ECG Review

What Is the Frequency?

By Ken Grauer, MD, Professor, Department of Community Health and Family Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.


The ECG shown above was obtained from a 66-year-old woman with chest discomfort. What is going on in this tracing?


The difficulty interpreting this 12-lead tracing results from the lack of a lead II rhythm strip. There are a total of 13 beats on this 3-channel, simultaneously recorded 12-lead tracing. QRS morphology of the normal, sinus-conducted complexes changes every 3-4 beats with each lead change. This makes it challenging to distinguish the new QRS morphology of sinus beats resulting with each lead change from the frequently occurring premature ventricular contractions (PVCs) that are seen on this tracing. Looking along the lower row of beats, the first complex in lead III is a PVC. The 5th, 9th, and 13th beats are also PVCs. Each of these beats occurs early, is obviously widened, and not preceded by a premature P wave. In contrast, even though QRS morphology of the 1st complex in leads V3 and V6 differs markedly from QRS morphology just preceding the lead change, we know that the 1st complex in leads V3 and V6 is a sinus beat, because the QRS is of normal duration, it is on time (rather than early), and the QRS is preceded by a sinus P wave. The rhythm is therefore ventricular quadrigeminy (sinus rhythm with every 4th beat a PVC). Assessing Q-R-S-T morphology of the sinus-conducted beats suggests that this ECG is otherwise normal.