There is a sinus rhythm with a frequency of approximately 75 beats/min. The PQ interval is normal and the majority of QRS complexes show a left bundle branch block (LBBB). The 3rd, 6th, and 11th complexes are narrow and look quite normal (with the exception of the absence of an initial ‘R’ in the right precordial leads). These complexes are all preceded by a P-wave that is buried in the preceding T-wave (i.e. atrial extrasystoles). Figure 2 shows a ladder diagram with the potential explanation of the narrow QRS complexes. The first two complexes are sinus beats with an LBBB, the 3rd beat (6th and 11th) is (are) preceded by an atrial extrasystole and has (have) a slightly longer PR interval enabling conduction through the right bundle as well. In this case both bundle branches conduct slowly, but the final result is that they conduct and the resulting QRS complex is narrow. This phenomenon is referred to as ipsilateral conduction slowing. The 9th complex is also the result of an atrial extrasystole but this one comes earlier and finds the right bundle still refractory, resulting in a broad (LBBB) complex.
Fig. 2
Schematic drawing of a possible explanation for the occurrence of narrow QRS complexes
