The cardiovascular parameter which is the best indicator of vagal tone?

There are central influences for hea rate. Medullary centers are cardioinhibitory center and vasomotor centre. There is a constant discharge of impulses from CIC through the vagus which influences the activity of the hea.Continuous stream of impulses from CIC to hea is responsible for vagal tone which is very prominent in humans. Ref: Textbook of medical physiology by N Geetha, 2nd edition, page no. 161,162