The Cardiac Cycle

It is generally accepted that when identifying the periods of the cardiac cycle, they are referred to as (ventricular) diastole and (ventricular) systole. It must be noted that both the ventricles and atria undergo the same electrophysiologic events.

Start with the P wave of the EKG

  1. Atrial Systole - The P wave signals the beginning of atrial systole which is followed by atrial contraction (atrial systole), also called the "A Kick", which adds an additional 20-25% more volume to the ventricles. This short period is call end-diastole where the ventricles have been completely filled.
  2. Isovolumic Contraction - The QRS begins the depolarization of the ventricles. Closure of the atrioventricular valves (mitral and tricuspid) occurs as the pressures in the ventricles begin to rise. There in no blood flow occurring, yet.
  3. Ejection - Pressure has risen in the ventricles to overcome the diastolic pressures holding the semi-lunar valves (aortic and pulmonic) closed, and blood begins ejection into the pulmonic and systemic circulation.
  4. Proto-Diastole - Forward motion of blood begins to wane and ventricular pressures begin to fall. The is a slight retrograde flow of blood which closes the semi-lunar valves.
  5. Isovolumic Relaxation - The aortic and pulmonic valves close and the ventricles begin to relax. There is no blood flow at the period. Ventricular repolarization begins and the T wave occurs around this period.
  6. Passive Filling - Ventricular pressure has fallen below the pressure in that atria and the atrioventricular valves (mitral and pulmonic) open and blood flow moves passively into the ventricles.
  7. Diastasis - The pressure in the atria and ventricles have equalized and there is a reduction in blood flow into the ventricles.

Then, like every cyclical event, it all starts over again.