Heart Rate Variability: What is it good for?

This data point has been on trend in the last few years, thanks largely to its increased availability from the multitude of fitness devices now available. What does it mean, and is it useful?

What is HRV?

Heart rate variability (HRV) reflects tiny fluctuations in the beat to beat timing of each cardiac cycle. The heart rate is regulated by nerves in our autonomic nervous system (ANS), which includes the sympathetic nervous system (SNS, fight or flight, correlated with “low frequency” HRV signals) and the parasympathetic nervous system (PNS, rest and relax, often correlated with “high frequency” HRV signals). Higher PNS activity leads to higher HRV. Higher SNS activity reduces HRV. Poor sleep, alcohol, stress, anxiety and depression reduce HRV as well. Interestingly, one study indicates that GLP-1 receptor agonists (similar to how drugs like Ozempic work) may downregulate the parasympathetic tone and decrease HRV (caveats: this study was in mice, didn't use any of the GLP-1 pharmaceuticals available for humans, and applied the study drug directly into their brains…so it was both disturbing and not exactly representative of current drugs).

How is HRV measured?

The most precise way to measure HRV is with an electrocardiogram, however chest strap heart monitors are considered quite reliable. Photoplethysmography (PPG) is commonly used by watches (that’s the green dot on the underside of your smart watch) but the accuracy tends to vary by manufacturer. Time of day can affect HRV, and HRV tends to be highest at night. Most devices, such as consumer devices like the Garmin 965 that I use, measure HRV at night or early in the morning over a set period of time and uses a statistical operation called RMSSD (root mean square of successive differences). The data is often reported in the form of a stress or recovery score. One recent study suggests a high correlation in resting conditions between PPG derived HRV from a Garmin watch and ECG derived HRV.

What is the relationship between training and HRV?

In general, more training over time = increased HRV. Studies have shown that endurance athletes have a higher HRV than sedentary healthy individuals. However, overtraining can lead to reduced HRV. The degree of impact on HRV is influenced by the duration, intensity, and even modality of training. HRV can reflect recovery as well- in a well recovered person, the HRV is higher.

It is important to note that so many non-modifiable factors appear to affect HRV- such as gender, age, genetics, etc. Everyone appears to have their own HRV “set point” and deviations from that range for that individual are more important than the raw number compared between individuals.

Some data suggest that using HRV to guide training may be useful. Generally this means reserving moderate and high intensity activity for days when the athletes’ HRV is in their normal range. When applied this way, some athletes had similar results to controls despite less intense training over the study interval.

Hot take

I admit it. I was a skeptic. My biases were on full display. Having not been part of the curriculum for my core medical and cardiology training, I initially dismissed heart rate variability as a silly, possibly even misleading indicator. My skepticism came largely from a desire to simplify the tasks of patients in front of me- “Don’t worry about all that stuff, just get moving.” I think this is still true for the large majority of people, but after taking a closer look, I am intrigued by what we can learn from this data point. It probably shouldn’t govern what we do, but there seems to be plenty of evidence that it is a useful signal to pay attention to. All the more enticing is the fact that the data is readily available and reasonably accurate in devices we already use.

Acknowledgements/References

• “Practices and Applications of Heart Rate Variability Monitoring in Endurance Athletes”- Lundstrom et al

• www.garmin.com

• Heart Rate Variability: An Old Metric with New Meaning in the Era of using mHealth Technologies for Health and Exercise Training Guidance. Part One: Physiology and Methods- Singh et al