Rick Schreurs

28 Chapter 2 Pacing location The size of hemodynamic response to CRT depends on the location of the LV lead. Usually the best location is in the latest activated region of the LV free wall [28]. Intriguingly, most studies fail to indicate an anatomical location that is clearly the best. A study in LBBB dogs showed that pacing in a rather large area of the LV free wall (±43%) yielded an dP/ dt max larger than 70% of the maximal observed value [29]. This, coupled with the fact that anatomy will limit the number of available pacing sites, may explain the lack of a clear anatomical defined location for pacing. Moreover, LV activation patterns vary considerably between patients. Therefore it seems advisable to estimate the ideal LV pacing region in each individual patient [28]. To this purpose, the CardioInsight technique ( Figure 1 ), other electro-anatomical techniques [30], as well as echocardiographic speckle tracking [31] may be used to determine the latest activated regions. Currently, the site of latest activation is determined during intrinsic activation, but it is probably wiser to determine it during RV apex pacing, since during CRT the RV will be paced which may shift the site of latest activation ( Figure 1 ; compare intrinsic conduction and RV pacing) [32, 33]. Myocardial scarring Myocardial scarring plays a dual role in CRT response. First, the response to CRT is inversely related to the total amount of ventricular scar tissue [35]. Secondly, CRT response is poor, and potentially adverse if the LV lead is positioned in a region with extensive scar tissue [35, 36]. This may be related to the fact that scar tissue slows electrical conduction and may give rise to ventricular arrhythmias. However, a study in canines indicates that CRT should still be considered for patients with a myocardial scar [34]. In this study CRT reduces LV TAT and increases LV dP/dt max to similar values in LBBB dogs with and without infarction when pacing sites are selected outside of the infarcted region ( Figure 4 ). Multiple pacing sites Since BiV pacing leads to electrical resynchronization of the LV, it has been argued that increasing the number of LV pacing sites (multi-LV) will further improve resynchronization and cardiac function. The prospective TRIP-HF study shows that reverse remodelling is more pronounced after 3 months of triventricular pacing (2 LV leads) compared to conventional BiV pacing, with higher ejection fraction and lower end systolic volume in patients who did not respond to conventional BiV pacing [37]. Experiments using a strategy to maximally resynchronize the LV in the canine LBBB model show that additional pacing sites (up to seven) consistently decrease LV TAT, but only increase dP/dt max if the hemodynamic effect of pacing the initial single-site is small ( Figure 5A ) [38]. Similar findings were reported later for CRT patients ( Figure 5B ).