Programmed versus effective VV delay during CRT optimization: when what you see is not what you get.

Background: In cardiac resynchronization therapy (CRT) devices, the interventricular (VV) delay denotes the time interval between left (LV) and right ventricular (RV) pacing. This study aimed to determine the proportion of patients in whom the effective VV delay (VVeff , delay between LV and RV depolarization, being induced either by pacing or intrinsic conduction) is different from the programmed VV delay during a standard VV delay optimization procedure.

Methods: Thirty-three patients with heart failure and left bundle branch block configuration without total atrioventricular (AV) block receiving CRT were prospectively included. VVeff was calculated from intrinsic AV intervals, programmed optimal AV delay, and programming system. Intrinsic AV intervals were measured on intracardiac electrograms. The optimal AV and VV delays were determined by highest increase in maximum rate of LV pressure rise (dP/dtmax ). VV delays of 20-80 ms LV and RV preactivation were tested.

Results: Calculated maximum possible VVeff was shorter than 80 ms LV preactivation in up to 46% of patients and shorter than 40 ms LV preactivation in up to 3% of the patients. These proportions were 6% and 0% during 80 and 40 ms RV preactivation, respectively.

Conclusions: In CRT patients with left bundle branch block without total AV block, the effective VV delay is shorter than the programmed VV delay during a standard optimization procedure in approximately half of the patients and this phenomenon is encountered predominantly during LV preactivation by 40 ms or more. Calculation of the individual maximum VVeff in advance can shorten the VV delay optimization procedure.