ECG Challenge: A 64-year-old woman with a history of hypertension for which she is receiving therapy with a β-blocker and diltiazem (having been intolerant of an angiotensin-converting enzyme inhibitor and angiotensin receptor blocker) presents to her primary care physician for a routine physical examination. Her blood pressure is noted to be elevated (170/90 mm Hg); hence, the doses of β-blocker and diltiazem are increased. She presents for a follow-up 1 week later, and her pulse is noted to be slow. An ECG is obtained.
There is a regular rhythm at a rate of 36 bpm. The QRS complexes are narrow (0.08 second), and they have a normal morphology. The axis is about −30° (positive QRS complex in lead I, negative QRS complex in lead aVF, and isoelectric in lead II). The QT/QTc intervals are normal (480/370 milliseconds). P waves are seen (+), and they have a stable PP interval (└┘) at a rate of 75 bpm. The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, this is a normal sinus rhythm. However, the PR intervals are not constant, although they are only slightly different in the beginning. However, the 10th P wave (^) is premature and has a different morphology that the other P waves (it is negative in leads V1–V3). This is a premature atrial complex. As a result of this complex, there is a resetting of the sinus node, and it can be seen that the relationship between the last 2 P waves (*; which have a PP interval [┌┐] that is the same as the other PP intervals [└┘]) and the QRS complexes is variable. Therefore, atrioventricular dissociation is present, which may be attributable to complete heart block or to an accelerated junctional pacemaker. Because the atrial rate is faster than the ventricular rate, this is complete heart block with an escape junctional rhythm. The pathogenesis of the escape rhythm is based on the QRS complex morphology, not the rate of the escape rhythm (which in this case is slow at 36 bpm). The other reason for atrioventricular dissociation is an accelerated lower pacemaker, and in this situation, the atrial rate is slower than the ventricular rate.
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- © 2015 American Heart Association, Inc.