Author's reply

We very much appreciate the interest in our studies1,2 performed on haemodynamic effects of vector selection of quadripolar electrodes in cardiac resynchronization therapy (CRT). Currently,3 vector selection in clinical practice is mainly based on technical issues, i.e. optimization of pacing thresholds and avoidance of phrenic nerve capture. Solely based on this, with more options to effectively deliver biventricularpacing,4 incremental benefits of CRT may be expected. The variety of available left ventricular pacing configurations (LVPCs) however also prompts the search for the optimal LVPC from a haemodynamic viewpoint. After we could show that selection of the LVPC matters in this regard,2 we tried to find a clinically feasible way to guide vector selection with the use of the surface ECG. The results of this study1 have been put up for discussion by van Everdingen et al. Anodal capture is an interesting phenomenon, well-known and unwanted in CRT when using parts of the right ventricular electrode as the anode, and certainly incompletely studied with quadripolar LV electrodes. As van Everdingen et al. mentioned, this will affect both QRS width and acute haemodynamic response (AHR) (in an analogous manner), therefore we would not expect a large effect when comparing these two, as performed in our study. It however cannot be dismissed that it might be a misconception that the labelled cathode always denotes the place where the stimulated wavefront originates. This deserves further investigation, and it might hamper device-based algorithms which intend to guide vector selection (as discussed below). We apologize for the typographical error in Figure 1A, correct QRS width for vector D1M2 is 136 ms. Acute haemodynamic response is not solely affected by the choice of the LVPC, but also by time delays, especially the AV and VV delays. As testing all possible combinations of these is impossible due to the multitude of measurements, certain limitations need to be accepted in any stimulation protocol and are always prone to criticism. A general pre-excitation of proximal LVPCs, as suggested, however might disregard differences in conduction velocities depending on the underlying heart disease (another major ‘confounder’) and therefore would also serve as an approximation only. We however agree that further individual optimization can be achieved when combining the choice of VV delays with LVPC selection. The Q-LV interval5,6 may predict AHR in CRT recipients, however, is not validated for quadripolar electrodes. Q-LV intervals, as partly measured by the surface ECG, and interventricular delays, exclusively measured by intracardiac electrograms, in our opinion are not equivalent. Caution should be applied when choosing LVPCs based on these measurements, even if already integrated in device-based algorithms. Further validation of these algorithms is necessary. The problem of anodal capture (as discussed above) also would not be detected by these measurements. Optimization of vector selection remains challenging. Open questions need to be addressed in further studies and we are glad that we might have sparked interest in this issue.