Should Cardiac Resynchronization Therapy Be the Standard Treatment for Patients with Atrioventricular Block Who Require Pacing?

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Citation
US Cardiology - Volume 5 Issue 1;2008:5(1):77-79

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Cardiac pacing remains the only effective treatment for patients with symptomatic atrioventricular block (AVB). However, recent concern over the detrimental effects of chronic right ventricular (RV) pacing has motivated clinicians to look into the role of cardiac resynchronization therapy (CRT) in this group of patients. Chronic RV pacing causes ventricular dyssynchrony, which may lead to atrial and ventricular remodeling, mitral regurgitation, and diastolic and systolic dysfunction (see Figure 1).1,2 Tolerance to chronic RV pacing appears to be influenced by baseline left ventricular (LV) function, the cumulative amount of ventricular pacing (VP), the duration of pacing, and other factors that are not currently understood.

Clinical Trials of Chronic Right Ventricular Pacing

Chronic RV pacing has been associated with a higher incidence of heart failure (HF) and atrial fibrillation (AF) than atrial-based pacing modalities in randomized clinical trials.3–10 In the MOST trial, the cumulative percentage of ventricular pacing (cum% VP), rather than the specific pacing mode, was a strong predictor of HF hospitalization and AF. A cum% VP >40% conferred a 2.6-fold increased risk for HF hospitalization in the dual-chamber (DDD) group compared with a lower percentage of pacing in similar patients. Similarly, the risk for AF was increased by 1% for each 1% increase in cum% VP.3,4 In a study by Andersen et al., atrial pacing was associated with a significantly higher survival, less AF, less HF, and fewer thromboembolic complications.5 Fixed-rate single-chamber ventricular pacing (VVI) pacing was associated with a significant increase in LV end systolic diameter and dilatation of the left atrium (LA).6 These findings also appeared to be time-dependent, since there were no significant differences in mortality or HF during the initial three years of follow-up.7
The effects of ventricular dyssynchrony induced by RV pacing appear to be more dramatic in patients with LV dysfunction or a previous history of HF. In the Dual Chamber and VVI Implantable Defibrillator (DAVID) trial, in which all patients had an LV ejection fraction (EF) <40% at enrollment, the primary combined end-point of hospitalization for HF or death was significantly increased in the DDD group compared with the VVI-40 group during a relatively short period of follow-up (mean eight months).8,9 The worse outcome in the DDD group correlated with cum% VP >40%.10

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