How Should Investigators Compare Different Perfusion Modes or Different Types of Pulsatile Flow During Chronic Support?We read with great interest the article entitled "Hemodynamic and Pressure-Volume Responses to Continuous and Pulsatile Ventricular Assist in an Adult Mock Circulation" by Koenig and associates. 1 The authors investigated the hemodynamic and left ventricular pressure-volume loop responses to continuous versus pulsatile assist techniques at 50 and 100% bypass flow rates during simulated ventricular physiologic and pathophysiologic states in an adult mock circulation. Koenig and associates concluded that hemodynamic responses to continuous and pulsatile assist during simulated heart failure differed from normal and recovery states, and their findings suggest the potential for differences in endocardial perfusion between assist techniques. 1 We have a few comments on this important investigation.The controversy over the benefits of pulsatile perfusion during chronic and acute support still continues because of the lack of understanding of the definition and quantification of arterial pressure and pump flow waveforms. 2-4 Without a precise quantification, it is impossible to make direct and meaningful comparisons between different perfusion modes or different types of pulsatile flow (physiologic pulsatile versus diminished pulsatile flow) during chronic support. [5][6][7] Generation of pulsatile flow depends on an energy gradient. 8 Therefore, the precise quantification of pressure flow waveforms in terms of hemodynamic energy levels is a must, not an option. Readers of any article on pulsatile versus nonpulsatile perfusion should know whether there is any difference in hemodynamic energy levels between different perfusion modes. If there is a difference between the energy levels, then performing additional tests is warranted.To make meaningful comparisons between different perfusion modes, investigators should use a method that takes into account energy differences, such as Shepard's energy equivalent pressure (EEP) formula, to precisely quantify pressure flow waveforms. 8 The EEP formula is based on the ratio between the area beneath the hemodynamic power curve (͐ fpdt) and the area beneath the pump flow curve (͐ fdt) during each pulse cycle:where f is the pump flow rate, p is the arterial pressure (mm Hg), and dt is the change in time at the end of flow and pressure cycles. The unit for the EEP measurement is mm Hg. Therefore, it is possible to make real time and direct comparisons between the EEP and mean arterial pressure (MAP). The difference between the EEP and MAP is the extra energy generated by each pump. In a normal adult heart, the difference is approximately 10%. 4 If the pump flow is nonpulsatile, the EEP is very similar to the MAP, so there is no extra hemodynamic energy.It is possible to convert the units of mm Hg to units of dynes/cm 2 using Shepard's total hemodynamic energy formula, ͓͑ergs/cm 3 ͒ ϭ ͑1,332 ͐ fpdt͒/͑͐ fdt͔͒ The constant 1,332 changes pressure from units of mm Hg to u...