Wednesday, January 19, 2022

Heart Rate Recovery Predicts... What?

An entire health assessment industry evolved out of one scientific journal article published in the prestigious New England Journal of Medicine in October 1999. The title was “Heart-Rate Recovery Immediately after Exercise as a Predictor of Mortality.” It was submitted by five people from the renowned Cleveland Clinic Foundation, and since then its publication has been cited by 1,145 science journal articles, meaning that it attracted and continues to attract a lot of attention. What, exactly, did it report, and how was that interpreted within and outside the medical community?

The value for the recovery of heart rate was defined as the decrease in the heart rate from peak exercise to one minute after exercise stopped. Why one minute? Standard exercise tests such as a treadmill test are used to assess cardiovascular health by gradually increase exercise intensity to a level of vigorous exercise, and then drop to a near-resting state, i.e., slow walking. What the researchers did was record for 2,428 middle-aged people how much their heart rate dropped at the end of one minute, and then followed their health for an average of six years. An abnormal value for the recovery of heart rate was arbitrarily defined as a reduction of 12 or fewer beats per minute (bpm) from the heart rate peak; 26 percent had recovery of less than or equal to 12 bpm, but in the follow-up period, these people accounted for 56 percent of the deaths. Incrementally, above a recovery of 15 or more bpm there was no improvement in reducing risk of death for higher values. Lastly, people who were more physically fit were less likely to have an abnormally low bpm recovery.

Smart watches provide many functions, including
heart rate and heart rate recovery at one minute.
These results led to an explosive popularity of the heart rate monitor industry, first as a device strapped around one’s chest, but evolving to wrist devices such as Apple Watch and recently to the Oura Ring. The newer devices send data to your phone, pad or personal computer. The concept behind all this heart rate fascination is a leapt-to conclusion that being more physically fit reduces risk of death (true), and that fitness can be assessed by heart rate recovery (maybe). The real question may be whether – if faced with a low heart rate recovery – to start exercising more, or make sure your will is up to date?

Exercise is associated with increased sympathetic and decreased parasympathetic nervous system activity, two components of the autonomic nervous system. The period of recovery after vigorous exercise is characterized by a combination of sympathetic signals decreasing and parasympathetic reactivation. To other than researchers in exercise physiology, one minute of recovery feels like the time it takes for fast breathing to slow and the sense of acute fatigue to temper. Which feels like a fitness measure that makes sense. HOWEVER, a research article published in 2018 assessed heart rate recovery (HRR) at 10, 20, 30, 40 and 50 seconds in 40,727 middle-aged adults who had no history of cardiovascular disease. Subjects were followed for an average of six years. Only HRR at 10 seconds remained predictive of both all-cause and coronary artery disease mortality. This fast timing suggests that renormalization of the autonomic nervous system signals is a better predictor of heart health than the slower stuff we subjectively feel: the slower breathing and lessening fatigue, associated with recovering from oxygen dept and lactic acid build-up.   

So, back to question of whether exercise can improve heart rate recovery and reduce risk of death. The answer appears to be the former. One study looked at people who had had a heart attack. Exercise training improved HRR, and increases greater than 12 bpm had better cardiac survival. In another, for people in cardiac rehabilitation who started with a HRR averaging less than 12 bpm, 41 percent raised their HRR above 12 and had reduced risk of death compared to those who did not. This still begs the question of what is going on in people who take up exercise but do not have an improvement in heart rate recovery. Is autonomic nervous system senescence irreversible?

The term “heart rate variability” (HRV) comes up in this research arena. The autonomic nervous system, drives heart rate higher in times of physical or emotional stressors and lower when stress is over. A larger range is described as having a high HRV. Neurodegenerative diseases that present as mild cognitive impairment, progressing to dementia, may include damage to the neural networks controlling the autonomic nervous system, and by doing so, lower HRV. High HRV is associated with better cardiovascular health, and interestingly, with sleep health, which is also dependent on the autonomic nervous system. A common denominator here is that exercise improves cardiovascular health and sleep quality, and some evidence that exercise is of benefit to people with mild cognitive impairment or dementia. From all this, it is possible that aging of the autonomic nervous system is a driving force behind diseases of aging.

How much exercise is enough? The consensus is that a large fraction of total health benefits is reached with 150 minutes per week of moderate exercise or 75 minutes per week of vigorous exercise. The exercise should be a mix or aerobic and muscle building. More will make one stronger and faster, but that is a sports performance benefit, not a health benefit.   

Mark says that during his peak bicycling years – more than 3,000 miles per year – he had handlebar devices measuring average and maximum speed, distance traveled and heart rate. In later years he took all that off, to spend more time looking around and less looking down.


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