ECG Review
Why is the QRS Wide?
By Ken Grauer, MD, Professor Emeritus in Family Medicine, College of Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
Figure Right-sided MCL-1 monitoring lead rhythm strip. Why might beats #4 through #7 be wide?
Scenario: Interpret the rhythm strip shown above. The widened beats on the tracing are not ventricular. What else might they be?
Interpretation: The rhythm strip shows an irregularly irregular rhythm in this right-sided MCL-1 monitoring lead. No P waves are seen the underlying rhythm is atrial fibrillation. Fine undulations in the baseline represent "fib waves." The interesting part of the rhythm strip is intermittent widening of the QRS complex.
Although at first glance one might be tempted to interpret the run of widened beats (beats #4 through #7) as accelerated idioventricular rhythm (AIVR) subsequent rhythm strips proved this not to be the case. AIVR is often an "escape rhythm" that arises when the patient's underlying rhythm slows. AIVR is typically (although not always) a regular rhythm. In contrast to this, the widened beats in the Figure do not manifest the delayed timing of escape beats, nor is the run (beats #4 through #7) regular. An alternative explanation for the QRS widening seen in the Figure is rate-related bundle branch block (BBB). While admittedly difficult to be certain of this diagnosis from inspection of the single rhythm strip shown, the important point is to be aware of this entity. QRS morphology of the widened beats is consistent with the predominantly negative QS or rS complex expected in right-sided lead V1 when there is left bundle branch block (LBBB). Rather than two competing rhythms, the overall irregular irregularity of the rhythm in the Figure suggests that all beats seen represent atrial fibrillation. Rate-related BBB characteristically begins when heart rate speeds up. In atrial fibrillation, it is typically seen following a longer R-R interval, since the relative refractory period is dependent on the length of the preceding R-R interval. Beat #3 in the Figure is slightly widened. This beat follows a longer R-R interval (between beats 1-to-2), and represents the onset of LBBB conduction (albeit with an incomplete form of LBBB). The run of rate-related LBBB conduction continues until beat #8 when the rate of atrial fibrillation slows. Beat #11 at the end of the tracing represents a final widened beat that manifests LBBB conduction as a result of its short coupling interval with beat #10. Subsequent rhythm strips proved beyond doubt that LBBB conduction consistently occurred during periods of more rapid atrial fibrillation and consistently resolved soon after the rate slowed down. Of interest (and further complicating diagnostic recognition of this important but uncommon phenomenon) is the fact that the rate of onset of BBB conduction is often not the same as the rate where normal conduction resumes (i.e., rate-related BBB may begin when heart rate exceeds 90 or 100/minute but normal conduction may not resume until heart rate goes back down to 80/minute or less).
Right-sided MCL-1 monitoring lead rhythm strip. Why might beats #4 through #7 be wide?You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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