Apple Watch and other fitness trackers have the capability to provide us with information on cardiovascular parameters that reflect the activity of the autonomic nervous system. Measures of that activity reflect the balance between the sympathetic nervous system (which activates fight and flight responses) and the parasympathetic nervous system (which activates "rest and digest" activities) and have been shown to be powerful predictors of mortality.
Most of the attention in this area has been on heart rate variability with various wearables trying to promote it as a surrogate marker for stress. The OURA ring people, for example, without citing evidence that "high heart rate variability is an indication of especially cardiovascular, but also overall health as well as general fitness."
Although unimpressed with the heart rate variability data from the Apple Watch or the , I have recently discovered that I can get a more useful parameter of autonomic nervous system tone from my Apple Watch -- heart rate recovery.
What Is Heart Rate Recovery?
Heart rate recovery (HRR) is the rate of decline in heart rate after the cessation of exercise. Basically, you measure your heart rate right when you stop exercising and again a minute later (and/or 2 minutes later) and subtract one from the other.
Unlike heart rate variability, you don't really need any high-tech devices to make this simple but highly reproducible measurement. You can simply measure your pulse the old-fashioned way by putting a finger on your carotid or radial artery and counting the beats.
What happens to the heart rate during exercise has long been considered to be due to the combination of parasympathetic withdrawal and sympathetic activation.
The fall in heart rate immediately after exercise to be a function of the reactivation of the parasympathetic nervous system. It is accelerated in athletes and blunted in patients with heart failure.
A Predictor Of Mortality
A found that abnormally low HRR was associated with double the risk of dying over 6 years.
The study examined outcomes in 2,428 consecutive adults (mean age 57 years, 63% men) without significant prior cardiac disease who were referred to the Cleveland Clinic cardiac lab for nuclear stress testing. Patients underwent symptom-limited exercise on a treadmill using a standard or modified Bruce protocol.
Heart rate was recorded at peak exercise, and then patients were walking at a speed of 1.5 miles per hour at a grade of 2.5% when heart rate was checked a minute later.
Median HRR was 17 beats per minute, with a range from the 25th to the 75th percentile of 12 to 23 beats per minute. Abnormally low HRR was selected as <13 beats/min and was found in 639 patients (26%).
In univariate analyses, a low value for the recovery of heart rate strongly predicted death, conferring a four-fold increased risk. After adjustment for multiple confounding factors, including age and exercise capacity, patients with HRR <13 beats/min had a two-fold risk of dying.
This 20-year-old study and HRR remain highly relevant. The paper has been cited 1,001 times since publication. Thus far in 2019, 58 papers have referenced it.
In a follow-up this same Cleveland Clinic group looked at nearly 10,000 patients undergoing treadmill ECG testing and found HRR <13 beats/min doubled the 5-year risk of death. In the figure below from that JAMA paper, mortality jumps markedly as HRR drops below 13 and quite dramatically if <10 beats/min.
Subsequent studies from different investigators confirmed that HRR is associated with mortality, independent of workload and myocardial perfusion defects, treadmill risk score, and even after adjusting for left ventricular function and angiographic severity of coronary disease.
There has been a lack of consistency among these studies in stress protocols, activity post-exercise and optimal duration of heart beat measurement post exercise.
This determined that a 2 minute HRR <22 beats/min provided a better cut-point than 1 minute HRR <13 beats/min in predicting mortality at 7 years in male veterans. Individuals underwent maximal treadmill followed by lying down, and those with an abnormal HRR were 2.6 times more likely to die. The HRR was equivalent to age and exercise capacity for predicting death.
HRR with Apple Watch
It's not entirely obvious how to view the heart rate recovery data on your Apple Watch, but it is routinely logged if you record an activity and end it precisely at the end of the activity. To see it, you must leave the activity app and open the .
Scroll to the bottom of the screen and you will see heart rate data on your most recent activity, including the peak heart rate and 1-minute recovery heart rate.
Click on that tab and the full and awe-inspiring graph of your recovery heart rate over 3 minutes is revealed. Here is mine following a 1.5 mile run at 6 to 7 MPH. I did not walk at 1.5 MPH on a 2.5% grade in recovery, which would be needed if one wanted to more carefully compare a personal HRR to the numbers from the 1999 NEJM study.
My data shows a peak heart rate of 121 BPM that dropped to 90 BPM at 1 minute (121-90=31). Two minute recovery is 121-78 or 43 bpm. Both values are within normal limits.
The watch only stores data on your last workout. But if you go to the activity app on your iPhone (something I had never previously done), you will find under the workouts tab a complete listing of all previous workouts.
Click on the workout of interest and all the data from the workout is wondrously revealed, including cadence, pace, and (near the bottom) heart rate changes. Swipe the heart rate changes during exercise to the left, and the heart rate recovery graph is revealed. This time you will have to do the subtraction for yourself.
A Simple, Powerful, And Intuitive Measure of Autonomic Tone
So there you have it. Heart rate recovery (unlike heart rate variability) is a simple parameter, easy to understand and measure. It yields information on your vagal/parasympathetic tone and has been proven to be a powerful and independent predictor of your overall mortality.
It makes more sense to pay attention to HRR if one wants a measure of your body's autonomic tone than heart rate variability.
If your 1-minute HRR is <13 beats or 2-minute HRR <22 beats per minute, this is a bad prognostic sign. If you have not been diagnosed with significant cardiovascular disease, consider seeing a physician for evaluation.
For those who have been sedentary and are deconditioned or overweight, consider an abnormal HRR as a wake-up call to modify your lifestyle and improve your mortality.
For healthy, asymptomatic individuals, the HRR can serve as a marker for your overall cardiovascular fitness. Monitor it along with your exercise capacity, peak heart rate, and resting heart rate to raise your awareness of how your exercise is influencing your overall autonomic nervous system balance.
is a private practice noninvasive cardiologist and medical director of echocardiography at St. Luke's Hospital in St. Louis. He blogs on nutrition, cardiac testing, quackery, and other things worthy of skepticism at , where a version of this post first appeared.