Thus far, the discussion of supraventricular arrhythmia with abnormal QRS complex has included preexisting conduction defects, the preexcitation syndrome (Wolff-Parkinson-White syndrome), and functional bundle-branch block due to sudden shortening of cycle length, often combined with prolongation of preceding cycle; the latter is popularly called the "Ashman phenomenon." We will now present examples of other causes of aberrancy for which the mechanistic explanations may not be as clearly delineated as for those entities previously presented.
Aberrancy Occurring at Slow Rates
The contour of QRS complexes which escape after long pauses may differ from the QRS complex inscribed during the normal dominant rhythm; yet, the duration of these escape systoles, which is less than 0.12 seconds, strongly, but not unequivocally, indicates an origin above the bifurcation of the bundle of His (Fig 1 and 2). Basically, four theories exist which attempt to explain the cause of such aberrantly conducted junctional escape beats.
Douglas P. Zipes, Charles Fisch. Supraventricular Arrhythmia With Abnormal QRS Complex. Arch Intern Med. 1972;130(3):426–429. doi:10.1001/archinte.1972.03650030098022