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J Thorac Cardiovasc Surg 1998;116:530-531
© 1998 Mosby, Inc.

Letters To The Editor

The relation between pump flow rate and pulsatility on cerebral hemodynamics during pediatric cardiopulmonary bypass

To the Editor:

We read with great interest the report by Chow and associates ¹ about the effects of pulsatile versus nonpulsatile flow on the cerebral hemodynamics in children. Pulsatile and nonpulsatile flow were tested for six 5-minute intervals at three different pump flow rates in 40 patients. Chow and associates concluded that pulsatile flow delivered with a Stöckert pump (Stöckert, Munich, Germany) did not increase cerebral blood flow (CBF) or alter hemoglobin concentration during cardiopulmonary bypass in children.

Although pulsatile flow settings such as pump rate, baseline flow rate, and run time were included in their study, these parameters are inadequate to quantify the pulsatility. To compare pulsatile versus nonpulsatile flow or different types of pulsatility, first investigators have to establish common criteria for pulsatile flow. The criteria should be easy to understand and apply without using complex mathematical calculations. Although several attempts have been made to establish common criteria, no consensus has been reached.

^2,5Until a common definition is reached, investigators should be specific about the quality of pulsatility by including pressure and flow waveforms, rate of pressure rise, ejection time, pulse pressure, and stroke volume, along with the brand name of the pulsatile device.In addition to the pump, an acceptable membrane oxygenator and aortic cannula are extremely important to produce sufficient pulsatility. ^5a Hollow-fiber membrane oxygenators dampen pulsatile flow significantly less than other types of membrane oxygenators. ⁶ Since the diameter of the aortic cannula is significantly smaller for neonates and infants than for adults, the geometry of the aortic cannula is also extremely important in producing adequate pulsatile flow. It has also been proven that the shorter the tip of the aortic cannula, the better the pulsatility.

Referring to an article by Sadahiro and coworkers, ⁷ Chow and associates stated that the higher CBF was obtained during pulsatile pump flow at cerebral perfusion pressures (CPPs) below 50 mm Hg in dogs. Referring to an article by Hindman and colleagues, ⁸ however, they stated that this higher CBF has not been reproduced in rabbits. In the rabbit model, the mean arterial pressures were above 60 mm Hg, not lower than 50 mm Hg. It has also been proven that the cerebral pressure/flow autoregulation is intact if the CPP is greater than 50 mm Hg. In addition, Hindman and coworkers ⁸ had a severe technical limitation in their rabbit model. The distal aorta was ligated and cannulated in retrograde fashion, which produced artificially higher pressures and CBFs in rabbits. This may be the reason that they could not show the benefits of pulsatile perfusion in the rabbit model. However, they have produced physiologic pulsatile pressures with a unique two-chamber hydraulically driven pump system (Medical Engineering Consultants, Bishop, Calif.) during cardiopulmonary bypass. The first chamber was placed between the venous reservoir and the oxygenator, and the second pumping chamber was placed after the oxygenator. Thus the membrane had no effect on the quality of the pulsatility in this particular model. However, this physiologic pulsatile pump system has not been yet approved for clinical trials by the Food and Drug Administration.

It has already been proven that CBF is improved with pulsatile flow at CPP of 40 mm Hg in piglets. ^9,10 Although pulsatile perfusion had no beneficial effects on global and regional CBF at a CPP of greater than 50 mm Hg, myocardial and renal blood flow were better maintained with pulsatile flow than with nonpulsatile perfusion in a piglet model. ¹¹

Chow and associates have also stated that "preliminary studies from our group suggest that pulsatile flow does not increase CBF in adults either." ¹² Once again, they have used pulsatile and nonpulsatile flow with 5-minute intervals, which was a limitation of their model. However, systemic vascular resistance with pulsatile flow was lower in five of seven adult patients, but results were not statistically significant.

Certainly, there is an urgent need to design better pulsatile pumps other than roller pumps, which generate only 12.4% of the pulsatile power of a human heart. ¹³ In addition, investigators should have common criteria for pulsatility when comparing results from different institutions.

