How should investigators compare different perfusion modes or different types of pulsatile flow during chronic support?

Abstract

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. 5–7 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. EQUATION It is possible to convert the units of mm Hg to units of dynes/cm2 using Shepard’s total hemodynamic energy formula, EQUATION The constant 1,332 changes pressure from units of mm Hg to units of dynes/cm2.8 We recently quantified pressure flow waveforms in terms of EEP and total hemodynamic energy levels in a Penn State adult mock loop using a pulsatile ventricular assist device (VAD) (70 ml Pierce-Donachy pneumatic VAD). 9 With a constant pump flow rate of 5 L/min; pump rates of 65, 70, and 80 bpm; and aortic pressures of 80, 90, and 100 mm Hg, we quantified the EEP and total energy levels at each experimental stage. Our results have clearly shown that this particular pulsatile VAD generates physiologic hemodynamic energy levels at each experimental stage. The differences between the EEP and MAP at different experimental stages were from 9 to 12%. 9 What happens to this extra energy generated by the pulsatile devices? The literature clearly suggests that end organ recovery and capillary perfusion are significantly better maintained with pulsatile perfusion compared with nonpulsatile perfusion during chronic cardiac support. 10–12 Current literature also suggests that several experimental limitations and design errors exist when pulsatile and nonpulsatile flow are compared during chronic support. 2,3,5–7,13,14 We have some questions for this important paper’s authors. Have Koenig and associates quantified the pulsatile and nonpulsatile pressure flow waveforms in terms of hemodynamic energy levels? What are the EEP levels of different perfusion modes at normal, failing, and recovery stages? If they have not calculated the hemodynamic energy levels yet, we strongly suggest that they consider using Shepard’s EEP and total energy formulas for precise quantification of pressure flow waveforms. Akif Ündar Gerson Rosenberg John L. Myers Departments of Pediatrics, Surgery, and Bioengineering, Penn State College of Medicine, Department of Pediatrics—HO85, 500 University Drive, P.O. Box 850; Hershey, PA 17033-0850.