Managing fluid status in patients receiving renal replacement therapy.

PURPOSE OF REVIEW: Net ultrafiltration (UFNET) may help correct fluid overload in patients receiving renal replacement therapy (RRT). Yet, how to set it safely at the bedside remains unsettled. This review addresses the hemodynamic aspects of UFNET setting, its time-dependent adjustments in relation phase-dependent balance target, and the contribution of fluid inputs to fluid balance control. RECENT FINDINGS: Hemodynamic instability associated with RRT, the principal obstacle to active fluid removal, arises from vasomotor and cardiac mechanisms, which may or may not be related to UFNET. Fluid balance management, including UFNET setting, evolves across the resuscitation, optimization, stabilization and evacuation phases of care. The U-shaped association between UFNET and mortality is observational and should not be interpreted as a validated bedside target, while fixed prescriptions may prove difficult to deliver. Cardiac output monitoring, preload-dependence assessment and perfusion indices may help predict hemodynamic instability, although comparative evidence that these strategies improve outcomes is lacking. Finally, fluid overload is generated predominantly by nonresuscitation fluids, so that UFNET rate intensity is principally determined by the cumulative volume administered. SUMMARY: Setting UFNET safely requires its titration using repeatedly reassessed hemodynamic tolerance, awareness to phase-dependent fluid balance targets and reduction of fluid inputs.