Rick Schreurs

29 Exploring CRT * Infarcted region 120ms 40ms 60ms 80ms 100ms 20ms Pacing location CRT Intrinsic conduction LBBB LBBB + LADi LBBB + CXi * * * * Figure 4. Three-dimensional reconstruction of electrical activation times of the LV and RV during intrinsic conduction (LBBB, top) and CRT using midlateral LV wall pacing (bottom) in representative hearts with LBBB (left), and LBBB combined with an infarction of the LAD (LBBB+LADi, middle) or left circumflex artery (LBBB+LCXi, right). Reproduced with permission from the American Heart Association [34]. While multisite pacing would often require leads in multiple veins, the recently introduced quadripolar leads enable multipoint pacing at two LV sites in one cardiac vein. The first studies on multipoint pacing have shown a small but significant increase in dP/dt max and decrease in QRS duration compared to BiV pacing [39, 40]. Although these results are promising, all studies so far relate to individual optimization using invasive hemodynamic measurements and no long-term benefits and clinical implications have yet been reported. Endocardial pacing Previous paragraphs mostly discussed (multiple) LV epicardial pacing sites, because the conventional lead position in the coronary vein is epicardial. However, both animal and patient studies indicate that LV endocardial pacing is more promising ( Figure 5 ) [40, 41]. Several other studies corroborated these findings [42, 43]. The better resynchronization with endocardial CRT can be explained by three factors: 1) a shorter path length for the depolarization wave to reach all regions of the ventricles, 2) more rapid impulse conduction in the endocardium than in the epicardium, and 3) a more rapid transmural conduction from endocardium to epicardium than in the opposite direction [41, 44]. Unfortunately, practical implementation of endocardial CRT is still problematic, because currently leads placed in the LV cavity require anticoagulation and also show significant dislodgement [45]. 2