A study of the pacemaker-induced ventricular complexes can be of help in determining the site of stimulation. The first step consists in separating the stimulus artefacts from the QRS complexes. Right ventricular pacing usually produces a complete left bundle branch block (CLBBB) morphology. The electrical axis can be left, normal or even right, as pacing is performed from the apex to the outflow tract. Rarely an S1-S2-S3 pattern (with negative deflection in V1 and V2) appears during right ventricular apical pacing. Left ventricular stimulation usually produces a right bundle branch block pattern with inferior and rightwards deviation of the AQRS, although a superior axis is rarely seen with inferior stimulation. In the absence of pacer failure the basic QRS morphology produced by late diastolic stimuli can change if: (a) there are significant respiratory movements; (b) stimulation occurs during the relative refractory period; and (c) fusion beats occur. The classical Wolff-Parkinson-White beat was interpreted as a fusion beat, as long as exclusive Kent bundle conduction was not present. In the latter instances pacing from the preexcited area appeared to have produced similar QRS complexes.
