Answer

The premature depolarisation following the second QRS complex occurs late in the cardiac cycle (CL = 1000 ms) with a QRS duration of 120 ms followed by a complete compensatory pause (Fig. 1 in the Question article, doi: 10.1007/s12471-015-0655-z). The QRS complex is not preceded by a P wave, excluding an atrial ectopic origin. The amplitude of the R wave is large in leads V2–V4, suggesting an anterior axis deviation of the QRS complex in the precordial leads, which makes an AV nodal origin unlikely in the presence of a left anterior fascicular block during sinus rhythm. Since the QRS width is borderline at around 120 ms, a premature ventricular depolarisation cannot be excluded as origin of the ectopic focus. But what is the most likely explanation for this depolarisation?

Figure 1 shows the proposed mechanism of activation from a spot in the specific His-Purkinje system where the time of slow anterograde conduction over a diseased left anterior fascicle is equal to the normal conduction time of the impulse, travelling over a longer distance through the posterior fascicle and right bundle. This results in an almost synchronous activation of both right and left ventricle. Already in 1973 [1], electrophysiological investigations were performed to prove this paradoxical phenomenon of premature beats in patients with conduction disease in the left bundle branch (Fig. 1).

Fig. 1
figure 1

Mechanism of QRS complex by a fascicular ectopic depolarization. The speed of conduction of the impulse is delayed anterogradely over a diseased LAF. A normal speed of conduction occurs through the LPF and RB over a longer distance towards the ventricular myocardium. When both conduction times are equal, the result is a nearly synchronous activation of right and left ventricle. (H common His bundle, LAF left anterior fascicle, LPF left posterior fascicle, RB right bundle)

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The second part of the question was why does the QRS complex following the first sinus beat in V1–V6 have a different QRS morphology than the other QRS complexes during sinus rhythm? Most likely this beat is a fusion beat, since the QRS complexes in leads V4–V6 are nearly identical with the ectopic focus from the diseased left anterior fascicle.