The correct diagnosis is sinus rhythm, RBBB, and V1-V3 lead switch (Figure 2).
Figure 2. Courtesy of Philip J. Podrid, MD.
The rhythm is regular at a rate of 64 beats/min, and the P waves (+) are positive in leads I, II, aVF, and V4-V6. This is a normal sinus rhythm.
The QRS complex is wide (0.14 s). A terminal S wave in leads I and V5-V6 (^) is consistent with terminal forces moving from left to right, suggesting RBBB. However, the QRS complex morphology in lead V1 — a tall R wave (→) and a terminal S wave (^) — is not consistent with RBBB.
Conditions associated with a tall R wave in lead V1 include a posterior wall myocardial infarction, right ventricular hypertrophy, Wolff-Parkinson-White syndrome, septal hypertrophy (ie, a hypertrophic cardiomyopathy), right-sided leads, dextrocardia, or lead switch or misplacement.
Of note, the QRS complex morphology in lead V3 is typical for RBBB as would be seen in lead V1 with an initial R wave (↓), followed by an S wave and a tall R' (←). In addition, the P wave in lead V1 is all positive, which suggests a right atrial abnormality or hypertrophy, while the P wave in V3 is biphasic and has normal morphology usually seen in lead V1. This indicates a V1-V3 lead switch. The QRS complex in lead V3 is actually V1, and the V1 QRS complex is actually V3. The QRS complex morphology represents a typical RBBB.
Although the axis appears to be rightward, it is caused by a prominent and broad terminal S wave in lead I (^), which represent right ventricular activation and should not be considered when determining the axis in the frontal plane (defined by the direction of the impulse conduction through the left ventricle). If the S wave is not considered, the axis is normal. The PR interval appears to be short but measures 0.12 s, which is within the lower limits of normal.
Philip J. 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.
Image 1: Philip J. Podrid, MD
Image 2: Philip J. Podrid, MD
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