Transport Characteristics of the Slow Therapies: Implications for Achieving Adequacy of Dialysis in Acute Renal Failure

Slow continuous renal replacement therapy is used primarily to treat critically ill patients with acute renal failure. Knowledge of the small solute transport characteristics in continuous renal replacement therapy (CRRT) provides a rational basis for the manipulation of operating conditions to achieve desired clearances and, hence, desired metabolic control. These manipulations simply involve varying the dialysate inflow rate, the ultrafiltration rate, and the substitution fluid replacement rate. For the diffusion-based therapies, the condition of dialysate saturation, ie, exiting dialysis fluid urea nitrogen (FUN) approximating inflowing plasma water urea nitrogen (BUN, ie, the FUN/BUN ratio ≈ 1), simplifies these manipulations. Examples of the calculations required to compute urea generation rate, protein catabolic rate, and the desired clearances are given. However, all of these calculations leave unanswered the question of "what is adequate dialysis" in the critically ill patient with acute renal failure. In the absence of the data that are required to answer this question, one can only say that if dialysis makes a difference, the persistently high mortality rate in intensive care unit acute renal failure suggests that patients are not receiving enough Kt/V. The reasons why CRRT is inherently more efficient than intermittant hemodialysis are explored. The usual reasons why prescribed therapy is less than delivered therapy with respect to slow continuous therapy are also discussed.