ECG Review

How Wide Is the QRS?

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 lead II rhythm strip shown above was obtained from an elderly patient with shortness of breath. How wide is the QRS complex?


The answer as to how wide the QRS complex is in the above lead II rhythm strip is: "It depends." The underlying rhythm is rapid and irregularly irregular. No P waves are seen. Thus, the underlying rhythm is atrial fibrillation with a rapid ventricular response. Of interest is the constantly changing QRS duration and morphology throughout the rhythm strip. The QRS complex is at its most narrow immediately following the two relative pauses that occur in the rhythm strip. The first of these follows the fifth beat in the tracing. The second relative pause occurs a bit after the middle of the tracing. In both cases, QRS narrowing occurs after slight slowing of the rate.

This suggests that the tracing represents rapid atrial fibrillation with a rate-related bundle branch block conduction disturbance. The "width" of the QRS is dependent on the rate of the atrial fibrillation at the time in question. With rate-related conduction disturbances, bundle branch block tends to develop at a certain heart rate due to corresponding reduction in recovery time of the affected part of the conduction system. Interestingly, the rate of onset of the rate-related conduction disturbance often differs from the rate of offset. For example, QRS widening may develop at a heart rate of 120/min, but may not disappear until heart rate has dropped to a rate that is significantly below this (say to 100/min). Because the rate of onset and offset in any particular patient may vary, definitive diagnosis of the rhythm disturbance may at times be difficult.

In the above case, serial additional tracings on this patient confirmed that the underlying rhythm was truly atrial fibrillation, and that QRS widening was not reflective of ventricular ectopy, but rather of intermittent rate-related bundle branch block. Clues in support of this diagnosis are the irregular irregularity throughout the tracing with QRS narrowing immediately after the two relative pauses, and greatest QRS widening at times when the rate is at its most rapid.