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Let’s get to it! Heart rate variability (HRV) is an emerging biomarker that has the potential to provide diagnostic and prognostic information for a variety of different disease states.
🤷♂️ Problem
An ECG (tracing of the hearts electrical activity) is the ‘gold standard’ way to measure HRV. These are often conducted in healthcare settings (inconvenient).
Despite multiple possible use-cases across the wellness and clinical space, a lack of evidence has limited the widespread use of HRV either alone or in conjunction with other biomarkers.
💡 Solution
Use wearable technology that leverages photoplethysmography (see this HealthTrends post) to measure HRV in a convenient, accessible and continuous way.
Use this data to accelerate the research into HRV, in order to understand where it can influence real health outcomes.
📖 Terms
What is HRV exactly?
The heart doesn’t beat like a metronome, there’s constant variation between beats. This variation is called heart rate variability:
Heart rate variability (HRV). A measure of the variation in time between each heartbeat (measured in milliseconds).
HRV is seen as a way to identify imbalances in the autonomic nervous system:
Autonomic nervous system (ANS). A part of the nervous system thats works without conscious input. Regulates a lot of bodily functions including digestion, breathing, even sexual arousal. Made up of several divisions:
Sympathetic nervous system (SNS). Stimulates the body’s flight or fight response. Kicks in when under threat to raise heart rate (among other things)
Parasympathetic nervous system (PNS). Often called the ‘rest and digest’ system as it works to slow the heart rate and increase intestinal activity (among other things).
HRV reflects the constant ‘battle’ between the SNS and PNS. A high HRV is good indicating that the body is ready and able to adapt to sudden environmental changes. A low HRV is bad, suggesting something is subduing one or more parts of the ANS leading to less beat-to-beat variability.
Measuring HRV has become more accessible with the development of wearable and PPG technology. But wearables using this tech are actually only providing a surrogate measure for HRV, called pulse rate variability:
Pulse rate variability (PRV). A measure of the variation in the pressure waves generated by the heart and conducted to the measurement site (wrist or finger) by the blood vessels.
Research has suggested that PRV can only provide an accurate estimate for HRV in health young people...
👊 Impact
Modifiable
Exercise, good nutrition and high quality sleep are all associated with higher HRV.
Stress is associated with lower HRV (which is why some think it could be used to evaluate psychological health).
Non-modifiable
Genetics, age and gender all have an impact. HRV declines with age and is slightly higher in males than females of the same age.
📚 History
HRV has been kicking around for decades. Here’s a quick rundown of some key milestones:
1965 - Clinical relevance first appreciated when researchers found foetal distress was preceded by alterations to the baby’s HRV
1970s - HRV is used to detect autonomic neuropathy in diabetic patients (where prolonged high sugar levels causes nerve damage)
1977 - Researchers discover that there is a higher risk of death in those who’ve had heart attacks where the HRV is reduced.
1980s - Further evidence is published of a strong and independent relationship between HRV and mortality after heart attacks.
Despite all this, HRV still isn’t widely used in clinical practice. Perhaps reflecting a preference for more established, better evidenced biomarkers and investigations.
💼 Use cases
The ability of popular wearables to measure HRV has put renewed focus on HRV as a useful biomarker. Although the evidence base is still emerging, multiple use-cases are being investigated and proposed.
Consumer
Readiness. HRV is commonly used by wellness and lifestyle brands to provide a measure of ‘readiness’ . A low HRV might prompt advice for the user to ‘take it easy’ while a high HRV might encourage someone to tackle difficult tasks.
Clinical
Depression. Research suggests those with major depression have lower HRV compared to healthy controls which normalises once antidepressant therapy is started.
Anxiety. Research has also shown anxiety disorders are associated with significant reductions in HRV.
Mortality. In an elderly cohort, a lower HRV was associated with higher risk of death. In patients who’d suffered a heart attack, lower HRV was associated with a four-fold higher risk of death over the next 3 years.
Diabetes. Studies have shown that HRV is lower in type 2 diabetics. Higher HbA1c levels correlate with lower HRV (possible reflecting increased nerve damage).
Cancer. HRV might have a role in predicting breast cancer survival as well as cancer survival more generally.
Sports science. Evidence suggests there is a dose dependent impact of exercise on HRV.
👥 Players
HRV is yet to become an established clinical biomarker. It currently remains a prominent tool in the arsenal of various fitness, performance and lifestyle wearable brands. Here’s a few:
Firstbeat. A sensor and digital platform for HRV monitoring (provides corporate and pro sport solutions)
Whoop. A fitness wearable that measures HRV (among other metrics). Used to guide exercise regimes.
EliteHRV. A sensor and app for HRV monitoring (suggests HRV can provide a ‘snapshot’ of autonomic nervous system function).
Ithlete. Awful name. Provides sensors and apps for HRV monitoring (focuses on athletic performance).
Oura. Probably the only ring wearable available. Monitors HRV amongst other metrics to provide a readiness score and help evaluate sleep quality.
Fitbit. Requires no introduction. Has started funding research into HRV and the insights it can provide.
🔮 Predictions
Predictive capabilities. Machine learning techniques will improve the predictive accuracy of HRV in specific disease states. This will move HRV from a consumer device metric to a useful clinical tool.
PRV accuracy. There are concerns about the ability of PPG signals to accurately reflect HRV. Although acceptable for consumer lifestyle products, clinical use cases will need higher accuracy. PPG technology will probably improve to allow for more widespread use of PRV in clinical scenarios.
Self care. Use of HRV to detect early signs of depression, worsening diabetic control, and other problematic declines will become more common. Changes in HRV will serve as a prompt to intervene at an earlier stage in a disease process.
Employee wellness. Companies are looking for solutions to monitor their workforce (health and otherwise) particularly now ‘work from home’ is more common. HRV might be one such measure to track employee wellbeing.
🌅 Opportunities
Behaviour change. Building on the approaches of fitness trackers, HRV could serve as a useful behaviour change tool, encouraging users to explore mindfulness, meditation, better sleep and increased physical activity.
Measurement techniques. Not all wearables are using PPG technology to measure HRV. The Apple Watch is now able to perform a single lead ECG for the purposes of detecting an irregular heart rhythm. The same approach could be used for HRV and might be more accurate.
More research. The jury is still out on whether monitoring HRV over the longer term can change health outcomes. Longitudinal population studies will help unearth whether theres a meaningful role for HRV beyond a healthy adult population.
Insights. A huge quantity of continuous HRV data is being collected by a bunch of different companies. They should make this data available for researchers to accelerate the research effort.
🔗 Links
This Lancet paper (funded by Fitbit) which probably represents the largest analysis of HRV measurements collected via PPG based wearables
This Harvard Health blog that serves as a useful primer for the possible role of HRV in wellbeing