heart rate variability (hrv)
the human heart beats at ever changing rates. hence, the time interval between heart beats is always changing, or varying. this variance is captured by heart rate variability (hrv), measured in milliseconds. hrv is useful as a measure of both acute and chronic physical and mental stress, quantified by lower relative hrv values.¹
hrv represents the level of coherence and balance in the systems of the body (the autonomic nervous system). hence why chronically low hrv values are associated with issues such as emotional dysregulation.² one aspect of this coherence is between the heart and the breath, show in the figure below.⁴ the dark lines represent the breath, whereas the shaded region represents heart rate. the first graph shows a physiologically (and emotionally) regulated person. we see their heart rate follows their breath, increasing on the inhale and decreasing on the exhale. this is a normal and healthy result.
stress upsets this balance, causing heart and breath rate to increase and misalign slightly, as shown in the second graph. this is a normal response in a healthy individual responding to stress. the third graph shows an individual with ptsd responding to stress, where we see a very different response. heart rate is significantly diminished with little variance, whereas the breath is rapid and shallow. the heart and breath are very much out of sync. the shallow peaks and troughs in the heart rate are characteristic of low variability (hrv), indeed a characteristic of ptsd.³
there are many other ailments that are associated with such autonomic dysfunction, such as depression, anxiety, insomnia, irritable bowel syndrome, pain, hypertension⁵ and inflammation.⁹ conversely, greater average hrv values are associated with better physical and mental health, such as stress tolerance and emotional regulation (reduced negative emotions on exposure to acute stress).⁶
heart rate variability biofeedback, a protocol in which an individual is trained to enhance the coherence between their breath and heart, has been shown to improve hrv and help alleviate the above-mentioned ailments.² ⁵ ⁶ the implementation can be very involved, with complex expensive devices that feed heart rate, breathing, blood pressure and skin temperature measurements to the individual to guide them towards greater coherence.
a key aspect of the biofeedback protocol is the breath rate of 5.5 breaths per minute, also known as resonance breathing.⁷ a recent randomised controlled trial showed that 20 minutes of resonance breathing a day leads to measurable improvements in hrv after 4 weeks.⁸ prana implements this based on biofeedback from instantaneous hrv (hp) measurements.
prana also converts hrv to health points (hp), based on the user’s biological sex and age (shown below). the blue dashed line in the centre of the hp bar represents the median value of the population (given the configured biological sex and age). the shaded circle represents the average hp of the past 48 hours.
the hp bar is intended to simplify hrv for the user, so that they can more easily monitor the swings of their nervous system. aside from improving a chronically low hp, the objective is not to maximise hp, but instead to monitor instantaneous and average values, and see how they are affected by lifestyle choices such as diet, sleep and drugs. for example, sleeping and waking late will yield lower hp values compared to sleeping and waking early. you may find yourself feeling stressed and anxious, and see your hp drop - signalling that it’s time to do some self-care such as meditation, after which you may see your hp increase. drugs such as alcohol are detrimental to hp, and seeing this may help you cut back your usage.
try practising some meditation or pranayama (breathwork) and monitor the effects it has on your hp and how you feel!
References
1 Kim HG, Cheon EJ, Bai DS, Lee YH, Koo BH. Stress and Heart Rate Variability: A Meta-Analysis and Review of the Literature. Psychiatry Investigation. 2018 3;15:235– 245. Available from: http://psychiatryinvestigation.org/journal/view.php?doi= 10.30773/pi.2017.08.17.
2 Lehrer P, Kaur K, Sharma A, Shah K, Huseby R, Bhavsar J, et al. Heart Rate Variability Biofeedback Improves Emotional and Physical Health and Performance: A Systematic Review and Meta Analysis. Applied Psychophysiology and Biofeedback. 2020 9;45:109– 129. Available from: https://link.springer.com/10.1007/s10484-020-09466-z.
3 Schneider M, Schwerdtfeger A. Autonomic dysfunction in posttraumatic stress disorder indexed by heart rate variability: a meta-analysis. Psychological Medicine. 2020 9;50:1937–1948. Available from: https://www.cambridge.org/core/product/ identifier/S003329172000207X/type/journal_article.
4 van der Kolk B. The Body Keeps the Score. Penguin Books; 2015.
5 Lehrer PM, Gevirtz R. Heart rate variability biofeedback: how and why does it work? Frontiers in Psychology. 2014 7;5. Available from: http://journal.frontiersin.org/ article/10.3389/fpsyg.2014.00756/abstract.
6 Goessl VC, Curtiss JE, Hofmann SG. The effect of heart rate variability biofeed- back training on stress and anxiety: a meta-analysis. Psychological Medicine. 2017 11;47:2578–2586. Available from: https://www.cambridge.org/core/product/ identifier/S0033291717001003/type/journal_article.
7 Nestor J. Breath: The New Science of a Lost Art. Penguin Life; 2021.
8 Chaitanya S, Datta A, Bhandari B, Sharma VK. Effect of Resonance Breathing on Heart Rate Variability and Cognitive Functions in Young Adults: A Randomised Controlled Study. Cureus. 2022 2;.
9 Aeschbacher, Stefanie et al. “Heart rate, heart rate variability and inflammatory biomarkers among young and healthy adults.” Annals of medicine vol. 49,1 (2017): 32-41. doi:10.1080/07853890.2016.1226512