A 76-year-old man is admitted for a nonhealing ulcer and cellulitis of his right leg. He has known diabetes mellitus that has been poorly controlled. He has no known cardiac disease but has hypertension for which he is taking a β-blocker and an angiotensin-converting enzyme inhibitor (Figure).
There is a regular rhythm at a rate of 42 bpm. The QRS complex has a normal duration (0.08 seconds) and a normal morphology, although there is poor R-wave progression in leads V1 and V2. The axis is approximately 0° (positive QRS complex in lead I and biphasic in lead aVF). The QT/QTc intervals are normal (460/385 milliseconds). There are nonspecific ST-T wave changes in leads I, II, aVL, and V3 through V6 (^). There is a P wave before each QRS complex (+), but the PR interval is very short and not constant but is variable. The P wave is positive in leads I, II, aVF, and V4 through V6. Hence, there is an underlying sinus bradycardia at a rate of 42 bpm. The variable PR interval (without any pattern) is defined as atrioventricular (AV) dissociation. There are 2 basic causes of AV dissociation. The first is complete heart block (third-degree AV block) in which the atrial rate is faster than the ventricular rate because the QRS complexes are attributable to an escape rhythm (either ventricular or junctional). The etiology of the escape rhythm is based upon the morphology of the QRS complex and not the rate of escape rhythm. The second is an accelerated lower pacemaker (either junctional or ventricular) in which the ventricular rate is faster than the atrial rate. When the atrial and ventricular rates are identical, as seen on this ECG (when recorded at a normal speed of 25 mm/sec), the reason for the AV dissociation cannot be established. This is called isorhythmic dissociation.
- © 2013 American Heart Association, Inc.