J Thorac Cardiovasc Surg 1995;109:810
© 1995 Mosby, Inc.
Biophysics of pulsatile perfusion
Ray C. J. Chiu, MD, PhD, FRCSC
Division of Cardiovascular and Thoracic Surgery
McGill University
Montreal, Quebec, Canada
To the Editor:
I was intrigued by the discussions on the long-standing controversies of pulsatile versus nonpulsatile flow perfusions by Drs. Runge and Trinkle (1994;107:642-3) and by Dr. Bartlett (1994;107:644-6) in this JOURNAL. Drs. Runge and Trinkle emphasized the importance of using "physiologic" pulsatile waveforms, particularly in relation to rate of pressure rise (dP/dt) and ejection time. Dr. Bartlett deduced from the existing literature that differences in "adrenergic response" at moderate flow rates can account for the reported advantages of pulsatile perfusion and that at very low or high flow rates such advantages become insignificant.
Although these insights shed much light on the controversy, I wish to propose that some of the mysteries may also be related to our neglect in considering one of the physical laws of rheology. We are used to thinking of flow F as R =
P/F, where R is "resistance" andP "pressure gradient." In fact, flow between two points is determined not by P alone, but by the total energy difference (E), which has two components: potential energy and kinetic energy. 
1 Potential energy is our familiar P, whereas kinetic energy depends on the flow velocity (mv2 /2) and acceleration (related to rate of pressure rise [dp/dt]). Pulsatile flow is associated with higher kinetic energy: witness the higher gasoline consumption of driving your car fast and slow in the city compared with that of steady speed driving on the highways. It has been repeatedly observed that pulsatile perfusion reduces peripheral resistance R. If flow is maintained constant, with higher kinetic energy in the pulsatile perfusion, a lower P will be recorded and decrease the calculated R. This may account in part for the reduced "adrenergic response" described by Dr. Bartlett, although I suspect baroreceptors 2 also may respond to E. In turn, if mean perfusion pressure P is held constant, pulsatile perfusion will result in higher F; thus if the study is being done in the low to moderate flow range, pulsatile perfusion may demonstrate physiologic advantage. Finally, because the magnitude of kinetic energy depends on the waveform of the pulsatile flow (whereas the mean pressure does not), it may offer a theoretic basis for the observations of Drs. Runge and Trinkle and many other investigators.
References
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Burton AC. Physiology and biophysics of the circulation. Chicago: Year Book, 1966:174.
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Minami K, Vyska K, Korfer R. Role of the carotid sinus in response of integrated venous system to pulsatile and nonpulsatile perfusion. J THORAC CARDIOVASC SURG 1992;104:1639-46.[Abstract]
