The correct diagnosis is sinus tachycardia (Figure 2).
Figure 2. Courtesy of Philip J. Podrid, MD
The rhythm is regular at a rate of 120 beats/min, and a single premature complex is seen (^). The QRS complexes are wide (0.18 sec). An RSR' morphology occurs in lead V1 (←) and a broad terminal S wave in leads I and V5-V6 (→).
Although this is consistent with a right bundle branch block (RBBB), the morphology is not completely typical because there is a prominent R' in leads V2-V3. In addition, the QRS complex is wider than usually seen with RBBB.
The wide QRS complex is consistent with a dilated cardiomyopathy and possibly an intraventricular conduction delay to the right ventricle rather than a true RBBB. The axis is extremely leftward between -30° and -90° (positive QRS complex in lead I and negative QRS complex in leads II and aVF).
There are two etiologies for an extreme left axis: a left anterior fascicular block (rS morphology in leads II, III, and aVF) and an old inferior wall myocardial infarction (QS or qR morphology in leads II, III, and/or aVF, ie any two of the inferior leads). In this case, there is a QS complex in leads III and aVF consistent with an old inferior wall myocardial infarction.
The QT/QTc intervals appear prolonged (400 msec/570 msec) but are normal considering the prolonged QRS complex duration. Because the QRS complex is 80 msec wider than the normal QRS width (up to 0.10 msec), 80 msec should be subtracted from the QT intervals as measured and then corrected for heart rate. The corrected QT/QTc is normal (320 msec/450 msec).
A single premature complex (^) has a different morphology; this is a premature ventricular complex. Although the P waves are not obvious, there appears to be a P wave on the downslope of the T wave that is most obvious in lead II (*). More important, a pause occurs after the premature ventricular complex during which a clear P wave can be seen (+).
The PR interval is 0.16 sec, the same PR interval seen in lead II. When P waves are not obvious, look for them on the T wave. The P wave may also be within the ST segment, causing a bump in this segment. Of note, you should also look for P waves during any pause in the RR interval. With tachycardia, and a long PR interval, the P wave may be superimposed on the T wave, resulting in a notching or bump on the T wave as in this ECG. Therefore, this is a sinus tachycardia.
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.
You can follow Dr Podrid on Twitter @PPodrid
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Cite this: ECG Challenge: Heart Racing and Probable Heart Failure - Medscape - Jul 18, 2022.