Urea kinetic modeling for CRRT.

Urea kinetic modeling (UKM) for dialysis quantification and prescription, although widely used in chronic renal failure (CRF), has been largely absent in the acute setting. A quantitative approach to prescription of continuous renal replacement therapies (CRRTs) for acute renal failure (ARF) based on UKM is presented. For patients with a relatively constant urea generation rate, G, who are receiving a fixed dose of CRRT, blood urea nitrogen (BUN) falls in an exponential fashion, approaching a plateau level after 3 to 4 days of continuous treatment. The CRRT clearance, K, necessary to achieve a desired plateau value of BUN, Cgoal, may be computed as G/Cgoal x K for all but predilutional CRRT modalities may be calculated as equal to the effluent (dialysate plus ultrafiltrate) flow rate from the filter. Urea mass balance equations are proposed for the determination of patient G value either during the pretreatment rise in BUN or during the decline in BUN with CRRT. In the absence of a reliable estimate of patient G, a reasonable CRRT starting prescription is to set the filter effluent rate in liters per hour (approximately K) to 1.2 times the patient's body weight in kilograms divided by the desired Cgoal in milligrams per deciliter. This relationship assumes moderate hypercatabolism (normalized protein catabolic rate = 2.0 g/kg/d) and patient urea distribution volume equal to 60% of body weight. For Cgoal = 60 mg/dL, this reduces to an easily remembered formula for K (in L/hr) of twice the patient's body weight divided by 100.