Heart rate, regularity, and synchronicity in heart failure: a tale of three brothers.

The electrical consequences of heart failure and their impact on disease progression have frequently been underestimated. It is long known that a significant per cent of heart failure patients develop disease of the conduction system and atrial fibrillation (AF). Conduction disturbances are not only a marker of disease progression but may lead to further haemodynamic deterioration, thus representing another vicious circle in the heart failure syndrome. It has also been recognized that AF may impair haemodynamics by the loss of atrial contribution to stroke volume, by reducing filling time especially if the ventricular response to AF is uncontrolled and by rate irregularity itself. In fact, AF with rapid ventricular conduction may ultimately lead to left ventricular (LV) dysfunction, i.e. to a ‘tachycardiomyopathy’, which can be reversed by controlling the ventricular rate, e.g. by atrioventricular (AV) node ablation with consecutive pacemaker implantation. The elegant study from Melenovsky et al. shows that there are two synergistic effects of this procedure in heart failure patients: rate control and rate regularization. However, the positive effect of rate regularization is only operative at high heart rates. This may be another explanation for the negative results of studies comparing rate with rhythm control and may prove the common belief untrue that heart failure patients benefit more from sinus restoration compared with less sick patients. In fact, there are many good clinical reasons to leave a heart failure patient in AF: the recurrence rate after cardioversion is high, the drugs needed to stabilize sinus rhythm may be pro-arrhythmic especially in heart failure, and there is apparently no clear haemodynamic benefit of re-establishing sinus rhythm as long as the ventricular rate during AF is well controlled. Melenovsky et al. used biventricular (BiV) pacing in order to reduce the influence of pacing-inducing ventricular dyssynchrony on haemodynamics. However, right ventricular (RV) apical pacing is usually used after AV node ablation. In the era of cardiac resynchronization therapy (CRT), we need to take a new look at this approach. With chronic RV pacing, the ventricular activation sequence resembles left bundle branch block, i.e. the RV is activated before the left ventricle (interventricular dyssynchrony) and the LV septum before the LV free wall (intraventricular dyssynchrony). Dyssynchronous ventricular activation leads to local differences in myocardial loading conditions which has detrimental effects on LV function: experimental and clinical evidence shows a reduction in stroke volume, an inhomogeneous distribution of perfusion and metabolism, and regional differences in protein expression. Therefore, patients with heart failure undergoing AV node ablation may pay a price for the procedure. This danger is real as data from several trials in patients with preserved sinus rhythm indicate that chronic RV pacing increases the likelihood of hospitalization for heart failure or death. However, it is not clear whether BiV pacing significantly improves patients after AV node ablation for chronic AF. This is highlighted by the OPSITE study presented by Brignole et al.. In their study there was no significant improvement with LV pacing compared with RV pacing and only modest benefit with BiV pacing after AV node ablation for drugrefractory AF. This result is somewhat disappointing but in agreement with previous data from the MUSTIC trial