Heart rate variability turns out to be a generalized, deep measure of health. That’s because higher HRV is a strong indicator of resilience to stress, while low HRV is a sign of reduced capacity to tolerate stress. And at the deepest level, health is resilience, and diseases in various ways compromise resilience.~ Todd Becker from, http://gettingstronger.org/2014/07/track-your-hrv-to-boost-adaptive-reserves/
This is a REALLY good article on understanding heart rate variability. Turns out, the MORE your heart rate varies — in terms of the variation of the timing from one beat to the next — the more that indicates good cardiovascular health. I found that idea to be counter-intuitive. I would have guessed, wrongly, that the more regular the heart beats were, the better.