Introduction

ECG Challenge: A 48-year-old man with a history of hyperlipidemia, hypertension, diabetes mellitus, and cigarette smoking presents to his primary care physician for a routine physical examination. The results of his examination are normal, but his pulse is noted to be irregular, resulting in an ECG.

The rhythm is regular at a rate of 90 bpm. However, there are 2 long RR intervals (rate, 58 bpm; ↔) that are equal to each other. Therefore, the rhythm is regularly irregular. The QRS complex duration is normal (0.08 s), and there is a normal morphology. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). There is a P wave before each QRS complex (+). Although the P wave is not always obvious, by using the PP interval of two sequential P waves that are apparent, it can be seen that the on-time P wave is occasionally within or on top of the T wave, accounting for the changes in the P wave morphology. The P wave is positive in leads I, II, aVF, and V4–V6. Therefore, this is a sinus tachycardia at a rate of 100 bpm. However, the PR interval is not stable. The baseline PR interval (ie, the PR interval after each long RR interval) is 0.24 ms (^), thus representing a first-degree atrioventricular block or prolonged atrioventricular conduction. After this, there is a gradual prolongation in the PR interval (0.30 s, 0.34 s, 0.36 s; ▴). This is a pattern of Mobitz type I or Wenckebach, a form of a second-degree atrioventricular block. With a second-degree atrioventricular block, there is a stable atrial rate (ie, constant PP interval) with an occasional nonconducted or blocked P wave. With Mobitz type I, there is a progressive lengthening of the PR interval with only 1 nonconducted P wave and pause, after which the PR interval shortens back to its baseline level. However, it can be seen that the pause is not the result of an on-time but nonconducted sinus P wave, because no P wave is seen during the pause. Close inspection of the last QRS complex before the pause demonstrates that there is a negative deflection at the very end of the QRS complex (↑), not present in the other complexes. This is a premature P wave, ie, a premature atrial complex, that is not conducted, and, hence, it accounts for the pause rather than the end of a Wenckebach cycle. In addition, there is shortening of the RR interval, seen for the first few QRS complex cycles. This on occasion occurs with Wenckebach and is due to the fact that the increment of PR interval lengthening becomes less and less with each complex. This is most apparent during the first few complexes of the Wenckebach cycle. As seen here, the baseline PR interval is 0.30 s, then increases to 0.34 s and then to 0.36 s. As there is a stable PP interval, the RR interval will shorten.

Please go to the journal’s Facebook page for more ECG Challenges: http://goo.gl/cm4K7. Challenges are posted on Tuesdays and Responses on Wednesdays.