Heart Rate Variability (HRV) in Autonomic Assessment

Dr Graham Exelby May 2025

Overview:

Heart Rate Variability (HRV) reflects the dynamic interplay between the sympathetic and parasympathetic branches of the autonomic nervous system. It measures the variability in time intervals between heartbeats (typically R-R intervals on ECG) and provides a window into autonomic adaptability, vagal tone, and overall homeostatic balance. In the context of Long COVID, POTS, and dysautonomia, 24-hour Holter monitoring with HRV analysis offers a valuable method for autonomic phenotyping.

https://commons.wikimedia.org/wiki/File:Heart_rate_variability_(HRV).svg#/media/File:Heart_rate_variability_(HRV).svg

Source: Jason Moore: Normative HRV Scores by Age and Gender [Heart Rate Variability Chart] https://elitehrv.com/normal-heart-rate-variability-age-gender 

SDNN by Age: Standard deviation of all normal R-R intervals over 24 hours. Reflects overall autonomic tone and is a global marker of HRV. Normal SDNN declines with age and is generally higher in females under 55 due to oestrogen-mediated vagal tone.

Source: Umetani K, Singer DH, McCraty R, Atkinson M. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. J Am Coll Cardiol. 1998 Mar 1;31(3):593-601. doi: 10.1016/s0735-1097(97)00554-8. PMID: 9502641.

Samples HRV (QML Holter):  

Physiology and Mechanisms:

• The parasympathetic nervous system (PNS) slows the heart rate and increases HRV via vagal activation of the SA and AV nodes.

• The sympathetic nervous system (SNS) accelerates the heart rate and reduces HRV through adrenergic stimulation of cardiac myocytes.

• HRV is highest when parasympathetic tone dominates and lowest during sympathetic dominance or vagal withdrawal.

Clinical Relevance:

• Low HRV is associated with increased biological age, reduced resilience to stress, and poorer cardiovascular outcomes.

• In Long COVID and dysautonomia, low SDNN and RMSSD are frequently observed, often with blunted circadian variation.

• Autonomic rigidity (low HRV) may reflect brainstem dysregulation, vagal suppression, or persistent inflammatory signalling (e.g., TLR4, NF-κB activation).

Neurovisceral Integration:

• The prefrontal cortex (PFC), insula, hypothalamus, and brainstem form a regulatory network that modulates HRV.

• Hyperactivity in limbic regions or impaired PFC regulation may suppress vagal tone and shift the balance toward sympathetic dominance.

• HRV serves as an accessible proxy for monitoring this brain–heart axis in conditions involving central sensitization, cognitive fatigue, and neuroinflammation.

Interpretation and Use:

• 24-hour HRV analysis provides better resolution of autonomic patterns than single-point ECGs.

• Abnormal patterns may include:

  • Persistently low SDNN (<50 ms): Autonomic rigidity, common in POTS/Long COVID.

  • Absent nocturnal rise in HRV: Indicates failure of parasympathetic recovery during sleep.

  • Inverted circadian rhythm: Suggests central dysautonomia or limbic overactivation.

Application:

• HRV profiling assists in treatment personalization (e.g., vagal stimulation, beta-blockade, pacing, neurocognitive retraining).

• Consider SDNN trends alongside amino acid profiles and postural testing (NASA lean, standing ECG).

• Use HRV to monitor therapeutic response to interventions like propranolol, LDN, nicotinamide,

Conclusion:

HRV, particularly 24-hour SDNN analysis, is a powerful non-invasive tool for assessing autonomic function and central integration in complex disorders such as Long COVID, POTS, and ME/CFS.

Its application bridges neuroimmune regulation, cardiovascular dynamics, and systemic resilience. Early HRV analysis can guide stratification, treatment planning, and long-term monitoring.