Biventricular pacing in heart failure: right is not wrong!

Abstract

Cardiac resynchronization therapy (CRT) improves left ventricular (LV) function, symptoms, and prognosis in selected patients with heart failure [1][2][3]. Those with left bundle branch block (LBBB) and the widest QRS complexes seem to have the highest rate of response [4]. However, a significant percentage of patients with guideline-approved criteria remain "nonresponders." Improving the CRT response rate using imaging techniques has remained an elusive goal. This is a worthy area of research since a suboptimal response rate affects the cost-effectiveness of CRT unfavorably and subjects patients to the risks of the procedure without its benefits.Our current knowledge base of CRT mechanisms indicates that response depends on a complex interplay among many known and potentially unknown factors [5]. Despite a substantial body of literature attesting to the benefits of CRT in selected patients with heart failure, surprisingly little data exist pertaining to the mechanisms of benefit. One presumes logically that CRT produces its salutary effects by improving LV mechanical synchrony, but the large multicenter studies that established the field did not explore mechanisms.Several small, single-center studies have explored CRT mechanisms. In a study of 100 patients chosen for conventional CRT indications and the presence of LV dyssynchrony defined as opposing wall delay of >65 ms on tissue Doppler echocardiography, Bleeker and colleagues from Leiden reported that a significant improvement in LV synchrony after CRT was mandatory for chronic remodeling effects to occur [6]. They also found that when resynchronization occurred it was evident immediately, and there was little if any "recruitment" on follow-up. A pilot study from our group suggested that the acute response to CRT can be heterogeneous in patients undergoing CRT for guideline-based indications, with approximately one third of patients having worse LV synchrony after CRT [7]. Patients who had significant LV dyssynchrony, noncritical extent and location of LV scar, and a strategic LV lead position in viable, late-activated myocardium had the highest chance of improved synchrony after CRT. Importantly, patients in whom LV synchrony deteriorated after CRT had a worse composite outcome compared to patients who improved or remained unchanged. A positive association between each of the above component requirements for successful acute resynchronization and a favorable long-term outcome has been demonstrated in other small and mostly retrospective studies [8][9][10]. In the longer term, the acute salutary effect of CRT on LV dyssynchrony appears to be associated with chronic LV reverse remodeling and improved prognosis [11]. Thus, the mere presence of LV dyssynchrony at baseline may not translate into a benefit of CRT, but rather a constellation of conditions should be met to make the dyssynchrony remediable.In this issue of the European Journal of Nuclear Medicine and Molecular Imaging, Hung and colleagues report their exploration of an interesting and clinically rel...