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A 65 year old woman with hypertension and permanent atrial fibrillation had suffered two brief syncopal episodes. She was on losartan, digoxin, and aspirin. The ECG showed atrial fibrillation and incomplete left bundle branch block; the QT interval was 0.36 s and the QTc 0.414 s. The echocardiogram revealed mild dilatation and dysfunction of the left ventricle (ejection fraction = 40%). Laboratory tests were normal; serum potassium was 4.1 mEq, magnesium 2.2 mEq, and digoxin 0.95 ng/ml.
The Holter recording (right) revealed an episode of ventricular fibrillation (VF) occurring during sleep. Surprisingly, VF ceased after 1.16 mins; an asystolic pause of 16.08 s then ensued, followed by resumption of a supraventricular rhythm.
Ventricular fibrillation seldom terminates spontaneously, since several re-entrant wavefronts, independent from each other, coexist, and the simultaneous extinction of all the circuits is unlikely. In the present case, fibrillation waves are initially tall and regular, and later become irregular and reduced in voltage (third strip). From the second half of the fourth strip, however, the tracing shows again regular and high voltage waves. Although the term “ventricular fibrillation” is appropriate to describe this rhythm disorder, fragmentation of the electrical activity, as deduced from the ECG, is not very pronounced, to the point that in the section preceding termination the pattern resembles a fast ventricular tachycardia rather than a true VF. This may help to explain the unexpected spontaneous interruption of the arrhythmia.