HRV stands for heart rate variability and refers to the time between contractions of the heart muscle. It is viewed on an electrocardiogram as the spike in the waveform, or "R" wave. Variations would be the differences (if any) in the time between one R wave and the next. It’s sometimes called RR variability or cycle length variability. Heart rate variability may be a predictor of altitude sickness and is one metric physiologists watch closely at the higher elevations.