What is the physiological cause of low HRV variability? Again, the autonomic nervous system regulates many cardiovascular parameters, including both heart rate and HRV. Recall that heart rate can be elevated by increasing sympathetic activity or decreased by increasing parasympathetic (vagal) activity. The balance between the effects of the sympathetic and parasympathetic system, the two opposite-acting branches of the autonomic nervous system, is referred to as the sympathovagal balance and is reflected in the beat-to-beat changes of the cardiac cycle (19).