Can the circadian variation in sympathetic and parasympathetic tone explain nocturnal increases in capture thresholds in patients with a leadless pacemaker?
Scientific Answer:
Yes, the autonomic nervous system’s circadian dynamics can significantly influence capture thresholds in patients with a leadless pacemaker, especially at night. The body’s natural 24-hour rhythm governs fluctuations in sympathetic and parasympathetic tone, which directly affect myocardial excitability and pacing capture efficiency.
1. Circadian Shifts in Autonomic Tone
During the night, parasympathetic tone (vagal activity) increases, while sympathetic output decreases.
These shifts lead to bradycardia, increased atrioventricular refractoriness, and reduced myocardial excitability.
Increased vagal tone also elevates the capture threshold, which may lead to intermittent loss of capture if the device's output isn't adjusted.
2. Sympathetic Withdrawal and Device Behavior
In the early morning hours (e.g., 2:00–5:00 a.m.), sympathetic drive reaches its nadir.
This creates an autonomic window in which capture thresholds may transiently exceed the programmed output.
If safety margins are narrow or the electrode-tissue interface is borderline, this can result in nocturnal pacing failure.
3. Clinical Considerations
Threshold testing should consider time-of-day variability.
Some patients may benefit from evening testing or 24-hour remote monitoring to identify time-specific vulnerability to loss of capture.
Conclusion
The nocturnal rise in capture threshold is a physiologic phenomenon linked to parasympathetic predominance and low sympathetic tone. Device programming strategies and timing of threshold testing should account for this circadian behavior to avoid undiagnosed nighttime pacing failures.