What is Heart Rate Variability?
Heart rate variability (HRV) measures the variation in time between consecutive heartbeats. Contrary to what might seem intuitive, a healthy heart does not beat with metronomic regularity but rather shows constant subtle variation in response to autonomic nervous system input.
Key Measures
- RMSSD: Root mean square of successive differences - reflects parasympathetic activity
- pNN50: Percentage of successive RR intervals that differ by >50ms
- LF/HF Ratio: Balance between sympathetic and parasympathetic tone
HRV in PAIS
Reduced HRV has emerged as one of the most consistent objective findings in ME/CFS and Long COVID:
- Lower resting HRV compared to healthy controls
- Blunted HRV response to postural change
- Reduced HRV during sleep (when it should be highest)
- Correlation with symptom severity and PEM
Clinical Value: HRV provides an objective biomarker that can help differentiate PAIS from deconditioning and track treatment response over time.