Understanding HRV: What It Reveals

At the Empowered Brain Program we place a high importance on the dynamic relationship between the brain and the heart. Is that strange? In fact, it is one of our most important biometrics and is central to how we assess and guide our neurocognitive therapies.

The reason being, is that while the brain controls the heart, the heart has its own intrinsic nervous system that communicates back to the brain. In actuality, the heart’s intrinsic nervous system influences and directs brain function. (Topic for a later discussion). Heart Rate Variability is a measure of that two-way communication.

Heart Rate Variability (HRV) measures the variance in timing in between consecutive heartbeats. Of note however, the information we extrapolate from HRV is far more than its rhythm.

HRV offers unique clinical insights into the balance of autonomic nervous system activity (sympathetic and parasympathetic systems), cardiovascular health and neurocognitive health- relating to your health state today and predictive of your health state later-in-life. A well-regulated HRV reflects the nervous system’s capacity to adapt to ongoing changes, both physiological and environmental.

A healthy heart is not a metronome.

In fact, a robust HRV score expresses a high level of time interval fluctuation between heartbeats. (Some pathological conditions cause a high HRV, but that is beyond the scope of this article).

HRV is more than a single number, it involves different values, each relating to a different facet of neurocardiac function. The purpose of this article is to offer a clear understanding of their significance, especially as it relates to rehabilitating cognitive decline and dementia – the core clinical focus at the Empowered Brain Program.

HRV and Biological (Heart) Age

In our previous article, Heart Rate Variability (HRV): A Key to Optimal Brain Health, we explored the fundamental role HRV plays in the dialogue between the heart and brain. Beyond its neurological significance, HRV provides insights into biological age- our physiological resilience and overall health status.

The research consistently demonstrates a clear association between low HRV and chronic degenerative conditions:

• Mood and sleep disorders - Lower HRV is linked to conditions such as anxiety, depression and sleeping disorders.¹
• Systemic and cardiovascular dysfunction – HRV serves as a sensitive and non-invasive biometric for detection of diabetes, sleep apnea, myocardial infarction, cardiac arrhythmia, hypertension, renal failure and psychiatric disorders.²
• Neurological and cognitive disorders - Abnormal HRV values are observed in mild cognitive impairments, dementia, mild traumatic brain injury, migraine, COVID-19, stroke, epilepsy and psychological conditions.³

What do these diverse dysfunctions have in common?

They largely represent (age-related) breakdown in brain-body communication, involving a gradual decline in adaptability and function across multiple physiological systems.

HRV should be viewed as a biological age indicator. In younger individuals, the heart rate intervals are characterized by a higher variance- reflecting a flexible and adaptive autonomic nervous system. As we advance in age this natural fluctuation steadily declines.⁴

Importantly, higher HRV values are not only predictive of better cardiovascular, cognitive function and autonomic function — they also correlate with a younger biological age relative to one’s chronological age.⁵

Ultimately the amount of HRV an individual has relative to age indicates overall vitality or system depletion. 

How to Read Key HRV Metrics

There are two distinct ways we can observe HRV, time domain analysis and frequency domain analysis.

Time domain analysis refers to the overall variance between successive heartbeats over a given time period.

Frequency domain analysis evaluates the quality of that variance, meaning the heart rhythm patterns and waveforms. This reveals their wave oscillation coherence or non-coherence.

• Time Domain = HRV: Wave amplitude
• Frequency Domain = Coherence: Pattern of heart rhythm, is independent of heart rate, most reflective of physiological and emotional synchronization.

In simple terms:

• Time Domain = HRV Amplitude (how much variability there is)
• Frequency Domain = spectral pattern of HRV (from which coherence can be derived)

The gold standard of HRV measurement is the continuous 24 hour period recording. This reflects the influences from the circadian rhythms, sleep cycle, core body temperature, metabolism and blood pressure changes making it the most representative autonomic function. However, reliable HRV data can also be obtained from short-term (~5 min) and ultra-short (<5 min) recordings.^6,7

At the Empowered Brain Program, we utilize a five-minute paced breathing assessment with HeartMath™ emWave Pro Plus technology. While McCraty et al. demonstrated that a one-minute deep-breathing test strongly correlates with 24-hour HRV metrics, the broader literature (including Shaffer and Ginsberg, 2017) supports the validity of short-term (five-minute) recordings as accurate and clinically representative of 24-hour HRV indices.

Time Domain Analysis 

SDNN – Standard Deviation of Normal to Normal (artifacts removed) interbeat intervals  

• SDNN represents an aggregate HRV value score; it reflects input from both sympathetic and parasympathetic nervous system activity.
• It is highly correlated with Ultra Low Frequency (ULF), Very Low Frequency (VLF) and Low Frequency (LF) band power, and total HRV power – to be explained shortly.
• The 24 hour SDNN measurement is considered a gold standard. Clinically speaking, patients with SDNN values of
  • <50 ms = unhealthy
  • 50-100 ms = compromised health
  • >100 ms = healthy autonomic function
• Short-term (1–5 min) SDNN measurements are more reflective of parasympathetic modulation, particularly during slow, rhythmic breathing.⁷
• Research demonstrates that the 1 min deep breathing HRV test is highly correlated and representative of the 24 hour measurement.⁶

MHRR – Mean Heart Rate Range, indicates the largest difference between highest and lowest heart rate during a 1min or 5min HRV assessment.

SDRR – Similar to SDNN but includes data prior to artifact or abnormality detection and removal.

