The heart rate (HR) of a healthy individual is irregular. This irregularity, i.e. heart rate variability (HRV), is due to beat-to-beat variation. The autonomic nervous system (ANS) controls the heart's sinoatrial node (SA) which is the pace-making unit of the heart. HRV is known to be a valid indicator of stress.
In calves the use of HRV as an indicator of sympathovagal balance has been validated with autonomic blockade tests. HRV has also proved to be a useful tool for instance in assessing pain in animals
The physical activity and body posture (standing vs. lying down) also affects HRV.