ECG Challenge: A 75-year-old man with a previous history of complete heart block for which he received a dual-chamber pacemaker presents to his cardiologist for a routine follow-up and pacemaker check. He denies all symptoms. Physical examination is unremarkable, although an irregular pulse is found. An ECG is obtained.
The rhythm is irregular, but there is repeating pattern of equivalent long and equivalent short intervals (grouped beating). Hence, the rhythm is regularly irregular with an average rate of 84 beats per minute. The first QRS complex of the pair is preceded by both an atrial (+) and ventricular (^) pacing stimulus. Therefore, this is AV sequential pacing. There is a P wave (↓) that follows the atrial stimulus. After the ventricular stimulus, there is a QRS complex with duration of 0.12 s. The AV delay is 0.16 s. The QRS complex has an extreme left axis between –30° and –90° (positive QRS complex in lead I and negative complex in leads II and aVF). The QT/QTc intervals are prolonged (400/475 ms) but are normal when the prolonged QRS complex duration is considered (380/450 ms). The second QRS complex of the pair is preceded by a P wave (↑) that is negative in leads II, III, aVF, and V4 through V6. It is not preceded by an atrial pacing stimulus. Therefore, this is a premature atrial complex and, because every other QRS complex is a premature atrial complex, this is termed atrial bigeminy. After the P wave there is a ventricular pacing stimulus (*), and the PR interval is 0.16 s. This ventricular stimulus results in a QRS complex that is wider (0.16 s) and has a broad R wave in lead I (←). It is completely paced as a result of a ventricular stimulus, which originates in the right ventricle. Although the narrower QRS complex is also preceded by a pacemaker stimulus, this complex is not completely captured, that is, there is pseudofusion. This occurs when the programmed AV delay of the pacemaker is the same as the intrinsic PR interval. Hence, there is conduction through the AV node and His Purkinje system, and this supraventricular impulse fuses with the impulse generated from the ventricular stimulus, because the 2 impulses activate the ventricles almost simultaneously. The second QRS complex, which is also paced, is wider as it results entirely from activation by the pacemaker stimulus. This is due to the fact that the premature atrial impulse is conducted through the AV node at a slower rate as a result of decremental conduction (ie more rapid stimulation of the AV node results in slower conduction through this structure). Since AV nodal conduction is slower, there is a delay in myocardial activation through the normal AV node His-Purkinje pathway resulting in more myocardial activation from the pacemaker stimulus; hence the QRS complex is entirely paced or captured by the pacemaker.
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- © 2014 American Heart Association, Inc.