The term heart rate variability, or HRV for short, is gaining notoriety lately. The definition is quite intuitive to the name — it’s a measure of the variability between heartbeats, a function controlled by your autonomic nervous system. The autonomic nervous system (ANS) has two parts: the sympathetic nervous system (fight or flight or freeze) and the parasympathetic nervous system (rest and digest).
What’s the connection between stress and heart rate variability?
When we get stressed, our heart rate, breathing, and blood pressure go up so we can quickly respond to a threat. Similarly, when we are calm and not in the presence of danger, our heart rate, breathing, and blood pressure should be at a comfortable resting rate — which is the body’s way of maintaining the balance between thriving and surviving over time. Along with the autonomic nervous system, heart rate variability is affected by the vagus nerve. The vagus nerve activates the parasympathetic nervous system and is kind of like a button you can press to reduce stress.
Heart rate variability is a metric of how your body is recovering from stresses throughout the day and an evaluator of overall fitness. An even more straightforward definition is that HRV is a metric of our well-being. Analysis of heart rate variability could be the next step to getting your well-being under control.
What does low heart rate variability mean? What about high HRV?
It may feel a bit counterintuitive, but low HRV indicates that we are not recovered — your body isn’t bouncing back from stress and is spending too much time in fight-or-flight mode. The goal is to increase parasympathetic activity; AKA spend a lot more time in rest-and-digest. Sounds nice, right?
A high HRV means the variance between heartbeats is more significant and predicts more consistent performance and better recovery from stress. In seeking high HRV, we’re really after balance in the autonomic nervous system. Understanding the connection between our heart rate variability and our nervous system requires tracking small changes (milliseconds) in the intervals between consecutive heartbeats, also called R-R intervals. Understanding R-R intervals is different from tracking heart rate, which averages only the number of beats per minute.
Having consistently low heart rate variability indicates that our bodies are not adapting to or recovering well from stress. This could mean that we aren’t sleeping well, we’re overworked, emotionally depleted, getting sick, or all of the above. It’s generally that feeling that we’re running on empty. As described by Drs. Lehrer & Gevirtz in their comprehensive 2014 review article on HRV biofeedback, those of us with consistently low HRV have a higher likelihood of developing insomnia, chronic pain, cardiovascular illness, and anxiety-related disorders, to name just a few. A lifetime of chronic stress resulting in low HRV can be dangerous to the cardiovascular system, leading to heart disease, heart attack (myocardial infarction), and, ultimately cardiac death. Mental stress isn’t our friend.
What does my heart rate variability say about my heart health?
Healthy individuals with a high HRV tend to have improved focus, access to calm, performance (athletic and otherwise), breathing, pain tolerance, blood pressure, and resilience. Regular physical activity and fitness certainly indicate a healthy heart and a healthy heart = high HRV. Tracking resting heart rate can also indicate fitness and well-being (a lower resting heart rate would indicate strong overall fitness).
Heart Rate Variability is one of the most reliable, non-invasive ways to monitor the body’s balance between parasympathetic activity (as measured by high HRV, low respiratory rate, and low heart rate) and sympathetic activity (as measured by reduced HRV, elevated heart, respiratory rates, and high blood pressure) systems. The vagus nerve and parasympathetic nervous system are where you’re seeking balance and an increased HRV and vagal tone.
How do I understand my HRV measurements?
The gold standard, and most common in clinical use, is an electrocardiogram, or ECG. ECG is the most accurate means of measuring, but more recently, there are HRV trackers you can use at home for less precise measurements. There are wearables for your chest, wrist, ankles, and fingers that all collect HRV data. Awareness of your heart rate variability and engaging in HRV analysis can be a motivator for behavioral change. We all like to get good grades, right?
Once you dive into heart rate variability, there’s a lot to discover, like RMSSD (a measure of high-frequency HRV), interbeat intervals (the time interval between individual beats), SDNN (standard deviation of NN intervals which Apple uses to measure HRV), and VLF (very-low-frequency band in HRV). Oof, we won’t go on. The most important thing to know as a general rule is that a greater high-frequency HRV (HF-HRV) generally indicates higher parasympathetic rest and digest activity and greater low-frequency HRV generally indicates more sympathetic fight or flight activity. When wearables tell you they measure HRV, they’re typically talking about HF-HRV, but it’s good to look it up just to be certain you know what you’re dealing with. The world of tracking HRV is fascinating and seemingly endless. At the end of the day, it’s a great way to understand your well-being and quite possibly, the limits of your human potential.