The interval from the pacemaker stimulus to the onset of the earliest paced QRS complex (latency) may be prolonged during left ventricular (LV) pacing. Marked latency is more common with LV than right ventricular (RV) pacing because of indirect stimulation through a coronary vein and higher incidence of LV pathology including scars. During simultaneous biventricular (BiV) pacing a prolonged latency interval may give rise to an ECG dominated by the pattern of RV pacing with a left bundle branch block configuration and commonly a QS complex in lead V1. With marked latency programming the V-V interval (LV before RV) often restore the dominant R wave in lead V1 representing the visible contribution of the LV to overall myocardial depolarization.
When faced with a negative QRS complex in lead V1 during simultaneous BiV pacing especially in setting of a relatively short PR interval, the most likely diagnosis is ventricular fusion with the intrinsic rhythm. Fusion may cause misinterpretation of the ECG because narrowing of the paced QRS complex simulates appropriate BiV capture. The diagnosis of fusion depends on temporary reprogramming a very short atrio-ventricular delay or an asynchronous BiV pacing mode.
Sequential programming of various interventricular (V-V) delays may bring out a diagnostic dominant QRS complex in lead V1 that was previously negative with simultaneous LV and RV apical pacing even in the absence of an obvious latency problem. The emergence of a dominant R wave by V-V programming strongly indicates that the LV lead captures the LV from the posterior or the posterolateral coronary vein and therefore rules out pacing from the middle or anterior coronary vein.
In some cardiac resynchronization systems LV pacing is achieved with the tip electrode of the LV lead as the cathode and the proximal electrode of the bipolar RV as the anode. This arrangement creates a common anode for both RV and LV pacing. RV anodal capture can occur at a high LV output during BiV pacing when it may cause slight ECG changes. During LV only pacing (RV channel turned off) RV anodal pacing may also occur in a more obvious form so that the ECG looks precisely like that during BiV pacing. RV anodal stimulation may complicate threshold testing and ECG interpretation and should not be misinterpreted as pacemaker malfunction. Programming the V-V interval (LV before RV) in the setting of RV anodal stimulation cancels the V-V timing to zero. (Cardiol J 2011; 18, 6: 610–624)