A very long time ago, when I ran clinical labs, one of the most ordered tests was the “sed rate” (aka ESR, the erythrocyte sedimentation rate). Easy, quick, and low cost, with high sensitivity but very low specificity. If the sed rate was normal, the patient probably did not have an infectious or inflammatory disease. If it was elevated, they probably did, but no telling what. Later, the C-reactive protein (CRP) test came into common use. Same general inferences: If the CRP was low, the patient was unlikely to have an inflammatory process; if high, they were sick, but we didn’t know what with.
Could the heart rate variability (HRV) score come to be thought of similarly? Much as the sed rate and CRP are sensitivity indicators of infectious or inflammatory diseases, might the HRV score be a sensitivity indicator for nervous system (central and autonomic) and cardiovascular (especially heart rhythm) malfunctions?
A substantial and relatively old body of heart rhythm literature ties HRV alterations to posttraumatic stress disorder, physician occupational stress, sleep disorders, depression, autonomic nervous system derangements, various cardiac arrhythmias, fatigue, overexertion, medications, and age itself.
More than 100 million Americans are now believed to use smartwatches or personal fitness monitors. Some 30%-40% of these devices measure HRV. So what? Credible research about this huge mass of accumulating data from “wearables” is lacking.
What is HRV?
HRV is the variation in time between each heartbeat, in milliseconds. HRV is influenced by the autonomic nervous system, perhaps reflecting sympathetic-parasympathetic balance. Some devices measure HRV 24/7. My Fitbit Inspire 2 reports only nighttime measures during 3 hours of sustained sleep. Most trackers report averages; some calculate the root mean squares; others calculate standard deviations. All fitness trackers warn not to use the data for medical purposes.
Normal values (reference ranges) for HRV begin at an average of 100 msec in the first decade of life and decline by approximately 10 msec per decade lived. At age 30-40, the average is 70 msec; age 60-70, it’s 40 msec; and at age 90-100, it’s 10 msec.
As a long-time lab guy, I used to teach proper use of lab tests. Fitness trackers are “lab tests” of a sort. We taught never to do a lab test unless you know what you are going to do with the result, no matter what it is. We also taught “never do anything just because you can.” Curiosity, we know, is a frequent driver of lab test ordering.
That underlying philosophy gives me a hard time when it comes to wearables. I have been enamored of watching my step count, active zone minutes, resting heart rate, active heart rate, various sleep scores, and breathing rate (and, of course, a manually entered early morning daily body weight) for several years. I even check my “readiness score” (a calculation using resting heart rate, recent sleep, recent active zone minutes, and perhaps HRV) each morning and adjust my behaviors accordingly.
Why monitor HRV?
But what should we do with HRV scores? Ignore them? Try to understand them, perhaps as a screening tool? Or monitor HRV for consistency or change? “Monitoring” is a proper and common use of lab tests.
Some say we should improve the HRV score by managing stress, getting regular exercise, eating a healthy diet, getting enough sleep, and not smoking or consuming excess alcohol. Duh! I do all of that anyway.
The claims that HRV is a “simple but powerful tool that can be used to track overall health and well-being” might turn out to be true. Proper study and sharing of data will enable that determination.
To advance understanding, I offer an n-of-1, a real-world personal anecdote about HRV.
I did not request the HRV function on my Fitbit Inspire 2. It simply appeared, and I ignored it for some time.
A year or two ago, I started noticing my HRV score every morning. Initially, I did not like to see my “low” score, until I learned that the reference range was dramatically affected by age and I was in my late 80s at the time. The vast majority of my HRV readings were in the range of 17 msec to 27 msec.
Last week, I was administered the new Moderna COVID-19 Spikevax vaccine and the old folks’ influenza vaccine simultaneously. In my case, side effects from each vaccine have been modest in the past, but I never previously had both administered at the same time. My immune response was, shall we say, robust. Chills, muscle aches, headache, fatigue, deltoid swelling, fitful sleep, and increased resting heart rate.
My nightly average HRV had been running between 17 msec and 35 msec for many months. WHOA! After the shots, my overnight HRV score plummeted from 24 msec to 10 msec, my lowest ever. Instant worry. The next day, it rebounded to 28 msec, and it has been in the high teens or low 20s since then.
Off to PubMed. A recent study of HRV on the second and 10th days after administering the Pfizer mRNA vaccine to 75 healthy volunteers found that the HRV on day 2 was dramatically lower than prevaccination levels and by day 10, it had returned to prevaccination levels. Some comfort there.
Another review article has reported a rapid fall and rapid rebound of HRV after COVID-19 vaccination. A 2010 report demonstrated a significant but not dramatic short-term lowering of HRV after influenza A vaccination and correlated it with CRP changes.
Some believe that the decline in HRV after vaccination reflects an increased immune response and sympathetic nervous activity.
I don’t plan to receive my flu and COVID vaccines on the same day again.
So, I went back to review what happened to my HRV when I had COVID in 2023. My HRV was 14 msec and 12 msec on the first 2 days of symptoms, and then returned to the 20 msec range.
I received the RSV vaccine this year without adverse effects, and my HRV scores were 29 msec, 33 msec, and 32 msec on the first 3 days after vaccination. Finally, after receiving a pneumococcal vaccine in 2023, I had no adverse effects, and my HRV scores on the 5 days after vaccination were indeterminate: 19 msec, 14 msec, 18 msec, 13 msec, and 17 msec.
Of course, correlation is not causation. Cause and effect remain undetermined. But I find these observations interesting for a potentially useful screening test.
George D. Lundberg, MD, is the Editor in Chief of Cancer Commons.
A version of this article first appeared on Medscape.com.