Bigeminal rhythm IV

The electrocardiogram in Figure 1 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.

Figure 1.

Twelve-lead electrocardiogram recorded soon after the patient's admission. See text for explication.

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 2). As Castellanos et al have pointed out, this may be a simplistic explanation for more complicated electrophysiological mechanisms (2).

Figure 2.

A portion of the lead V₁ (above) and lead II (below) rhythm strips from Figure 1 with a ladder diagram to show how two levels of second-degree block, with 2:1 block high in the atrioventricular junction and 3:2 type I block lower in the junction, explain the pattern of conduction. The nadir of each flutter wave is indicated by a vertical line in the atrial (A) portion of the diagram, and alternate flutter waves are blocked high in the atrioventricular (A-V) junction. Of those impulses making it through the high junction, one third are blocked low in the atrioventricular junction, and two thirds are conducted to the ventricles (V) in a bigeminal pattern. The QRS complexes following the short R-R intervals have aberrant ventricular conduction due to functional block in the right bundle branch.

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).

Table.

Some causes of bigeminal rhythm

1.	Alternate premature complexes: supraventricular with or without aberration, ventricular
2.	3:2 conduction
	a. 3:2 AV block (type I or II) during sinus rhythm
	b. 3:2 AV block (type I or II) during atrial tachycardia
	c. 3:2 type I AV response of an AV sequential pacemaker during sinus tachycardia or atrial tachyarrhythmia
	d. 3:2 SA block
	e. 3:2 exit block of atrial, junctional, or ventricular escape rhythm; 3:2 exit block of accelerated atrial, junctional, or ventricular rhythm
3.	Other causes
	a. Escape-capture bigeminy, i.e., sinus complexes follow junctional or ventricular escape complexes in cases of sinus bradycardia, SA block, or advanced AV block
	b. Atrial flutter with alternating 2:1 and 4:1 conduction block
	c. Nonconducted atrial premature complexes after every second QRS

AV indicates atrioventricular; SA, sinoatrial.