ECG Challenge: A 34-year-old man with a history of atrioventricular nodal reentrant tachycardia being treated with a β-blocker and verapamil presents to the emergency department with complaints of fatigue and a slow heartrate. A 12-lead ECG is obtained.
The rhythm is irregular as a result of 2 premature QRS complexes (↓). Therefore, the rhythm is regularly irregular. The other QRS complexes are occurring at a regular interval and a rate of 42 bpm. These regular QRS complexes are normal in width (0.08 second) and have a normal morphology and axis between 0° and +90° (positive QRS complex in leads I and aVF). There is a tall R wave in lead V2 (v) that is consistent with early transition or counterclockwise rotation of the electric axis in the horizontal plane. This is determined by imagining the heart as if viewed from under the diaphragm. With counterclockwise rotation, left ventricular forces develop early, and there is a tall R wave in lead V2. The QT/QTc intervals are normal (440/370 milliseconds). No P waves are seen before any of these QRS complexes, but there are P waves after each QRS complex (+) with an inconsistent relationship with the QRS complex; that is, there is a variable RP interval with a gradually prolonging RP interval consistent with retrograde or VA Wenckebach. Although the P wave might be considered to be retrograde resulting from the junctional complex, the P wave is positive in leads I, II, aVF, and V5. Hence, this is a sinus P wave, and there is a stable PP interval (└┘) at a rate of 32 bpm. Therefore, there is atrioventricular dissociation. There are 2 causes of atrioventricular dissociation: complete heart block (with an atrial rate faster than the rate of the QRS complexes) and an accelerated lower pacemaker (atrial rate slower than the rates of the QRS complexes). Thus, this is an accelerated junctional rhythm. The 2 premature QRS complexes are preceded by an on-time P wave (*), and the PR interval is the same (^). Hence, these 2 complexes are captured or conducted. These captured complexes have a right bundle-branch block with an RSR’ morphology in lead V1 (←) and a broad S wave in leads I and V5 (→). The right bundle-branch block is rate related, resulting from the shorter RR interval.
- © 2016 American Heart Association, Inc.