Heart racing then feeling dizzy

2026-11-10

The case

78-year-old man. Heart racing then feeling dizzy

A dual-chamber pacemaker was implanted two years ago for symptomatic sinus bradycardia. Regular medications include bisoprolol 2.5 mg daily.

On arrival

Heart rate: 50
Blood pressure: 104/62

Stage 1 of 1

He describes several minutes of rapid palpitations while sitting at home, followed by light-headedness. A lead II rhythm strip is recorded on arrival.

The tracing for the quiz “Heart racing then feeling dizzy”. — Lead II rhythm strip, 25 mm/s Open this tracing in the studio

In the middle of the strip the QRS complexes are narrow and upright at about 100 per minute with no pacing spikes. What best explains this section?

ASinus tachycardia with appropriate device inhibition Your answer
BConducted atrial flutter inhibiting the pacemaker Correct answer Your answer
CAtrial flutter tracked by the pacemaker at its upper rate Your answer
DVentricular oversensing causing pacing inhibition Your answer

There are flutter waves between the complexes, easiest to see where the gaps are longest, and no separate P waves as you would expect in sinus tachycardia. The QRS is narrow, with no spike in front of it, so these complexes are coming through his own AV node. If the pacemaker were tracking the flutter, the complexes would be wide and paced, at the device max rate (as it would try to track a atrial rate of 280/min). His own rate is faster than the 70 the device was pacing at, so it senses each beat and does not fire. That is normal function.

What best explains the rhythm in the final ten seconds of the strip?

AAtrial flutter with 6:1 conduction Your answer
BComplete heart block with ventricular escape Your answer
CAtrial undersensing with lower-rate ventricular pacing Your answer
DMode switch with appropriate ventricular pacing Correct answer Your answer

Each QRS has a pacing spike in front of it and looks the same as the paced beats at the start, so this is neither 6:1 conduction nor a native escape rhythm. The spikes also land at different points in the flutter cycle. The ventricle is paced at about 50, slower than the 70 at the start, with no atrial spikes. That fits a pacemaker that has detected a fast atrial rhythm and stopped following the atrium (mode switch). In the last part of the strip, the AV node does not conduct well, and the ventricular response is below the device rate, hence the ventricular pacing. With atrial undersensing it would keep pacing both chambers at about 70, with atrial spikes, as in the first few beats.

Diagnosis

Atrial flutter with pacemaker mode switch

The strip has three parts. It starts with dual-chamber pacing at about 70. Atrial flutter then begins and conducts at about 100 with narrow complexes, so the pacemaker stays quiet. After about nine seconds the flutter stops getting through to the ventricles, perhaps partly because of the bisoprolol, and the device paces the ventricle alone at about 50, with no atrial spikes.

None of this is a pacemaker fault. When the atrial rate is very fast, the device stops tracking the atrium so that it does not drive the ventricle at that rate, and paces the ventricle at a lower rate instead. The slower rate, the missing atrial spikes and the shifting relation between spikes and flutter waves are what separate this from undersensing or a high conduction ratio. A device check is needed to confirm the mode switch.

References

Glikson M, Nielsen JC, Kronborg MB, et al. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy. Eur Heart J. 2021;42(35):3427-3520.
Mulpuru SK, Madhavan M, McLeod CJ, Cha YM, Friedman PA. Cardiac pacemakers: function, troubleshooting, and management: part 1 of a 2-part series. J Am Coll Cardiol. 2017;69(2):189-210.

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