Despite these concerns, the study by Chow and associates is interesting. We were particularly pleased to see the use of near-infrared spectroscopy during pulsatile versus nonpulsatile perfusion for pediatric patients undergoing cardiopulmonary bypass. This adds another useful noninvasive tool in comparing pulsatile versus nonpulsatile perfusion. We look forward to future articles by the investigators with different settings of pulsatile versus nonpulsatile perfusion.

Congenital Heart SurgeryTexas Children's Hospital^a, Cullen Cardiovascular Research Laboratory, Texas Heart Institute^b, Section of Congential Heart SurgeryDepartment of Surgery, Baylor College of Medicine^c, Houston, Tex.

References

1. Chow G, Roberts IG, Edwards AD, et al. The relation between pump flow rate and pulsatility on cerebral hemodynamics during pediatric cardiopulmonary bypass. J Thorac Cardiovasc Surg 1997;114:568-77.[Abstract/Free Full Text]
   
2. Shepard RB, Simpson DC, Sharp JF. Energy equivalent pressure. Arch Surg 1966;93:730-40.[Abstract/Free Full Text]
   
3. Grossi EA, Connolly MW, Krieger KH, et al. Quantification of pulsatile flow during cardiopulmonary bypass to permit direct comparison of the effectiveness of various types of pulsatile and nonpulsatile flow. Surgery 1985;98:547-53.[Medline]
   
4. Wright G. Hemodynamic analysis could resolve the pulsatile blood flow controversy. Ann Thorac Surg 1994;58:1199-204.[Abstract]
   
5. Ündar A, Runge TM, Miller OL, et al. Design of a physiologic pulsatile flow cardiopulmonary bypass system for neonates and infants. Int J Artif Organs 1996;19:170-6.[Medline]
   
6. Ündar A, Lodge AJ, Daggett, Runge TM, Ungerleider RM, Calhoon JH. Design and performance of a physiologic pulsatile flow neonate-infant cardiopulmonary bypass system. ASAIO J 1996;42:M580-M583.[Medline]
   
7. Sadahiro M, Haneda K, Mohri H. Experimental study of cerebral autoregulation during cardiopulmonary bypass with or without pulsatile perfusion. J Thorac Cardiovasc Surg 1994;108:446-54.[Abstract/Free Full Text]
   
8. Hindman BJ, Dexter F, Ryu KH, Smith T, Cutkomp J. Pulsatile versus nonpulsatile cardiopulmonary bypass. Anesthesiology 1994;80:1137-47.[Medline]
   
9. Ündar A. Design and performance of physiologic pulsatile flow cardiopulmonary bypass systems for neonates and infants. PhD Dissertation, The University of Texas at Austin, May 1996.
   
10. Lodge AJ, Ündar A, Daggett CW, et al. Effects of pulsatile cardiopulmonary bypass on cerebral recovery from circulatory arrest in an infant model [abstract]. Circulation. In press.
    
11. Lodge AJ, Ündar A, Daggett CW, Runge TM, Calhoon JH, Ungerleider RM. Regional blood flow during cardiopulmonary bypass and after circulatory arrest in an infant model. Ann Thorac Surg 1997;63:1243-50.[Abstract/Free Full Text]
    
12. Chow G, Roberts IG, Harris D, Wilson J, Elliott MJ, Edwards AD, et al. Stöckert roller pump generated pulsatile flow: cerebral metabolic changes in adult cardiopulmonary bypass. Perfusion 1997;12:113-9.[Medline]
    
13. Wright G. The hydraulic power outputs of pulsatile and nonpulsatile cardiopulmonary bypass pumps. Perfusion 1988;3:251-62.
    

This article has been cited by other articles:

				A. Undar, K. M Koenig, O H Frazier, and C. D Fraser Jr Impact of membrane oxygenators on pulsatile versus nonpulsatile perfusion in a neonatal model Perfusion, March 1, 2000; 15(2): 111 - 120. [Abstract] [PDF]

				A. Undar, T. Masai, S.-Q. Yang, J. Goddard-Finegold, O.H. Frazier, and C. D. Fraser Jr Effects of perfusion mode on regional and global organ blood flow in a neonatal piglet model Ann. Thorac. Surg., October 1, 1999; 68(4): 1336 - 1342. [Abstract] [Full Text] [PDF]

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