The most important health metric is now at your fingertips

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For centuries, doctors have relied on a variety of vital signs to quickly assess the well-being of patients. A visit to the ER these days can result in up to five measurements, each offering unique clues about what’s going on inside the complex human body. Recently, however, a new number has emerged that may prove to be the most useful information available for understanding a person’s underlying health condition.

Heart Rate Variability (HRV) offers insight into recovery after illness, injury, or exercise, can track physical and emotional stress levels, and even act as a predictor of heart failure. Advances in technology, including image sensors, now make HRV measurement accessible to anyone with a chest strap or smartphone.

Of the most common measurements, heart rate and respiratory rate are the easiest to take – all you need is 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 quite simple. The history of blood pressure measurement dates back 300 years, when the Reverend Stephen Hales pushed tubes into a horse to see how high the column of blood would climb. Today, you simply need an armband attached to some electronic devices.

While heart rate provides beats per minute, variability shows the change in time interval between these heart contractions. Heart rate is strongly correlated with breathing: it speeds up when you inhale and slows down when you exhale – and this difference provides a measure of variability. But when the body is tired, the difference in heart rate between inspiration and expiration is reduced.

HRV is a bit more complicated to capture than traditional metrics because more precise instruments are needed to detect, time, and record heartbeats, then run statistical analysis to calculate variability. Two patients can have exactly the same heart rate (HR) but different deviations (HRV), so accuracy is crucial.

Hales had observed the connection between heart rate and respiration, while German physician Carl Ludwig later noted that it changed with the phases of the respiratory cycle. But it wasn’t until the mid-1990s that modern standardized measures of HRV began to gain widespread acceptance, just as research underscored the metric’s value as a predictor of mortality after a heart attack. Electrocardiography (ECG) devices are a gold standard in heart monitoring and can provide a measurement of HRV. But they are bulky and expensive.

More advanced semiconductors allowed better sensors to take accurate readings from a chest strap. Over the past decade, advancements have gone so far as to allow an iPhone’s camera and flash to be used to detect blood flowing from fingertips and accurately record pulses.

This ease of use has spurred a slew of new apps and gadgets, as well as 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 such as digestion, respiration, and heart rate. Within this system are two branches – sympathetic and parasympathetic – which 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), but the intrinsic heart rate for humans – when nothing is regulating it – is actually close to 100 bpm. The parasympathetic system lowers it at rest.

Causing 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 linked to psychological issues such as worry or fear. But even joyful activities can trigger a stress response if they cause tension: doing a 100-pound bench press, sprinting to the bus, or singing in a choir. Injury, illness, lack of sleep, a big night out 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 consumption, HRV will show a more marked change,” Marco Altini, who trained both data science and human movement, and founded the HRV4Training app, I said recently.

A major stumbling block is collecting accurate and usable data. Other measurements such as temperature or blood pressure can be taken instantly, compared to population averages, and implemented immediately. HRV, on the other hand, is very individual and requires the collection of benchmark figures over several days and under similar circumstances each time. The variability taken while drinking your 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 one to five minutes. Polar has a chest strap that pairs with a number of apps including Elite HRV or KubiosHRV, while others such as HRV4Training use the camera and flash to detect heartbeats.

Some devices such as Garmin smartwatches, fitness bands from Whoop, and the smart ring from Oura can automatically track HRV during sleep, allowing users to keep tabs without having to set aside time. But consumers should beware that some smartwatches are more sporadic in their sampling, which could lead to inaccurate data.

Integrating HRV into everyday life remains the biggest challenge. While a host of gadgets can now provide a stress score each day, users should resist the temptation to over-interpret single-day feedback or seek out a “perfect number”. This can lead to the deleterious nocebo effect.

“Using HRV as feedback can help us make meaningful adjustments before we dig ourselves into a hole,” Altini notes. “While over the past decade we’ve gotten much better at measuring it, over the next we could better implement meaningful interventions, starting with the usual suspects: exercise, diet , sleep and forms of mindfulness.”

Although we now have this powerful new measure of health care at our fingertips, life still cannot be reduced to single digits. Taking care of us cannot be outsourced to gadgets 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.

More stories like this are available at bloomberg.com/opinion

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