Several clinical and animal studies have demonstrated that pulsatile perfusion is more beneficial than nonpulsatile perfusion during short or long durations of extracorporeal circulation. Other investigators, however, have been unable to document these benefits. The issue remains controversial. Central to the debate is the issue of a precise definition of pulsatile flow. To help resolve the conflict, pulsatile flow may be quantified in terms of energy equivalent pressure. This formula contains both the arterial pressure and pump flow rate, which are the 2 most critical parameters for open heart surgery. This definition establishes common criteria for assessment of the effectiveness of extracorporeal support.