The correct diagnosis is a 2:1 left bundle branch block (Figure 2).
The rhythm is regular at 100 beats/min when measuring intervals from the beginning of the QRS complexes and not R to R. A P wave precedes each of the QRS complexes (+) with a stable PR interval (0.20 sec) consistent with a first-degree atrioventricular (AV) block because the PR interval is longer than expected for the sinus tachycardia at a rate of 100 bpm.
Every other QRS complex has a different morphology. One is narrow (0.10 sec) with a normal morphology and axis. The next QRS complex (^) is wide (0.16 sec) and has a morphology typical of a left bundle branch block.
There is a QRS morphology in lead V1 (→) and a broad R wave in leads I, V5, and V6 (←), reflecting all forces moving from right to left. Because every other QRS complex has a left bundle branch block morphology, this is termed 2:1 left bundle branch block. It may be intermittent or rate related. The QRS complex in leads V4-V6 is early (*) and is preceded by an early P wave (v) that has a morphology different from the sinus P wave. Hence, this is a premature atrial complex. This QRS complex also has a left bundle branch block morphology, suggesting that the left bundle branch block is rate related.
Philip Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time he enjoys photography, music, and reading.
You can follow Dr Podrid on Twitter @PPodrid
Image 1: Philip J. Podrid, MD
Image 2: Philip J. Podrid, MD
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Cite this: ECG Challenge: What's Behind This Man's Mid-systolic Murmur? - Medscape - Feb 14, 2022.