Influence of Pulsatile and Nonpulsatile Left Heart Bypass on the Hormonal Circadian Rhythm

To the Editor: We read with interest the report by Nishinaka and associates 1 on the effects of pulsatile and nonpulsatile left heart bypass on the hormonal circadian rhythm. These authors concluded that, in a chronic goat model, the circadian rhythm of melatonin was maintained during both pulsatile and nonpulsatile left heart bypass, while the circadian rhythm of stress sensitive cortisol and hemodynamic responsive renin was modified. In all the experiments, Nishinaka and associates used pulsatile flow for the first 2 weeks of left heart bypass and then used nonpulsatile flow for 4 weeks in the same animals. The fact that the researchers combined two perfusion modes in the same animals was a clear limitation of the study. Ideally, for direct comparison of pulsatile and nonpulsatile flow, pulsatile flow should be used continuously for 6 weeks; then a separate group of animals should undergo nonpulsatile perfusion alone for the same duration. In addition, at several conferences, Dr. Yukihiko Nosé, of Baylor College of Medicine, has stated that the most critical period for long-term mechanical circulatory support is the first 2 weeks after device implantation. Using pulsatile flow for the first 2 weeks, then switching to nonpulsatile flow for 4 weeks, does not support the conclusions of Nishinaka and associates regarding the impact of pulsatile versus nonpulsatile flow in this model. The pulse pressures were 37.8 ± 5 mm Hg and 17.4 ± 6.7 mm Hg during pulsatile and nonpulsatile left heart bypass, respectively. Because the pulse pressure was 17.4 ± 6.7 mm Hg during nonpulsatile perfusion, the investigators in fact compared two forms of pulsatile flow, not pulsatile versus nonpulsatile flow. Another of our concerns about this study involves the quantification of pulsatile and nonpulsatile flow. Quantification of the flow in terms of pulse pressure is inadequate for direct comparison, because the generation of pulsatile flow depends on the energy gradient rather than the pressure gradient. 2–4 We use the energy equivalent pressure (EEP) formula by Shepard et al. to quantify pulsatile and nonpulsatile flow, because this formula accounts for both pressure and flow waveforms, not pressure waveforms alone. 5 Using the EEP formula, we have clearly shown that pulsatile perfusion generates significantly higher hemodynamic energy than nonpulsatile perfusion, and this extra energy may enhance vital organ perfusion. 2,3,6–8 Dr. Nosé’s group at Baylor quantifies the arterial pressure waveforms in terms of pulsatility index and pulse power index, using a fast Fourier transform technique during nonpulsatile left ventricular assist device support. 9–11 This useful approach should be considered by other investigators for directly comparing pulsatile and nonpulsatile flow. We hope that Nishinaka and associates will consider our suggestions and use them in performing additional experiments for clarifying this important controversy. Akif Ündar Charles D. Fraser Jr.