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The Concept of Preload and Afterload Applied to Renal Hemodynamics

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The Concept of Preload and Afterload Applied to Renal Hemodynamics lower urinary protein leakage: Will such treat- The Concept of Preload ments also help to improve renal function out- come? J Am Soc Nephrob 1993:3:1333-1341. and Afterload Applied to 2. Smith TW. In: Wyngaarden JB, Smith LH, Ben- net JC, Eds. Cecil Textbook of Medicine. Phila- Renal Hemodynamics delphia: WB Saunders Company: 1992:188. To the Editor: I have enjoyed reading the fine review of de Jong Response: et at ( 1 ) on the therapeutic strategies to lower pro- We thank Professor Koenc for his comments on our teinunia in gbomeruban disease and on the possible concept of glomerular preboad and gbomerular after- influence of these treatments on the progression of load reduction. Indeed, he is obviously correct in that renal disease. However, I want to take issue with the function of the gbomerubar capillary loop cannot their proposal to borrow the terms “preload” and “af- be compared with that of the heart, the first being a tenboad” from cardiology in describing the events fol- specialized structure that affords the filtration of lowing changes in pregbomerular and postgbomerubar water and sobutes, whereas the batter is an active vascular resistance. In the Cecit Textbook of Medi- pump. cine, preload is defined, in Its strictest sense, as the We however used the concept of preboad and after- stretch of myocandiab fibers at the end of the diastole, load reduction not to directly compare the function whereas afterload is the resistance that the ventricle of the glomerulus with that of the heart. but to com- must overcome during systole in order to eject a pare the function of the vessels to and from the stroke volume (2). This concept thus implies that the gbomerubus with the function of the vessels to and heart is a pump that actively delivers work (as in from the heart. By modulation of the resistances of lifting a load). It is difficult to visualize such a func- these vessels, the function of the organ positioned tion for the gbomerubar tuft. Intermediate between the vessels can be influenced In clinical practice, the term preload is often used greatly. This holds true both for the heart and for the as a synonym for the venous return to the heart. gbomerulus. The function of the heart may be im- With this in mind, one could make a case to use a proved by either cardiac preboad or aftenboad reduc- similar terminology in describing the pregbomerular tion or by the combination of both. Similarly, a gb- arterial flow. However, the analogy becomes rather merular preload reduction (afferent vasoconstnic- confusing when the postgbomeruban vascular changes tion), a glomerubar aftenboad reduction (cfferent are described in terms of afterboad, because the gb- vasodilation), or a combination of both can influence merular tuft does not contract against an effenent the filtration properties of the gbomcrulus. resistance. We argue for this concept because It draws atten- The terminology, as proposed by the authors, cer- tion, as It has done in cardiology, to the fact that the tamnly has its attractiveness in that it attempts to function of the organ may be influenced in different describe complicated phenomena in a concise way. It ways. One should not rely only upon the treatments might seem an acceptable jargon for the knowledge- that influence gbomerular afterboad (such as anglo- able nephrobogist. but It will tend to confuse those tensmn-converting enzyme inhibitors) but also book who feel less well at home in the field of renal hemo- for treatments that bower gbomerular preload (such dynamics. Therefore, I would bike to make a plea to as bow-protein diets). If clinically feasible. the com- stIck to the more precise description of these phe- bination of both can, in some instances, be the treat- nomena in terms of the gbomeruban capillary hy- ment of choice. In that respect, it is noteworthy that draulic pressure gradient, influenced by systemic an- it has recently been shown that the antipnoteinunic tenial pressure and by the changes in afferent and effect of an angiotensmn-conventing enzyme inhibitor effenent vascular resistance. adds to that of a low-protein diet ( 1 ,2). Robert A.P. Koene Paul E. de Jong Department of Medicine Sharon Anderson Division of Nephrotogy Dick de Zeeuw University Hospitat Nijmegen Department of Medicine Nijmegen. The Net hertands Division of Nephrotogy University Hospital REFERENCES Groningen, The Netherlands and 1 . de Jong PE, Anderson 5, de Zeeuw D. Glomen- Oregon Health Sciences University ular preload and afterboad reduction as a tool to Porttand. Oregon Journal of the American Society of Nephrology 1087.
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