Rick Schreurs

135 General discussion the left sided AV-delay can be derived from the IAD and sensing of the LV wall, although this requires an LV lead and only gives information about activation of the posterolateral wall. Additional prospective studies are required to prove the benefits of these electrical markers and whether they support the importance of the mean effective AV-delay. Unraveling the SonR signal Mechanical signals of the heart can also be captured by intracardiac accelerometers and are currently used for ambulatory optimization of CRT. The SonR system uses the SonR1 signal, which corresponds to the first heart sound. Heart sounds are acoustic vibrations generated by mechanical interactions of the cardiohemic structures, like valves, myocardium and blood mass. In the 1960s Rushmer suggested that abrupt cessation of backward flow by each valve closure caused vibrations of the blood and surrounding tissue, including valve leaflets and atrial, ventricular and arterial walls [35]. In case of the first heart sound, the vibrations are then caused by closure of the mitral and tricuspid valves. The SonR1 amplitude increases during improved ventricular contraction, as shown in chapter 5 in a canine heart failure model, and previously in pigs and sheep [36-38]. In CRT, however, SonR1 behaves differently because changes in AV- and VV-delay create variations in diastolic filling and IVD. Like previously reported, the SonR1 amplitude increases at short AV-delays when atrial contraction occurs against closed or quickly closing atrioventricular valves [39, 40]. These data reveal that SonR1 amplitude does not only depend on the amount of contractility but also the interval between atrial and ventricular contraction, supporting Rushmer’s theory. The SonR algorithm is complex and defines the optimal AV-delay as the shortest AV- delay with the lowest SonR1 amplitude; the so-called infliction point. The optimal AV- delay as defined using SonR strongly correlates to the optimal AV-delay as derived using echocardiographic measurement, like longest diastolic filling time and aortic velocity time integral [41-43]. In the clinical setting the algorithm is non-inferior in terms of responder rate compared to echocardiography-based AV-optimization and decreases heart failure hospitalization by 35% [32, 34]. Our goal was to better understand the SonR algorithm. Chapter 5 shows that we replicated the sigmoid curve of the SonR1 amplitude at various AV-delays in a canine heart failure model. The AV-delay at which the infliction point of the AV-delay – SonR1 curve occurs was hard to determine due to measurement variability and due to the fact that this point is at the shallow part of the curve. However, the highest values of maximal LV pressure and LV dP/dt max occurred at AV-delays just longer that the one where SonR1 significantly increased (comparable to the infliction point), supporting the use of the SonR1 algorithm for AV-optimization. Additionally, chapter 5 reveals that the SonR1 signal does not seems to be influenced by IVD, as the amplitudes during BiV pacing are comparable as those during LV pacing with greater amount of IVD, suggesting that it should not be used for VV-optimization. 7