Dialysis, CRRT, CKD and Urosepsis

Indications for RRT and CRRT Modalities and Mechanics

Urgent renal replacement therapy is indicated when life-threatening disturbances cannot be corrected with medical therapy alone. In the intensive care unit, key indications for urgent initiation include refractory metabolic acidosis, life-threatening hyperkalemia, selected dialyzable drug toxicities, refractory fluid overload, and symptomatic uremia. While intermittent hemodialysis clears solutes rapidly, continuous renal replacement therapy is preferred for critically ill patients experiencing hemodynamic instability because it removes fluid and solutes gradually, minimizing sudden shifts in intravascular volume and osmolality.

Continuous modalities vary in their primary mechanisms of clearance and fluid movement. Slow continuous ultrafiltration removes plasma water via hydrostatic pressure gradients without utilizing dialysate or replacement fluid. Continuous veno-venous hemofiltration clears solutes primarily by convection; clearance of middle molecules depends on the membrane, whereas clearance of large molecules is limited. Continuous veno-venous hemodialysis clears small molecular weight solutes primarily through diffusion driven by a countercurrent dialysate flow. Continuous veno-venous hemodiafiltration combines both convection and diffusion by utilizing dialysate fluid and replacement fluid simultaneously.

The location of replacement fluid administration profoundly impacts both circuit longevity and clearance efficacy. Administering replacement fluid pre-filter dilutes blood entering the hemofilter, which prolongs filter life but reduces solute clearance efficiency. Conversely, administering replacement fluid post-filter maximizes solute clearance efficiency but increases blood viscosity within the circuit, raising the risk of circuit clotting. Because extensive solute removal occurs during continuous therapies, the critical care nurse must monitor for severe hypophosphatemia and hypokalemia caused by excessive clearance into the effluent fluid.

Continuous monitoring of circuit pressures is vital to anticipate filter failure and circuit disruption. A rising transmembrane pressure at unchanged flow settings may suggest reduced filter permeability from protein deposition or clotting; assess pressure trends and circuit settings rather than attributing the change to one cause. A rising filter pressure drop at a stable blood-flow rate suggests increasing resistance within the hemofilter, often from clotting, and warrants prompt circuit assessment. Pressure trends can suggest access or circuit problems: a more negative access pressure may indicate impaired inflow, and a falling venous return pressure may indicate a disconnection or leak; because alarms may not detect every disconnection, inspect the circuit and patient promptly. The nurse must assess the circuit and the patient promptly whenever these alarms occur.