.

CONCLUSION

Biventricular pacing prolongs QT interval. However, the occurrence of torsade de pointes is uncommon.

Cardiac resynchronization therapy: from creation to evolution--an evidence-based review

In the past decade, cardiac resynchronization therapy (CRT), achieved by simultaneous left and right ventricular pacing, has emerged as a potent therapeutic option for patients with congestive heart failure. Electrical dyssynchrony, most often manifested by left bundle branch block on the surface 12-lead electrocardiogram, results in mechanical dyssynchrony of the left ventricular septum and free wall, which decreases cardiac efficiency. In patients with ejection fractions <30%, New York Heart Association (NYHA) class III or IV, and QRS width >120 ms, CRT improves clinical parameters such as 6-minute walk distances, quality-of-life scores, and NYHA functional class. Long-term reverse remodeling of the failing ventricle results in reductions in congestive heart failure hospitalizations and mortality independent of defibrillator therapy. While most patients show significant improvement, a small proportion fail to respond. Appropriately identifying patients who will benefit most from CRT and timing the initiation of resynchronization therapy remain areas of intense investigation.

Left ventricular isovolumic flow sequence during sinus and paced rhythms: new insights from use of high-resolution Doppler and ultrasonic digital particle imaging velocimetry

OBJECTIVES

We sought to clarify the role of isovolumic intervals during a cardiac cycle by in vivo visualization of left ventricular (LV) intracavitary flow dynamics.

BACKGROUND

Asynchronous LV deformation during isovolumic contraction (IVC) and isovolumic relaxation (IVR) might represent a transient feature of myocardial wall mechanics that reverses the direction of blood flow.

METHODS

In 10 beating porcine hearts, the changes in LV intracavitary flow were recorded at baseline and after LV epicardial and right atrial pacing with high-resolution Doppler and contrast echocardiography. Two-dimensional vector flow fields were generated offline from B-mode contrast images with particle imaging velocimetry.

RESULTS

During IVC, flow from the LV apex accelerated toward the base, whereas blood from the base was redirected toward the outflow through formation of an anterior vortex. Conversely, during IVR, flow was initially directed toward the apex and then briefly reversed toward the base. Epicardial pacing from the LV base altered the stages of flow redirection during the pre-ejection period and delayed mitral valve closure (28 +/- 14 ms vs. 61 +/- 13 ms, p < 0.001) and aortic valve opening (77 +/- 18 ms vs. 111 +/- 18 ms, p = 0.004).

CONCLUSIONS

Isovolumic intervals are not periods of hemodynamic stasis but, rather, phases with dynamic changes in intracavitary flow. Experimentally induced aberrant epicardial electrical activation alters stages of flow redirection and prolongs the pre-ejection period. Normal electromechanical activation through the His-Purkinje system in mammalian hearts maintains an inherent synchrony with the sequence of intracavitary flow redirection.