Heart rate variability (HRV) provides insight into recovery from illness, injury or exercise, can track levels of physical and emotional stress and even act as a predictor of heart failure. Advances in technology, including imaging sensors, now make HRV measurement accessible to anyone with a chest strap or smartphone.
Among the most common measures, heart rate and respiration are the simplest to take – you only need a watch with a second hand to count. More specialized equipment is needed for body temperature, blood oxygen level and blood pressure, but they are also fairly straightforward. The history of measuring blood pressure dates back 300 years, when the Reverend Stephen Hales stuck tubes in a horse to see how high the column of blood would rise. Today, you just need an arm cuff connected to some electronic device.
While heart rate provides beats per minute, variability indicates the difference in the time gap between those cardiac contractions. Heart rate is closely related to breathing: it speeds up as you breathe in and slows down as you breathe out – and this difference provides a measure of variability. But when the body is tired, the disparity in heart rate between inhalation and exhalation narrows.
HRV is a bit more complicated to capture than traditional metrics because more precise instruments are needed to detect, time, and record heart rate, and then perform statistical analysis to account for variability. Two patients can have exactly the same heart rate (HR) but different gaps (HRV), so accuracy is essential.
Hales had observed the relationship between heart rate and respiration, while the German physician Carl Ludwig later noted that it varied according to the phases of the respiratory cycle. But it wasn’t until the mid-1990s that modern standardized measurements of HRV began to be widely agreed upon, just as research demonstrated the metric’s value as a predictor of mortality after a heart attack. Electrocardiography (ECG) machines are the gold standard in heart monitoring and can provide a measurement of HRV. But they are heavy and expensive.
More advanced semiconductors have enabled better sensors to get accurate readings from the chest strap. In the last decade, advancements have gone so far as to allow the iPhone’s camera and flash to be used to sense blood flow through a fingertip and accurately record pulses.
This ease of use has spurred a host of new apps and devices and an increase in research into how HRV works, what it measures, and how it can be used.
Among the many processes that regulate the human body is the autonomic nervous system, which controls functions including digestion, breathing, and heart rate. Within this system are two branches – sympathetic and parasympathetic – that function as a kind of yin-yang, balancing each other according to the needs of the body.
We tend to think of a “normal” heart rate as around 60 beats per minute (bpm), yet the internal heart rate for humans – when nothing is regulating it – is actually closer to 100 beats/minute. The parasympathetic system lowers it when you rest.
Triggering the ebb and flow between the sympathetic and parasympathetic systems is a simple thing called stress. This loaded term can sometimes be misunderstood and is often associated with psychological issues such as anxiety or fear. But even joyful activities can trigger a stress response if they cause strain: happily bench pressing 100 pounds, running for the bus, or singing in a choir. Injury, illness, lack of sleep, a big night on the town, and the trials of everyday life all trigger reactions in the autonomic nervous system. And HRV can track these changes.
“While heart rate may change only minimally outside of very strong stressors such as illness or excessive alcohol intake, HRV will show a more noticeable change,” Marco Altini, who is trained in both data science and in human movement, and founded the HRV4Training app. he told me recently.
A major obstacle is the collection of accurate and usable data. Other metrics such as temperature or blood pressure can be taken instantly, compared to population averages and acted upon immediately. HRV, on the other hand, is highly individual and requires collecting baseline figures over several days and under similar circumstances each time. Variability taken while drinking morning coffee is not comparable to a measurement taken after dinner.
The widely accepted approach is to take a reading immediately after waking up each morning – usually between one and five minutes. Polar has a chest strap that connects to a variety of apps including the Elite HRV or KubiosHRV, while others like the HRV4Training use the camera and flash to sense heart rate.
Some devices such as Garmin smartwatches, fitness bands from Whoop and Oura smart ring can automatically track HRV during sleep, allowing users to keep tabs without having to set aside time. But consumers should be wary that some smartwatches are more sporadic in their sampling, which can result in inaccurate data.
Incorporating HRV into everyday life remains the biggest challenge. While a range of devices can now provide a daily stress score, users should resist the temptation to over-interpret a single day’s feedback or chase a ‘perfect number’. Doing so can lead to the harmful nocebo effect.
“Using HRV as feedback can help us make meaningful adjustments before we dig ourselves into a hole,” notes Altini. “While in the last decade we got much better at measuring it, in the next decade we can get better at implementing meaningful interventions, starting with the usual suspects: exercise, diet, sleep and forms of attention. ”
While we now have this powerful new healthcare metric at our fingertips, life still can’t be distilled into a single number. Self-care cannot be outsourced to devices and data.
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This column does not necessarily reflect the opinion of the editorial board or of Bloomberg LP and its owners.
Tim Culpan is a Bloomberg Opinion columnist covering technology in Asia. Previously, he was a technology reporter for Bloomberg News.
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