A complex and only partially decrypted code of the heart: Heart Rate Variability

An emerging tool allowing for some visual to internal balance, stress resilience and healthy longevity.

The heart rate is defined by the number of heartbeats per minute (bpm). As with any biological measure a range of values is defined as normal taking into consideration the age and biological sex. In the case of heart rate this range (at rest) is for adults between 60 and 100 bpm with significant individual variations depending on fitness level, health conditions and idiosyncrasy. Even at a constant heart rate (bpm), the timely distance between singular heart beats varies despite the same final count per minute. This phenomenon is called Heart Rate Variability (HRV). The fluctuation of heart rate is the result of complex dynamic and non-linear interactions of several physiological systems.

HRV is considered a measure of the so-called neuro-cardiac function, which derives from the interaction between heart and brain, as well as from the dynamic interplay between the components of the autonomous nervous system, the sympathetic and parasympathetic nervous system (SNS and PNS accordingly). In short, the SNS is responsible for the so-called “fight-or-flight” reactions when perceiving a danger, the PNS is responsible for the so-called “rest-and-digest” behavior.

Brain and the Heart

Interactions between Brain (Autonomous Nervous System) and the Heart

A curiosity and a hint: after cardiac transplantation the HR at rest is almost constant, HRV is practically lost. Heart transplantation deprives per definition the transplanted heart from its nervous connections. The lost of HRV suggests that the autonomic nervous system through the vagus nerves (PNS) and the ganglia (SNS) is the single most important factor mediating HRV, with respiratory sinus arrhythmia, blood pressure variability and circadian rhythms as major contributors.

There are more than 50 measures of HRV included in 3 major domains:
1. Time: measured time intervals between heart beats, expressed in milliseconds (ms) or transformed as natural logarithm (ln) for more clarity)
2. Frequency: estimate of the potency of the oscillations of heart rate, i.e. the energy of the signal of heart rate oscillations within a frequency band. Measured in ms2/Hz.
3. Non-linear measures: non-linear measurements index the unpredictability of a time series, which results from the complexity of the mechanisms that regulate HRV. Non-linear indices correlate with specific frequency- and time-domain measurements when they are generated by the same processes.

A more practical approach to HRV is the time needed for a recording and what reliable measures we can gain from this.

Longer recordings (>24h) are needed for scientific research in this emerging topic. Short-term time frames (3 – 5 min in length) are already a standard and are currently well-accepted as suitable for extracting accurate HRV measurements, while some researchers were able to isolate some robust parameters also in 10 – 30 s recordings. Serial measurements of the same individual (after defining a basal set of values) can be accomplished in short intervals (1-3 min). In this context the most frequently used parameters are (both from the Time domain):

SDNN: standard deviation of intervals between normal heartbeats (ms)
RMSSD: root mean square of successive RR interval differences (ms)

More in-depth analysis can define indices, which can give more insight and visualize the relative representation of the SNS and PNS in the complex balance within the ANS: SNS- and PNS-indices (Kubios HRV analysis software).

There are many confounders in HRV measurement with a significant impact on the final result. To name a few: breathing pattern, posture, circadian rhythm, external optical, auditory or other influences during measurement, medicines, alcohol consumption, duration and quality of sleep. Therefore, to obtain reliable serial measurements we need to reduce as much as possible the presence and influence of such confounding factors to our recordings and interpretations.

Why do we care about HRV?

The interest in HRV is based on two factors:

1. HRV is a valid proxy of the stress response when measured under specific circumstances.

Measuring stress response gives us insight to the role and balance between the components of the autonomous nervous system. Response to stress (be it voluntary, such as athletic training or involuntary during our busy daily life) can have short- and long-term consequences. From not reaching the desired fitness level (due to overtraining as an example) to the development of arterial hypertension and other heart-related issues in the years ahead. Certain HRV measures were correlated even to mortality in the long-term or (better) in the opposite they might serve as a measure to achieve for healthy longevity.

2. The technological progress, especially regarding wearable micro-computers in the health / fitness sector (e.g. Apple Watch, Fitbit watch, Oura ring), made readings of heart rate and HRV feasible and more accessible to the broad public.

All mentioned serious brands limit their readings to one parameter and to more or less specific conditions trying to improve reliability. Serial measurements possibly under the same conditions can be helpful in understanding trends even with only one parameter measured. For more in-depth evaluations professional assistance is needed. In ambitious or professional athletes, HRV surveillance with good interpretation can prevent overtraining and contribute to a better performance.

Critical appraisal

Measured HRV is a proxy. It is the consequence and the result of what is happening “under the hood”. As with many biological parameters, the absolute value measured once is of almost no value. Serial measurements of HRV parameters under valid and specific conditions can compose a valuable individual profile over time. The primary visual should focus on analysis of stressors and on individual changes. Targeted daily interventions (focused breathing, meditation, effective after-work winding down, physical activity especially outdoor, sufficient and qualitative sleep) can enhance the relative role of the PNS (“rest-and-digest”), reduce the prevalence of the SNS (“fight-or-flight”) and thus contribute to better health and healthy longevity.

If you are interested in in-depth and detailed views on this topic (more focused on training) I’d recommend the Substack of Marco Altini, PhD.