Cardiac muscles have automaticity and hence are self exciting. Heart beat can be influenced by hormonal as well as stimuli from thesympathetic and vagus stimuli.
The systole is always triggered by impulses from pacemaker or SA Node and it takes 0.3 second regardless of cardiac cycle duration. When heart beat goes up the relaxation phase or diastolic time is reduced causing poor refilling of atrium thereby decreasing the cardiac output.A regular practitioner of pranayama has a positive cascading effect on different systems. It can reduce the heart rate from the normal rate of 72. Lesser heartbeat leads to longer diastolic periods which will allow the atrium to fill fully and better cardiac output.