The electrocardiogram is an essential tool in the follow-up of resynchronized patients in order to suspect a displacement leading to a loss of left ventricular capture even though the definitive diagnosis is made through interrogation of the device and chest x-ray. In order to facilitate follow-up by the cardiologist, it is useful to have a reference electrocardiogram from the patient in spontaneous rhythm, during right ventricular capture only, during left ventricular capture only and during biventricular pacing. This facilitates diagnosing loss of capture. Despite a potentially significant remodeling in terms of ventricular volume, the pattern in each configuration shows little change over time in terms of QRS width and pattern.
In order to potentially diagnose a loss of capture on the electrocardiogram, certain elements need to be known. The biventricular pacing pattern is dependent on the fusion of two different activation wave fronts originating from the respective right and left ventricular pacing leads. The biventricular pacing pattern varies between patients according to the position of the two pacing leads, the amount of myocardium depolarized by the right and left pacing leads respectively, the electrical and anatomical characteristics of the patients and the degree of fusion with spontaneous activation.
The most common and most important clinical condition to be diagnosed is the loss of left ventricular capture. QRS duration is generally shortened during a biventricular capture compared to a right or left single-ventricular pacing, although this criterion is not sufficiently discriminatory to demonstrate a loss of capture. Diagnosis is easier when the right ventricular pacing lead is located at the apex. Indeed, an apical right ventricular pacing is associated in a majority of cases with a positive QRS in lead I and a negative QRS in the inferior leads. The axis is usually deviated to the left and the QRS is almost never positive in V1. A negative QRS pattern in lead I and/or positive pattern in V1 must be sought. Indeed, the presence of one of these two elements (negativity in lead I and/or positivity in V1) guarantees the presence of an effective left ventricular capture. A compatible pattern with a right single-ventricular pacing does not eliminate the presence of an effective left ventricular capture but may be related to a significant predominance of the right pacing lead on the QRS pattern. QRS duration is therefore often shortened in biventricular pacing compared to the right single-ventricular pattern. When the right ventricular pacing lead is located in the upper septum or infundibulum, the diagnosis of loss of left ventricular capture is much more difficult since both V1 and lead I patterns are no longer typical.
The loss of right ventricular capture is a rarer occurrence although less hemodynamically deleterious. The QRS in left ventricular pacing is often very wide. The QRS axis is right or extreme-right. Its morphology is dependent on the position of the left ventricular pacing lead. The more lateral the pacing lead, the more negative the QRS complex in lead I with a right bundle branch block pattern. The more apical the pacing lead, the more negative the pattern in the inferior leads.