Urea disequilibrium contributes to underdialysis in the intensive care unit.
ASN 30th Annual Meeting, San Antonio
J Am Soc Nephrol (Sep) 8:287A 1997
A nice advance in determining solute removal by blood-based urea kinetics was the development of the arterial and venous rate equations . These were based on a regional blood flow model of urea kinetics and hold that, for an AV access, postdialysis urea rebound in terms of delta Kt/V can be expressed as 0.6 x K/V - 0.03. For a venous access, the comparable formula is 0.47 x K/V - 0.02. Rebound is greater with an AV access because it must also include the A-V urea gradient which develops during the dialysis session, and which has not yet closed at the point of blood sampling (assuming postdialysis blood is sampled at 10-20 seconds after slowing blood flow.
However, the regional blood flow model predicts that postdialysis urea rebound may be less in circumstances in which there is increased flow to muscle during dialysis, such as intradialytic exercise, and use of vasodilators, and that rebound will be increased when muscle blood flow is reduced or constriction to the muscle beds is present. The regional blood flow predictions were based on a hypothesized intradialytic cardiac index of 2.85 L/min/M2, and a fraction of blood flow to poorly perfused body compartments of 15%. It is entirely possible that assumptions which hold for chronic patients may not hold for ICU patients with acute renal failure.
In this paper, Depner's group tested the venous rate equation above in 10 patients undergoing acute dialysis using venous catheters. They measured BUN postdialysis and 1 hour postdialysis. They found that mean delivered single- pool Kt/V was only 0.82, 20% lower than the prescribed Kt/V of 1.0. Furthermore, equilibrated Kt/V, or eKt/V, was estimated by the venous rate equation to be 0.72, and was measured to be 0.68.
There were several conclusions. 1) Delivered eKt/V in an acute setting is far below that recommended by DOQI guidelines for chronic patients (> 1.0, if one assumes a standard rebound of 0.2 Kt/V units). 2) Delivered single-pool Kt/V is less than prescribed, by 20%, for reasons that are not entirely clear. In the NIH HEMO study, prescribed and delivered spKt/V are equal but a number of corrections are made, including use of KoA values based on actual measurement by the HEMO Study, a correction for blood flow prepump pressure effects, and absence of recirculation. It is unclear if these corrections were applied in the Depner study. 3) The point regarding predicted rebound is interesting also, and suggests that the rate equation underestimates rebound in an acute dialysis situation (by about 30%, it seems). This may well be due to some vasoconstriction in this population. These data provide a rational explanation for the results presented in another abstract at this meeting by Schiffl et al , suggesting that daily dialysis needs to be used in the treatment of acute renal failure. (John T. Daugirdas, M.D., University of Illinois at Chicago)
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ASN 30th Annual Meeting, San Antonio
Basic hemodialysis : (Intermittent) dialysis for ARF
Basic hemodialysis : Adequacy, prescription, urea kinetics