AV delay optimization
The various manufacturers offer in their defibrillator or CRT pacemaker platforms, a specific algorithm dedicated to the automatic optimization of the AV and VV delays. The AdaptivCRT algorithm proposed by MedtronicTM is available with 3 different programming modes: 1) Adaptive Bi-V: automatic optimization of AV delay and VV delay with biventricular pacing; 2) Adaptive Bi-V and LV: the device first dichotomizes patients between those with preserved atrioventricular conduction and those with impaired conduction (long PR or complete atrioventricular block); in the first instance, the patient is paced only in the left ventricle with the search for an optimal fusion between LV pacing and spontaneous activation; in the second instance, optimization of AV delay and VV delay with biventricular pacing; 3) Non adaptive CRT: no automatic adjustment by the device.
The Adaptive CRT algorithm is based on regular measurements of (1) the atrioventricular conduction time measured by the leads corresponding to the delay between the EGM sensed by the right atrial lead and the EGM sensed by the right ventricular lead, 2) the width of the P waves corresponding to the delay between the atrial EGM sensed by the atrial bipolar channel and the end of the atrial EGM measured on the defibrillator far-field channel and 3) the width of the QRS complexes corresponding to the delay between the right ventricular EGM sensed by the RV ventricular bipolar channel and the end of the ventricular EGM measured on the far-field channel.
This algorithm never leads to the programming of extreme values (very short AV delay or very long AV delay). The sensed AV delay range for the AdaptivCRT function is between 80 ms and 140 ms (never less than 80 ms or more than 140 ms). The paced AV delay range for the AdaptivCRT function is between 100 ms and 180 ms. The corresponding V-V pacing delay range for the AdaptivCRT function is between 0 ms and 40 ms (left or right pre-excitation).
The first step of optimization is an assessment of atrioventricular conduction. In this patient, the delay between spontaneous atrial EGM and spontaneous right ventricular EGM is greater than 200 ms, which corresponds to a conduction considered abnormal by the device. The patient is thus paced biventricularly, whether the programming is set to AdaptivCRT Auto Bi-V and LV or AdaptivCRT Auto Bi-V. The rationale for this choice is that a lone left ventricular pacing is probably more precarious in a patient with atrioventricular conduction disorder. It is preferable to select a biventricular pacing in this setting, the right ventricular pacing allowing effective pacing if the left lead is dislodged or displays an increase in threshold. The periodic sensing required for the various measurements may decrease the total percentage of biventricular pacing by 1 to 2%.
The functioning and philosophy of the algorithm subsequently differs completely according to this assessment. If the conduction is considered “normal”, the objective of the algorithm is to seek a fusion between spontaneous right ventricular activation and left ventricular pacing. This algorithm is therefore specifically adapted for patients with left bundle branch block that have normal right ventricular activation which should theoretically be preserved, and an asynchronous and delayed left ventricular activation which should conversely be changed and reversed. In these patients, the potential benefit of this algorithm is twofold: 1) reduction in energy consumption and prolongation of the lifetime of the device; indeed, the right ventricle is not paced, which allows reducing battery wear; 2) long-term hemodynamic and/or clinical benefit; an ongoing study on a significant number of patients with left bundle branch block will allow assessing the clinical impact of this type of algorithm and its putative effect on the percentage of nonresponse to resynchronization.
In order for a patient to be paced in pure LV mode, the following elements must be met: 1) the patient’s heart rate must be less than or equal to 100 beats per minute; 2) the conduction delay between spontaneous atrial EGM and spontaneous right ventricular EGM must be less than or equal to 200 ms; 3) the conduction delay between paced atrial EGM and spontaneous right ventricular EGM must be less than or equal to 250 ms. If one of these criteria is not found, the patient is paced biventricularly.