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The electrocardiogram in Figure Figure11 shows a bigeminal rhythm with each early complex having a wide QRS. Although the wide QRS superficially suggests that this is ventricular bigeminy, the perfect right-bundle-branch-block pattern (RS in lead I, qRS in lead V6, rsR′ in lead V1) suggests that the early complex is an aberrantly conducted supraventricular complex.
The atrial rhythm is typical counterclockwise atrial flutter with negative P waves in leads II, III, and aVF and positive P waves in lead V1. There are 6 flutter waves for every 2 QRSs. Atrial flutter only infrequently is conducted to the ventricles in a straightforward 3:1 pattern, but often appears to be conducted in a 2:1, 4:1 pattern. This produces a bigeminal rhythm, and here the short R-R interval is followed by a QRS complex with aberrant ventricular conduction of the right-bundle-branch-block type. The right bundle branch normally has the longest refractory period of any part of the conduction system (1).
Why would atrial flutter waves be conducted 2:1, 4:1 rather than 3:1? One explanation is that there is block at 2 levels in the atrioventricular junction with 2:1 block above and 3:2 type I block below (Figure (Figure22). As Castellanos et al have pointed out, this may be a simplistic explanation for more complicated electrophysiological mechanisms (2).
Atrial flutter waves frequently distort the electrocardiographic baseline and in so doing confuse the computer, which read this electrocardiogram as an acute inferior myocardial infarct. The patient is a 49-year-old man with systemic arterial hypertension, frequent binge alcohol abuse, including the night before admission, and failure to take his medications for 1 month, but no myocardial infarct.
Although ventricular premature complexes are the most common cause of a bigeminal rhythm, there are many other mechanisms, and frequent among these is 3:2 atrioventricular block of sinus-initiated impulses or of supraventricular tachycardias (Table) (3).