Bigeminal rhythm II

D. Luke Glancy, MD, and Mazen M. Kawji, MD

The simultaneously recorded lead II and lead V1 electrocardiographic rhythm strips show a narrow QRS followed by a wide QRS and then by a pause. The cycle then recurs repeatedly. Superficially, this sequence suggests sinus rhythm with ventricular premature complexes occurring in a bigeminal pattern. Closer observation, however, reveals P waves in front of the wide QRSs as well as in front of the narrow ones. This raises the possibility of atrial bigeminy with the atrial premature complexes being conducted to the ventricles with aberration of the left-bundle-branch-block type. Even closer inspection shows that each cycle contains a third P wave, which is best seen as a negative deflection at the end of each second T wave in lead V1. In lead II this third P wave appears as minimal widening, occasionally with a slight notch on the downslope, of the second T wave. Furthermore, the P-P intervals are regular. Thus, the rhythm is sinus tachycardia (rate = 118 beats/min), and second-degree atrioventricular block is present with a conduction ratio of 3:2. Two questions remain. First, is this type I or type II atrioventricular block? Second, why is the first QRS of each cycle narrow while the second QRS is wide?