Renal replacement therapy (dialysis) in acute kidney injury (acute renal failure): Metabolic and hemodynamic considerations
- Thomas A Golper, MD
Thomas A Golper, MD
- Section Editor — Dialysis
- Professor of Medicine
- Vanderbilt University Medical Center
Acute renal failure interferes with the excretion of water, electrolytes, and organic solutes (such as urea, creatinine, and uric acid). A few simple calculations will permit us to understand the limits on these processes and how they may be affected by dialytic procedures. The normal glomerular filtration rate is approximately 170 to 180 L/day, which is roughly the clearance of those solutes like creatinine that are excreted primarily by glomerular filtration.
Most patients with acute renal failure tolerate the solute retention that accompanies a glomerular filtration rate that is 10 percent of normal (17 L/day or 12 mL/min) [1-3]. This is therefore a reasonable initial target among patients who are not hypercatabolic. In comparison, hypercatabolic patients require more aggressive solute removal techniques to maintain acceptable or optimal steady-state or time-averaged concentrations of solutes.
The mechanisms of solute removal by dialysis and the hemodynamic changes that occur will be reviewed here. The indications for dialysis in patients with acute renal failure, the goals for dialysis delivery, concerns related to whether dialysis delays the recovery of renal function, and the effects of different hemodialysis membranes are discussed separately. (See "Renal replacement therapy (dialysis) in acute kidney injury in adults: Indications, timing, and dialysis dose" and "Renal replacement therapy (dialysis) in acute kidney injury (acute renal failure): Recovery of renal function and effect of hemodialysis membrane".)
MECHANISMS OF SOLUTE REMOVAL
For dialysis to remove a solute, it must be present in the circulation. The process of solute removal during dialysis can occur by two different mechanisms: passive diffusion down a favorable concentration gradient from the plasma into the dialysis fluid and during the ultrafiltration (or convection) of plasma water across the membrane of the hemofilter. The frictional forces between water and solutes (called solvent drag) result in the convective transport of small- and middle-molecular-weight solutes (less than 5000 daltons) in the same direction.
The dialytic clearance of a solute is dependent in part upon size, with larger molecules (including those that are protein bound) being less efficiently removed. The sieving coefficient (SC) is a measure of a solute's filterability during ultrafiltration, being equal to the ratio of the solute concentration in the filtrate to that in the arterial plasma water . The SC ranges from 0, for a solute that is completely rejected, to 1, for a solute that is freely filtered (such as urea and creatinine). The total clearance of a solute during ultrafiltration is equal to the product of the SC and the rate of fluid removal (the ultrafiltration rate). For a solute with an SC of 1, the concentration of the solute in the filtrate is roughly the same as that in the plasma water, and the solute clearance by filtration is equal to the net ultrafiltration rate. A more detailed discussion of these concepts can be found elsewhere. (See "Drug removal during continuous renal replacement therapy".)
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