Dialytic techniques used in decompensated heart failure. Slow continuous ultrafiltration (SCUF) uses a hydrostatic pressure difference (P) between the blood and nonblood sides of the membrane (dotted line within the dialyzer) to remove water and solutes from the plasma by ultrafiltration. Sustained low-efficiency dialysis (SLED) has the additional feature of dialysis fluid passed through the nonblood compartment in a countercurrent direction to the blood flow. Fluid removed from the blood must be replaced by transfer from the interstitial compartment. Failure of this fluid recovery will result in hemodynamic instability.