Artificial Intelligence Doctor

Question:

Why a patient with a single chamber leadless pacemaker set with a heart rate baseline of 50 bpm, has great discomfort sleeping that goes away at the end of the night when there is not capture threshold and the heart comes back to its natural heart rate of 28 bpm?

Answer:

The patient's discomfort during sleep is likely due to the pacemaker maintaining a heart rate of 50 bpm, which may be significantly higher than their intrinsic (natural) heart rate of 28 bpm. During rest or sleep, the body's metabolic demands are low, and a very slow natural heart rate (such as 28 bpm) might still be sufficient for adequate cardiac output in certain individuals, especially if they have high stroke volume or underlying athletic conditioning.

However, when the pacemaker enforces a minimum rate of 50 bpm, it may cause several issues:

• Pacemaker Syndrome-like Symptoms: In some cases, pacing at a rate that is inappropriately high for the body's current needs can lead to symptoms such as fatigue, palpitations, dizziness, or chest discomfort—collectively resembling pacemaker syndrome, even in single-chamber devices.
• Loss of Atrioventricular (AV) Synchrony: Leadless pacemakers are typically ventricular pacemakers (e.g., VVI mode). If the patient’s intrinsic atrial activity is not synchronized with the paced ventricular rate, it can result in retrograde conduction or cannon A waves, causing neck pulsations and discomfort.
• Reduced Cardiac Filling Time: Pacing at 50 bpm may shorten diastole relative to the patient’s natural rhythm, potentially reducing ventricular filling and stroke volume, especially if the intrinsic rhythm allows for longer filling periods.
• Increased Myocardial Oxygen Demand: An elevated heart rate increases myocardial oxygen consumption, which could be problematic if there is underlying ischemia or reduced coronary reserve.

Toward the end of the night, when the pacemaker stops capturing (possibly due to increased capture threshold during sleep or device algorithm behavior), the heart reverts to its intrinsic rate of 28 bpm. Although this rate is bradycardic by standard criteria, the patient may feel better because the heart is functioning in a rhythm that is more physiologically aligned with their resting metabolic state, minimizing artificial pacing-related hemodynamic inefficiencies.

Clinical Recommendation: This scenario warrants evaluation by a cardiologist or electrophysiologist. The pacemaker settings—especially the lower rate limit—should be reviewed and potentially adjusted to a more physiologically appropriate level during sleep (e.g., using sleep algorithms or rate-drop response features if available). Additionally, assessment of capture threshold, sensing, and battery status is essential to ensure proper device function.