Understanding the Fundamentals of Heart Rate Variability

Heart Rate Variability (HRV) demonstrates an organism’s adaptability. It represents a significant and largely uninfluenceable measure of health.
A variable heartbeat indicates good health, whilst a steady or rigid pulse should be considered a warning sign. Through 24-hour HRV measurements, we can detect these subtle differences between heartbeats and precisely assess an individual’s health and vitality status – long before any illness manifests!

— Dr Alfred Lohninger

What is Heart Rate Variability? How do 100,000 data points create a picture? Which factors influence HRV?

The fundamentals of HRV can be explained extensively. Here’s a concise overview addressing some frequently asked questions about the key facts.

As early as the 3rd century CE, physician Wang Shu-Ho (180-270) observed: “When the heartbeat becomes as regular as a woodpecker’s tapping or raindrops falling on a roof, the patient will die within four days.” However, the clinical significance of HRV wasn’t formally documented until 1963 by E.H. Hon and S.T. Lee. They noticed that changes in intervals between heartbeats preceded foetal distress, occurring before any alterations in heart rate itself became apparent. In their comprehensive review in 1996, the Task Force recognised HRV’s substantial potential in evaluating the autonomic nervous system’s role, both in healthy individuals and in patients with cardiovascular and non-cardiovascular conditions (Parekh & Lee, 2005).

“According to modern cardiology, Heart Rate Variability represents the most crucial prognostic parameter for cardiac and immune disorders, whilst also providing insights into the overall regulatory capacity and health of the entire organism. Individuals with restricted heart rate variability statistically develop significant health issues over time, including heart diseases, depression, neuropathies, and even cancer. Enhancing heartbeat variability through targeted lifestyle medical interventions enables the reduction of all types of medications, including psychotropic drugs, as it improves the adaptability of the entire organism.”
Medical Journal (37/2004)

The heartbeat is regulated by the internal clock, breathing patterns, emotions, and external influences; that is, the heart responds immediately to everything a person experiences externally and thinks and feels internally through finely tuned variations in the heartbeat sequence. Thus, heart rate increases during physical exertion or stress and decreases during rest or sleep. This phenomenon is called Heart Rate Variability, abbreviated as HRV.

HRV therefore describes the heart’s ability to continuously alter the temporal interval between successive heartbeats, thus flexibly adapting to constantly changing demands. It serves as a measure of an organism’s general adaptability. The regulation occurs through the activation of the sympathetic nervous system (for tension) and parasympathetic nervous system (for recovery). The autonomic nervous system is responsible for accelerating or decelerating the heartbeat (Shaffer et al., 2014; McCraty & Shaffer, 2015). It is calculated from the millisecond-precise intervals between individual heartbeats. The more variable the heartbeat, the healthier the organism. Herzschlägen eines Menschen. Je variabler der Herzschlag, desto gesünder der Organismus.

How is HRV calculated?

An electrocardiogram records the time series of RR intervals (Gramann & Schandry, 2009). The RR interval represents the duration between two R-waves, specifically the time span between electrical excitation (depolarisation) of the heart chambers. These time series are then quantified in terms of their strength, time scale, and internal patterns (for more detailed information, refer to “HRV Practice Textbook” by Dr Alfred Lohninger).

The system searches for R-waves 1,000 times per second. The distances between two adjacent R-waves are measured in milliseconds (ms). The numerical values of these intervals (approximately 120,000 per 24 hours) constitute the raw data. Specialised software transforms this data into numerical values and visual representations.

Heart Rate Calculation:
1 minute = 60,000 milliseconds

Different domains are utilised for heart rate variability analysis:

• Frequency domain (e.g., heart rate variability spectrum)
• Time domain (e.g., standard deviation of RR intervals)

Time Phenomena and Parameters of Heart Rate Variability

In 1981, Akselrod et al. introduced spectral analysis for quantitative assessment of cardiovascular beat-to-beat control. This is calculated using Fast Fourier Transformation (FFT).

The cornerstone of modern HRV diagnostics is the intuitively comprehensible graphical representation of the spectrogram (Sammito et al., 2014; Task Force, 1996; Berntson et al., 1997).

The intensity of HRV is expressed through colour coding and is measured in milliseconds squared (ms²). A dense, high-flaming, colour-intensive image, corresponding to the power bar on the right edge of the image, represents vitality. Following the colour spectrum of a gas flame, it transitions from light blue, medium blue, dark blue, dark red, light red, orange, yellow, and white to grey. Light blue corresponds to the highest intensity of at least 1,200 ms², red represents 240 ms², and grey indicates 0 ms².

This results in the following frequency ranges:

• ULF (ultra low frequency): < 0.0033 Hz; can only be calculated in long-term variability measurements due to cycle length. It reflects the circadian rhythm and remains largely robust against behavioural effects.
• VLF (very low frequency): 0.04 – 0.0033 Hz; captures cycle lengths from 25 seconds to several minutes. VLF fluctuations are influenced by breathing patterns, thermoregulation, vasoactive substances, altitude, and body position.
• LF (low frequency): 0.04 – 0.15 Hz; captures oscillations of approximately 10 seconds and corresponds to the periodic activity of the vasomotor part of the baroreflex loop (Mayer wave activity).
• HF (high frequency): 0.15 – 0.40 Hz; encompasses oscillations in the second range (2 – 7 seconds). It shows the parasympathetically determined oscillation component of respiratory sinus arrhythmia (RSA) and thus the breath-synchronous heart rate fluctuation.

TP (Total Frequency Power): 0 to 0.4 Hz; This serves as the measure for the influence of the vegetative system on the cardiovascular system.

Of course, there is much more to explore about Heart Rate Variability (HRV). For a deeper understanding, we recommend the “HRV Practice Handbook” in German by Dr Alfred Lohninger (2021). Published by Facultas Publishing and available for direct order through the website at https://www.autonomhealth.com/product/herzratenvariabilitaet-das-hrv-praxislehrbuch/