NDT Advance Access originally published online on February 20, 2008
Nephrology Dialysis Transplantation 2008 23(6):2106-2107; doi:10.1093/ndt/gfn057
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Reply
Correspondence and offprint requests to: E-mail: basile.miulli{at}libero.it
We read with interest the letter by Blankestijn et al. in response to our article [1]. We are very grateful to them for their letter, which confirms our findings as far as at least two points are concerned: (1) cardiopulmonary recirculation is higher in upper arm arteriovenous fistulae (AVFs) than in forearm AVFs; (2) a third-order polynomial regression model fits best the relationship between vascular access flow (Qa) and cardiac output. Furthermore, they found a decreased access resistance (AR) in upper arm AVFs. This finding is actually not unexpected if we take into account that Poiseuille's law states that the blood flow in any vessel, and therefore also Qa, is determined by the following relationship [1]:
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where 
P is the pressure difference between the extremities of the vessel, r the radius of the vessel, 
the viscosity of the fluid and l the length of the vessel. Now, we know that AR is expressed by the ratio
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We can rewrite this relationship in the following way:
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It is clearly evident that, among all factors involved, r (at power 4) of a vessel plays the most important role in determining AR. Now, the brachial artery utilized for an upper arm AVF must necessarily have a higher r than the radial artery utilized for a lower arm AVF. Consequently, the AR of an upper arm AVF must be evidently lower than that of a forearm AVF.
In conclusion, we think that the data presented by van der Mark et al. (with an elegant statistical inference) confirm our previous findings and that the evaluation of the peripheral, systemic and access resistance provides further interesting explanations about the determinism of the haemodynamic processes occurring in an AVF.
Conflict of interest statement. None declared.
1 Division of Nephrology, Miulli General Hospital, Acquaviva delle Fonti 2 Division of Nephrology, Hospital of Manduria, Manduria, Italy
References
- Basile C, Lomonte C, Vernaglione L, et al. The relationship between the flow of arteriovenous fistula and cardiac output in haemodialysis patients. Nephrol Dial Transplant (2008) 23:282–287.
[Abstract/Free Full Text] - Pfitzner J. Poiseuille and his law. Anaesthesia (1976) 31:273–275.[Medline]
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