Rick Schreurs

63 Effective mechanical atrioventricular delay INTRODUCTION The atrioventricular delay (AV-delay) is an important determinant of hemodynamic function in dual-chamber pacemaker therapy and in cardiac resynchronization therapy (CRT). Several studies have suggested that optimizing the delay between the atrial and ventricular contraction increases cardiac output by improving ventricular preload [1]. The optimal AV- delay varies from one patient to another and may change between rest and exercise [2-5]. The AV-delay plays an important role in ventricular filling. Diastolic filling exists of passive (E-wave) and active (A-wave) filling, the latter caused by atrial contraction. An AV-delay that is programmed too short leads to A-wave truncation because ventricular activation starts already before atrial contraction has finished. On the other side of the spectrum, a too long AV-delay (as in first degree AV-block) leads to fusion of the E and A-wave, which may enhance diastolic mitral regurgitation (MR). In general the optimal AV-delay is described as the shortest AV-delay without A-wave truncation [6]. It is important to realize that the AV-delay as programmed in the pacemaker is not the actual delay between activation of both atria and both ventricles. Interatrial and interventricular conduction delays may create considerable differences between right atrial-right ventricle (RA-RV) and left atrial-left ventricle (LA-LV) delays. Another important notice is that virtually all measurements of cardiac function are based on measurements in the systemic circulation. However, computer simulations in our research group emphasize that the systemic and pulmonary circulation are coupled in series, so that function of the entire circulation depends on function of both ventricles [7]. In optimization studies and in clinical practice it is known that optimal AV-delay differs substantially between atrial-sense and atrial-pace dual-chamber pacing, because atrial pacing creates a larger interatrial delay (IAD) [8-10]. As a consequence, the actual LA-LV delay is shorter than the programmed AV-delay, because the latter is based on the timing of stimulation of the RA appendage which creates a prolonged IAD. At ventricular level the pacing site is another determinant of the actual AV-delay. RV pacing leads to pre-excitation of the RV and delayed activation of the LV, while LV lateral wall pacing creates a delay in RV activation. Biventricular (BiV) pacing activates both ventricles simultaneously but changing the interventricular delay (VV-delay) will lead to differences in actual AV-delays between the ventricles. Here we propose and evaluate an overarching concept of determining optimal AV-delay: the effective AV-delay (eAVD). To this purpose we used a porcine total AV-block model to examine the role of AV-sequential pacing during atrial pacing and atrial sensing with different ventricular pacing sites on the effective right and left AV-delay. The objectives are: 1) to explore the diastolic filling pattern during various pacing conditions, 2) to examine whether atrial sensing improves overall cardiac function compared to atrial pacing, 3) to 4