Baylor University Medical Center Proceedings January 2014, Volume 27, Number 1 | Page 41

Electrocardiographic Report Group beating in a 69-year-old man with a previous silent myocardial infarct D. Luke Glancy, MD, and Viral N. Lathia, MD Figure. Electrocardiographic rhythm strips in a 69-year-old man taking amiodarone. See text for explication. n the electrocardiogram of this 69-year-old man (Figure), the rhythm strip of lead II best demonstrates the cardiac rhythm. There is sinus rhythm at a rate of 65 beats/min. The 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). The first P wave in this tracing does not find the atrioventricular junction refractory and captures the ventricles; the second P falls in the first wide QRS; the third P falls in the T wave of the second wide QRS; and then the cycle repeats itself. The second and third P waves find the atrioventricular junction refractory and are not conducted to the ventricles. Thus, 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 defined as identical repetitive groups of complexes, here three idioventricular complexes, separated by identical pauses or I Proc (Bayl Univ Med Cent) 2014;27(1):39 different complexes, here single capture complexes. There are many other causes of group beating, with repetitive premature complexes and second-degree atrioventricular block leading the list. The patient’s underlying disease is coronary arterial disease with an inferior myocardial infarct of indeterminate age, indicated by the pathological Q waves in the sinus-initiated complexes in leads II and III. The infarct was silent. The patient’s physician subsequently placed him on amiodarone because of ventricular ectopic beats. Amiodarone obviously has not prevented the ectopy, but has kept the rate slow when an idioventricular rhythm occurred. 1. 2. O’Keefe JH Jr, Hammill SC, Freed M, Pogwizd SM. The Complete Guide to ECGs. A Comprehensive Study Guide to Improve ECG Interpretation Skills, 2nd ed. Royal Oak, MI: Physicians’ Press, 2002:491. Surawicz B, Knilans TK. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 5th ed. Philadelphia: WB Saunders, 2001:375. From the Sections of Cardiology, Departments of Medicine, Louisiana State University Health Sciences Center and the Interim Louisiana State University Public Hospital, New Orleans. Corresponding author: D. Luke Glancy, MD, 7300 Lakeshore Drive, #30, New Orleans, LA 70124 (e-mail: [email protected]). 39