Group beating in a 69-year-old man with a previous silent myocardial infarct.

I n the electrocardiogram of this 69-year-old man (Figure), the rhythm strip of lead II best demonstrates the cardiac rhythm. Th ere is sinus rhythm at a rate of 65 beats/min. Th e 1st, 5th, 9th, and 13th QRSs are capture complexes. Capture complexes are separated from one another by three wide (0.13s) QRSs that occur regularly at a rate of 86 beats/min; this is an accelerated (rate 60–110 beats/min) idioventricular rhythm (1). Th e fi rst P wave in this tracing does not fi nd the atrioventricular junction refractory and captures the ventricles; the second P falls in the fi rst wide QRS; the third P falls in the T wave of the second wide QRS; and then the cycle repeats itself. Th e second and third P waves fi nd the atrioventricular junction refractory and are not conducted to the ventricles. Th us, an accelerated idioventricular rhythm, by being faster than the sinus rhythm, intermittently usurps the ventricular pacemaker role, and the atria and ventricles are temporarily dissociated (incomplete atrioventricular dissociation) (2). Because both the sinus rhythm and the accelerated idioventricular rhythm are regular and because the capture beats reset the idioventricular rhythm, the group beating is not happenstance (2). Group beating may be defi ned as identical repetitive groups of complexes, here three idioventricular complexes, separated by identical pauses or From the Sections of Cardiology, Departments of Medicine, Louisiana State University Health Sciences Center and the Interim Louisiana State University Public Hospital, New Orleans.