A recent study concludes that “normal” doesn’t exist when it comes to resting heart rate (RHR). Although the RHR for most people is relatively stable over time, rates vary by as much as 70 beats per minute (bpm) from person to person.
Although doctors routinely measure the RHR of people, it is still not clear what constitutes “normal,” and what variations to health mean from this norm.
Over the years there has been a consensus among large studies on what doctors should now consider the normal range.
For example, the National Health and Nutrition Examination Survey, which analyzed data from nearly 50,000 individuals, found that the adult RHR average is 72 bpm.
The American Heart Association (AHA) considers adult natural an RHR of 60–100 bpm.
RHRs, however, which fall well below the normal range can still increase the risk of mortality for a person.
Higher rates are generally associated with increased cardiovascular risk, but one study found that people with an RHR inferior to 65 bpm are also at risk. In the same study, the authors found that a substantial improvement in RHR also increased cardiovascular risk over the 3 year follow-up period.
The results are, in short, mixed. As the authors of the new study explain, “a single heart rate measurement provides very little useful information about an individual’s current health, unless it is well outside the expected normal range.”
The researchers behind the new study took a “long view of individual changes in cardiac output,” rather than taking RHR measurements at specific points in time.
We have now published their findings in the journal PLOS ONE.
Why does RHR matter?
Wearable devices today mean that accurate measurements of the heart rate can be taken continuously. It system also monitors sleep duration and activity levels, in addition to measuring heart rate each second.
As the authors of the study note, the prevalence of these sensors offers a “one-of- a-kind opportunity to better understand how RHR changes over time for and between individuals over the course of days, weeks, years, and potentially lives.”
If these all-pervasive wearable sensors can avoid the onset of disease, this would have a significant impact on population-level healthcare. For that reason, understanding what heart rate information can provide is important.
If we can decipher the secret messages taped by our hands, we may monitor our health constantly and receive early health warnings, theoretically.
The researchers had access to data across 50 states from 92,457 adults. Each participant wore a heart rate monitor between March 2016 and February 2018 for at least 2 days each week, for at least 35 weeks. They wore it every day, for at least 20 hours.
Those data provided a total of 33 million RHR daily values.
The average RHR overall was 65.5 bpm plus, or minus 7.7. For each individual the minimum and maximum RHRs were 39.7 and 108.6 bpm, respectively. In other words, “normal” could vary by around 70 bpm between individuals.
When the researchers delved into the data, a few significant relationships were found. For example, as the authors describe, there was a considerable difference between men’s and women’s RHRs:
“We found that 95 percent of men had an RHR between 50 and 80 bpm, whereas the corresponding range for women was between 53 and 82 bpm.”
The scientists also found significant differences in RHR with respect to age, body mass index (BMI), and duration of sleep. On average, with age, RHR steadily increased, peaked at 50, then a slow decline began into the older age. L
They found that individuals with moderate BMIs tended to have lower RHR than those with low or high BMIs when looking at the relationship between heart rate and BMI.
The scientists found that the lowest RHRs occurred in people who slept for about 7 to 7.5 hours each night, after analyzing the sleep duration.
Interestingly, they also found a person’s RHR tended to change by an average of 2 bpm with the seasons. Explaining:
“The RHR peaked for both men and women in the first week of January, after which the average RHR dropped to the annual minimum in late July. After this minimum, the average RHR has steadily increased until the end of the year.”
Scientists also note that for brief periods, such as a week, some people experienced minor but significant changes in their RHR. These minor alterations tended to be around 3 bpm.
People are simply different
Overall, the researchers showed that these variations were only “minimally correlated with individual characteristics.”
Although the RHR of a person varied with the seasons and for short periods, these changes were “an order of magnitude narrower” than the differences between the different RHRs of different people. Authors of the study write to:
“Individuals have a daily RHR that is normal for them but can differ from another individual’s normal by as much as 70 bpm.“
According to the writers, an “estimated 20 percent of consumers in the[ US] now have a smartwatch or fitness band capable of measuring continuous[ heart rate] passively and unobtrusively over long periods.”
Since exposure to RHR information is now so widespread, it makes sense to use it to its fullest potential. The researchers hope that health professionals will be able to use RHR data in the future to help diagnose conditions such as cardiovascular and pulmonary diseases and give insight into reproductive health.
“The ability to detect early acute diseases, such as infections, and early exacerbations of chronic diseases remains a promising avenue to explore,” the authors conclude.