Atrioventricular dromotropathy: an important substrate for complete resynchronization therapy

We read with great interest the article by Salden et al.1 that explored the effect of pacing therapy on atrioventricular (AV) dromotropathy. This work demonstrates haemodynamic improvement in cardiac output after restored AV coupling in a crystal clear fashion. The improved left ventricular stroke volume is related to an increased ventricular filling and reduced late diastolic mitral regurgitation. These results and data from previous clinical trials2,3 imply that biventricular (BiV) pacing might benefit patients with heart failure (HF) and prolonged PR interval. However, in our clinical experience, the long PR interval is not always associated with left-sided AV uncoupling. No data are provided on mechanical AV delay or uncoupling among the patient population in the present study. As only 19 out of 22 patients showed acute improvement of cardiac function after BiV pacing, it would be interesting to know whether the absence of mechanical AV delay was the reason for the lack of benefit in all patients. Could we speculate that identifying mechanical AV delay is of greater importance than the sole assessment of electrical AV delay derived from PR interval? The prolonged PR interval is associated with an increased risk of developing atrial fibrillation and HF.4 Mechanical AV delay causes temporal fusion of left atrial (LA) conduit and booster pump phases, prolongation of reservoir phase, and additional LA volume overload due to diastolic mitral regurgitation. In the present study, mean LA pressures did not significantly differ between prolonged and normal AV delay groups in a porcine model. However, do the authors have any data on the HF patient population's LA volumes and pressures? Namely, the LA unloading after restored AV coupling could be the mechanism behind the long-term beneficial effects of BiV pacing in this patient population. Finally, the present study showed that marked ventricular dyssynchrony caused by right ventricular pacing hampers the haemodynamic benefits of restoring AV coupling. While BiV pacing increased cardiac pump function in this study, it still relies on two non-physiological wavefronts, thus inducing some degree of ventricular dyssynchrony. In fact, BiV pacing is harmful in patients with narrow QRS.5 Since the mean baseline QRS duration in the present study was 128 ± 25 ms, it remains to be answered whether the beneficial effects of restoring AV coupling overcome the detrimental effects of inducing ventricular dyssynchrony in the long term. On this topic, what do the authors think of the interest of conduction system pacing? Conflict of interest: none declared.