Heart rate variability (HRV) is the variation in time between consecutive heartbeats. The acceleration and deceleration of the heart rate reflects the body’s ability to self regulate and maintain homeostasis. When we exhale, our heart rate decelerates and the parasympathetic branch becomes active. This naturally occurring phenomenon is called the Respiratory Sinus Arrhythmia, or RSA. Heart coherence reflects the pattern of the heart rate. HRV can be derived from ECG or Pulse (PPG). HRV is often combined with measurement of respiration.
To measure HRV, one needs to measure heart rate. This can be done with BVP or ECG, with BVP being easier to apply and ECG being more specific. Heart rate is often expressed in beats per minute [BPM]. There are different ways to quantify HRV, which can be grouped into Time domain methods and Frequency domain methods.
Time domain: SDNN or peak-to-through value (maximum – minimum over a time frame). Frequency domain: Power spectral analysis: The FFT algorithm decomposes the heart rhythm into the different frequencies which the heart rhythm is composed of and displays this in a graph. We distinguish the High Frequency (HF), Low Frequency (LF) and the Very Low Frequency (VLF).
Next, to illustrate RSA, one needs to measure respiration as well. We express the RSA by the correlation between the heart rate pattern and the breathing pattern, which is shown as a value between -1 and 1. -1 is a maximum coherence but out of phase (one goes up while the other goes down), 0 is no coherence and 1 means a maximum coherence with both signals going up and down at the same time. The goal is to reach a value going up to 1.