ECG Challenge: Fast Heartbeat With a Pacemaker

Philip J. Podrid, MD


January 25, 2022

The correct diagnosis is biventricular pacemaker with A-sensed, V-paced atrial fibrillation (Figure 2).

Figure 2.


The rhythm is regular at a rate of 120 beats/min. A pacemaker stimulus occurs before each QRS complex (→). This is ventricular pacing. Because the pacing rate is 120 beats/min, the mode of pacing must be A-sensed and V-paced because a VVI pacemaker would pace only at the programmed lower rate limit and not at a rate of 120 beats/min.

No clear P waves are seen, but in leads aVL and V1, irregular undulations (^) occur between each QRS complex, which are fibrillatory waves. Therefore, the underlying rhythm is atrial fibrillation. The pacemaker is tracking the atrial fibrillation but can only sense atrial activity based on the programmed post–ventricular atrial refractory period, which determines whether atrial activity is sensed; this determines the upper rate limit of the pacemaker. If atrial activity is sensed, the pacemaker commits to delivering a ventricular stimulus; if not, no stimulus is delivered.

With atrial fibrillation and atrial rates that are very rapid and irregular, only some impulses will be sensed by the atrial lead, and the ventricular pacing will be at the upper rate limit of the pacemaker. This leads to a regular paced rhythm at the upper rate limit of 120 beats/min.

The QRS complex morphology is not typical of a right ventricular pacemaker, especially noted in lead I, which is the only R-L bipolar lead that detects the impulse in a right-to-left direction. With a right ventricular lead, the impulse is directed from right to left, so there would be a positive complex in lead I, typical of a left bundle branch block morphology. In this case, the QRS complex in lead I indicates that the impulse goes from left to right. This is suggestive of a biventricular pacemaker with the impulse first originating from the left ventricular lead and moves toward the right.

A prominent R wave in lead V1 (←) also indicates that the impulse is directed to the right, consistent with a biventricular pacemaker. However, the tall R wave in lead V1 may also be present with a right ventricular pacemaker when the lead is at the septum, so this finding is less important than an initial Q wave or QS complex in lead I.

Last, the fourth QRS complex (+) has a different morphology and axis. This is a premature ventricular complex, which is not sensed by the pacemaker because there is a pacemaker stimulus occurring within the QRS complex (ie after the onset of the QRS complex).

Philip J. Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time, he enjoys photography, music, and reading.

You can follow Dr Podrid on Twitter @PPodrid

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