Understanding Heart Rate Variability (HRV): A Simple Guide

What is Heart Rate Variability (HRV)?

Heart Rate Variability (HRV) is a measure of the variation in time between each heartbeat. This variation is controlled by the autonomic nervous system, which has two main branches:

• The sympathetic nervous system, which prepares the body for stress or physical activity ("fight or flight" response).
• The parasympathetic nervous system, which helps the body to rest and recover ("rest and digest" response).

It's important to understand that different HRV metrics can provide different insights:

• RMSSD (Root Mean Square of Successive Differences) is the most used metric and sometimes it’s directly referred with the misnomer HRV: Higher values generally indicate better cardiovascular health and resilience to stress.
• LF (Low Frequency)and LF/HF (Low Frequency to High Frequency ratio): Higher values can sometimes indicate increased sympathetic activity and stress, although interpretations can vary depending on the context and individual circumstances.

Therefore, while some metrics like RMSSD suggest that higher HRV values are associated with better health, other metrics like LF and LF/HF may indicate higher stress levels when their values are elevated. Furthermore, note that HRV metrics are highly subjective and it’s best to compare such metrics with personal trends and variations over time, not comparing to other users.

How Do We Measure HRV?

HRV can be measured using two primary methods: Electrocardiography (ECG) and Photoplethysmography (PPG).

1. Electrocardiography (ECG):
   • ECG is the gold standard for measuring HRV.
   • It involves attaching electrodes to the skin to detect the electrical activity of the heart.
   • The ECG records the timing of each heartbeat with high precision, allowing for accurate calculation of HRV.
   • The intervals between consecutive R-waves (known as R-R intervals) are used to assess HRV.
2. Photoplethysmography (PPG):
   • PPG is a less invasive and more convenient method.
   • It uses a light source and a photodetector to measure blood volume changes in the microvascular bed of tissue.
   • Commonly used in wearable devices like smartwatches and fitness trackers.
   • While less precise than ECG, PPG provides a practical way to monitor HRV in daily life.

Differences Between ECG and PPG:

• Accuracy: ECG is more accurate than PPG due to its direct measurement of heart electrical activity. PPG, while convenient, can be influenced by factors like movement and skin perfusion.
• Convenience: PPG is more user-friendly and can be incorporated into wearable devices, making it suitable for continuous monitoring.

How Do We Measure Stress Using HRV?

HRV is a valuable indicator of stress. Here's how it works:

• When the body is stressed, the sympathetic nervous system becomes more active, reducing RMSSD.
• Conversely, when the body is relaxed, the parasympathetic nervous system dominates, increasing RMSSD.

Due to the potential for lower accuracy of HRV measurements caused by movement and other factors during the day, it is recommended to measure HRV metrics during the night when the body is at rest. The Resilient app follows this best practice by computing HRV metrics based on night time data. If the user does not wear the watch during the night or fails to sync the Resilient app, HRV metrics will not be computed.

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Practical Applications:

• Wearable Devices: Many modern wearables can track HRV to provide insights into stress levels, recovery, and overall well-being.
• Health and Wellness: HRV monitoring can be used to identify and manage stress, improving focus, energy and preventing burnout.

Sources:

1. Shaffer, F., & Ginsberg, J. P. (2017). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health, 5, 258. Link
2. By Yitzhak Nat - Own work + ECG wave from, CC BY-SA 4.0. Link
3. Altini M. (2021). Heart Rate Variability (HRV) trends: going beyond daily scores. Medium. Link