ECG Challenge: A 67-year-old woman with a history of hypertension treated with an angiotensin-converting enzyme inhibitor and β-blocker presents to her primary care physician for a routine physical examination. She has no complaints. Her blood pressure is normal, and the rest of her physical examination is unremarkable except for a grade II/IV harsh early peaking systolic ejection murmur at the second intercostal space. It radiates to her neck. Her carotid pulses are normal. Her pulse is noted to be irregular, and an ECG is obtained.
The rhythm is irregular, but all of the long RR intervals are the same and the short RR intervals are the same. Hence, the rhythm is regularly irregular. The average rate is 60 bpm. The QRS complexes are wide (0.14 second), and they have a left bundle-branch block morphology with a broad R wave in lead I and V5 to V6 (→) and a deep QS complex in lead V1 (←). The axis is about −30° (positive QRS complex in lead I, negative in lead aVF, and isoelectric in lead II). The QT/QTc intervals are normal (440/440 and 400/400 milliseconds when corrected for the prolonged QRS complex duration). There is a P wave before each QRS complex (+) with a stable PR interval (0.20 second). The P wave is positive in leads I, II, aVF, and V4 through V6. Hence, this is a sinus rhythm. There are other P waves (v) that are not followed by a QRS complex; that is, they are nonconducted. Occasionally, every other P wave is nonconducted, and there are episodes in which every third P wave is nonconducted. All the P waves have the same morphology, and the PP interval is regular (┌┐) with a rate of 100 bpm. Therefore, a sinus tachycardia is present. The occurrence of an occasional on-time but nonconducted P wave characterizes a second-degree atrioventricular block. When every other P wave is nonconducted, this is termed 2:1 AV block or AV conduction. The 2:1 AV block may be either a Mobitz type I (Wenckebach) or Mobitz type II. The etiology can only be established if there is a change in the pattern of conduction, for example 2 or more sequentially conducted P waves. As can be seen, there are occasionally 2 sequentially conducted P waves and the PR intervals associated with them are the same. Because all of the PR intervals are identical, this is a Mobitz type II with 2:1 atrioventricular block along with 3:2 atrioventricular block. Mobitz type II is a conduction abnormality within the His-Purkinje system. Because the His-Purkinje conduction is all or none, all the PR intervals are the same when there is conduction present.
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- © 2014 American Heart Association, Inc.