Today, most wearable health trackers measure heart rate continuously. From these continuous heart rate readings, we can measure Heart Rate Variability (HRV). HRV provides insights into how your heartbeats change over time and is now a crucial measure of overall well-being and physical fitness.
What is HRV?
Your heart rate measures the number of times your heartbeats per minute, which typically ranges from 60 - 100 beats per minute (bpm) at rest. However, your heart rate doesn’t stay at a constant speed. It changes based on your activity and even emotions. For example, even a reading or a heart rate at rest of 60 bpm, doesn’t necessarily mean that your heart only beats once per every second. In reality, there may be a half-second between some heartbeats and two seconds between others. HRV measures these beat-to-beat variations.
To better understand this, take a look at the echocardiogram (ECG) below.
An ECG is a standard test that doctors use to diagnose multiple heart issues. Each heartbeat results in a peak on an ECG referred to as an R peak. The time (typically in milliseconds) between each R peak is known as an RR interval and demonstrates the time between each heartbeat. The HRV measures the variation in time between RR intervals within a specific time frame, usually within 2-5 minutes.
There’s a few ways to calculate HRV, and the most common use the timing of RR intervals. At Basis, we use SDNN, which calculates the standard deviation of all RR intervals over a specific time frame. A different method, the RMSSD, analyzes the root mean square of successive RR intervals while the PNN50, divides the number of consecutive intervals that differ by more than 50 milliseconds by the total number of RR intervals.
The SDNN is the "gold standard" for medical stratification of cardiac risk when recorded over a 24 h period (Source). SDNN values predict both morbidity and mortality. Based on 24 h monitoring, patients with SDNN values below 50 ms are classified as unhealthy, 50–100 ms have compromised health, and above 100 ms are healthy (Source). Heart attack survivors, whose 24 h measurements placed them in a higher category, had a greater probability of living during a 31-month mean follow-up period. For example, patients with SDNN values over 100 ms had a 5.3 times lower risk of mortality at follow-up than those with values under 50 ms (Source).
What causes HRV?
Our bodies can change our heart rate from second to second depending on different stimuli. This is mediated by the autonomic nervous system (ANS), which automatically regulates the functions of our internal organs, such as the heart and stomach, without you having to think about it. Physical and mental stimuli directly affect ANS activity.
There are two branches of the ANS that signal to the heart: the parasympathetic and sympathetic nervous systems. The parasympathetic nervous system carries signals from the brain that prompt relaxation and digestion, known as the “rest and digest” response. This system slows down the heart rate. The sympathetic nervous system is on the opposite spectrum and carries signals from the brain to prepare our bodies for stressful situations, known as the “fight or flight” response. This system speeds up the heart rate.
Imagine taking a relaxing hike and suddenly coming across a bear. Your sympathetic nervous system will immediately turn on, raising your heart rate and diverting blood flow to your legs so that you can quickly escape. Once you have run to safety, the parasympathetic system will lower your HR and return your body to its normal resting state.
You are exposed to many different stimuli throughout the day, resulting in both branches competing to send signals to your heart. This constant competition is what causes HRV. A high HRV indicates a balanced ANS, meaning that your body quickly responds and adapts to different situations. However, in some people, such as those with chronic physical or mental stress, the balance is disrupted, and the sympathetic nervous system is constantly active, even at rest. While this is normal and beneficial for brief periods, such as while exercising or running away from danger, long-term imbalances can lower HRV.
How does HRV apply to overall health?
A low HRV indicates that the body is overworked, has fewer resources available for handling different situations, and cannot adapt well to changes in the environment. Research suggests that a lower HRV is an indicator of poor health and a risk factor for both physical and mental disorders. Specifically, a low HRV is associated with heart disease, inflammation, anxiety, post-traumatic stress disorder, depression, and increased risk of death in the elderly and following a heart attack.
In contrast, higher HRV indicates greater overall health, self-regulation, and psychological well-being. High HRV is also associated with greater athletic performance, recovery, and prevention of overtraining and is linked to better chances of survival in cancer patients. Therefore, it is important to monitor your HRV and take any necessary preventative measures.
How can you improve your HRV?
Many factors affect HRV. Some of these are biological, such as age, gender, genetics, illness, and hormone cycles, while some are related to lifestyle, such as exercise, diet, hydration, alcohol intake, sleep, and stress.
While some of these factors are out of our control, lifestyle changes can increase your HRV, such as:
So, what's a healthy HRV?
Unfortunately, defining a good HRV is complicated as it is unique to each person. HRV is highly influenced by age. A study of 260 healthy subjects found that HRV decreases rapidly with age when measuring HRV using RMSSD and PNN50. SDNN showed a more gradual decrease in HRV, from 153 milliseconds in people in their twenties to 121 milliseconds in people in their sixties. HRV is also influenced by gender, specifically in those between 35 and 54 years. Within the same age groups, women typically have higher HRV values for RMSSD and PNN50 than men. However, men and women have similar SDNN values.
It's most meaningful to compare your HRV to your recent history. To do this, use a wearable that tracks your heart-rate continuously throughout the day. Make sure to establish a baseline HRV range over a few weeks. Then, note how your HRV changes over time or with certain lifestyle changes. It's important to establish a baseline during a week that you know you're eating healthy, meeting the minimum of 150 minutes of moderate intensity activity per week and are not sick. (In Basis you can coordinate this with your health coach for more accuracy).
Once you've established the baseline, you can start monitoring HRV as a predictor of health. For example, a drop in HRV may mean that you are stressed, experiencing an illness, or have unhealthy behaviors. On the other hand, a rise in HRV may show that recent changes you implemented are improving your overall health and fitness.
HRV is a simple, non-invasive way to help you better understand your overall health, including nutrition, sleep, physical fitness, mental health, and risk of disease. Importantly, HRV monitoring is a preventative tool to motivate healthy behavioral changes.
Looking for more control over your health?
Sign up for the Basis Early Access program - a 1 month discovery into your health data and discover how HRV impacts your health risk alongside one of our Health Coaches.
Or sign up to the Basis 5-day email course into the lifestyle factors you can measure and control that have an immense impact on preventable disease.