ECG Challenge: A 72-year-old man with a history of ulcerative colitis, which has been stable, and hypertension, for which he has been taking an angiotensin-converting enzyme inhibitor and β-blocker, presents to his primary care physician for a routine physical examination. The results of the examination are normal, but a slow pulse rate is noted. An ECG is obtained (A). As a result, the patient is sent to the emergency department where a second ECG is obtained (B).
The rhythm in the first ECG (A) is regular, although there is 1 longer RR interval noted. The average rate is 36 bpm. The QRS complexes have a normal duration (0.08 s). There is low voltage present, defined as a QRS complex amplitude <5 mm in each limb lead and <10 mm in each precordial lead. There are Q waves (QS complex) in leads V1 through V3 (↓) and T-wave inversions (↑) consistent with an old anteroseptal myocardial infarction. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (480/370 ms). There are P waves seen before each QRS complex (+) with a constant PR interval (0.48 s) (⌈⌉). The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, there is a sinus rhythm with a prolonged PR interval (first-degree atrioventricular [AV] block or prolonged AV conduction). A second P wave (*) can be seen after each QRS complex within the T wave. The PP interval is constant (⌊⌋) with a rate of 68 bpm. This is a second-degree AV block with a pattern of 2:1 AV conduction. This may be either a Mobitz type I or Mobitz type II. During the longer RR interval, another (ie, second) on-time nonconducted P wave is seen (^). The presence of 2 nonconducted P waves is seen with a Mobitz type II. It is often referred to as high degree AV block. With Mobitz type I there is only one non conducted P wave.
The rhythm in the second ECG (B) is regular with a rate of 42 bpm. The QRS complex is wide (0.14 s) and the morphology resembles a left bundle-branch block, although the morphology is not completely typical. The QT/QTc intervals are normal (480/400 ms and 440 and 370 ms when corrected for the prolonged QRS complex duration). The T waves are tall and peaked, but they are asymmetrical with an upstroke that is slower than the downstroke; the T waves are therefore normal. P waves are seen (+) with a stable PP interval (⌊⌋) and a rate of 100 bpm. The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, there is a normal sinus rhythm. The PR intervals are not constant, indicating AV dissociation. There are 2 causes for AV dissociation: complete heart block in which the atrial rate is faster than the rate of the QRS complexes and an accelerated lower pacemaker in which the atrial rate is slower than the rate of the QRS complexes. In this case, the atrial rate is faster than the rate of the QRS complexes, and, hence, this is a complete heart block. Because the QRS complexes are wide and different from the conducted complexes seen in the first ECG (A), this is an escape ventricular rhythm. This is to be expected when a Mobitz type II exists before the onset of complete heart block.
Please go to the journal’s blog, OpenHeart, for more ECG Challenges: http://goo.gl/tQPNFp. Challenges are posted on Tuesdays and Responses on Wednesdays.
- © 2015 American Heart Association, Inc.