RMSDD - Root mean square of successive differences between normal heartbeats (RMSSD). Reflects heart rate variance beat to beat and estimates Vagal-Parasympathetic changes. It correlates with the High Frequency power band, making it a sensitive measure of rapid vagal modulation.⁷

Frequency Domain Analysis 

Frequency Domain analysis expresses HRV variance as a function of frequency, dividing the HRV spectrum into distinct bandwidths that reveal underlying physiological mechanisms. The three key bandwidths measured during a 5-minute or 1-minute assessment are High, Low and Very Low frequencies.  

High Frequency (HF):

• Range 0.15-0.40 hertz (Hz), (cycles every 2.5-7 seconds)
• Represents vagal or parasympathetic activity.
• Highest during periods of relaxation and sleep. Directly linked to respiratory sinus arrhythmia (RSA), the normal heart rate acceleration during inhalation and deceleration during exhalation.
• Low HF power is associated with stress, anxiety, panic, worry etc. whereas high HF power reflects healthy parasympathetic activity and is often higher at night.⁷

Low Frequency (LF):

• Range 0.04-0.15 Hz, (cycles every 7-25 s)
• Reflective of both sympathetic and parasympathetic activity in long term recordings, but mainly parasympathetic activity in short term sessions (<5 min).
• Breathing slower than 7 breaths per minute enhances LF amplitude.⁷
• Frequencies near 0.1 Hz represent optimal coherence levels- a resonant synchronization between the cardiac and brain rhythms. 0.1 Hz is considered the ideal coherence frequency for the cardiovascular system.⁶

Very Low Frequency (VLF):

• Range 0.0033-0.04 Hz, (cycles every 25-300 s)
• Reflects the activity of the heart’s intrinsic nervous system, with amplitude modulated in part by sympathetic activity and by mental, emotional, and physical states.
• Also influenced by hormonal and regulatory systems including te renin-angiotensin system (regulates blood pressure and fluid balance), thermoregulation and neuroendocrine rhythms.
• Low VLF power is associated with high inflammation, PTSD and arrhythmic death. ⁷
• Notably, low VLF power is strongly associated with all-cause mortality more than HF or LF measures.⁸

Rhythm Pattern Analysis

Rhythm-pattern analysis captures not only HRV magnitude but also the shape and order of heart-rhythm oscillations. These patterns closely reflect the individual’s emotional state and the degree of synchronization between higher brain centers and the autonomic nervous system. This interplay underlies executive functions such as attention, problem solving, creativity, emotional regulation, decision making and more.⁹

At the Empowered Brain Program, we routinely conduct standardized 5 min HRV assessments to measure SDNN, MHRR, HF, LF, VLF and coherence values. Each assessment is performed under controlled context including consistent time of day, at rest, not post meal or strenuous activity etc., using paced diaphragmatic breathing at 4.5-7.5 breaths per minute.

We also integrate HRV-Coherence biofeedback training to actively improve these measures for neuro-cardiac optimization. Through guided heart-brain synchronization exercises, individuals are able to enhance cognitive function, emotional regulation, autonomic balance and quality of sleep -all major components essential in slowing the advance of dementia.

HRV Can Be Changed—Here Are Proven Ways

HeartMath Coherence Training: A 2022 article in Applied Psychophysiology and Biofeedback reviewed the HeartMath Institute’s research pathway in HRV biofeedback and coherence training. Findings demonstrated that heart-focused breathing and emotion-regulation practices can reliably enhance HRV coherence, enhance vagal modulation and autonomic nervous system balance. These physiological changes are associated with improving cognitive and executive function, emotional regulation and stress resilience across diverse populations.¹⁰

At the Empowered Brain Program, we utilize HeartMath’s Inner Balance Coherence Plus technology as our HRV training tool of choice. This HRV biofeedback device guides individuals using real-time heart–brain coherence training and provides visual and audible feedback on HRV and coherence levels.

Through learning to synchronize breathing, emotional focus and cardiac rhythm, participants cultivate coherent heart-brain patterns that support neuroplasticity, emotional regulation, and cognitive restoration- all core objectives in our integrative rehabilitation model for dementia and cognitive decline.

Mindfulness Practice: A 10-day randomized controlled trial showed significant improvements in daytime and nighttime HRV with mindfulness training versus controls. Unique focal points of the study include utilizing test subjects unfamiliar with mindfulness practices, mindfulness practice interventions that were online based rather than in person and a specific focus on quality of sleep.¹¹

Exergame Training in Older Adults (RCT): Virtual reality-based cognitive-motor gaming improved HRV (SDNN and RMSSD) over treadmill or memory-only interventions.

This six month long randomized controlled trial used subjects age 70 years and older and divided them into three different test groups: virtual reality video game dancing (exergaming), treadmill walking with simultaneous verbal memory training and treadmill only. Two 1-hour training sessions were performed each week over the course of 6 months. Only the dance-exergaming group demonstrated a significant increase in HRV together with global and parasympathetic autonomic nervous system function.¹²

Action Steps- How to get started

Training and maintaining healthy HRV and Coherence levels is critical for rehabilitating the aging brain. Even more so in childhood through adulthood, to establish the bedrock for cognitive reserve. What is so unique about training HRV and Heart Brain Coherence is that it is simple, easy to practice and can be performed anywhere at anytime once the basic steps are learned.

At the Empowered Brain Program, we incorporate HeartMath™ technology for both HRV measurement and coherence training as part of our comprehensive neurorehabilitation approach. All HeartMath equipment and tools are included at no additional cost within our cognitive decline and dementia rehabilitation program.

One thing I find most meaningful about heart-brain coherence, and the broader field of neurocardiology, is the heart-centered meaning that it brings to healthcare. The dynamics that help individuals reconnect to themselves, the connections between loved ones and of course the therapeutic relationship between physician and patient.

Disclaimer. The information represented in this article is meant to provide concepts from evidence based research. It is not intended to treat or diagnose any health condition. For appropriate treatment methods please contact your healthcare provider.