Volume Removal Intolerance During Net Ultrafiltration in Acute Kidney Injury Patients

Fluid overload is a frequent and clinically relevant complication in critically ill patients with acute kidney injury (AKI). In this setting, continuous renal replacement therapy (CRRT) is frequently used not only for solute control but also as a strategy for controlled fluid removal through net ultrafiltration (UFNET). Although UFNET is central to de-resuscitation, the individual tolerance to fluid removal is highly variable and is not fully captured by blood pressure monitoring alone.

The concept of ultrafiltration intolerance remains poorly standardized. In clinical practice, intolerance is often recognized only after overt hemodynamic instability occurs, such as hypotension, escalation of vasoactive support, or interruption of fluid removal. However, reductions in cardiac output, impaired tissue perfusion, or worsening venous congestion may precede overt hypotension. Therefore, a multiparametric assessment may allow earlier identification of patients at risk.

This is a prospective, observational, analytical study in critically ill adult patients with AKI receiving CRRT with prescribed UFNET. The study does not assign or modify therapeutic interventions. CRRT modality, anticoagulation strategy, UFNET prescription, vasopressor management, fluid administration, and all other clinical decisions will remain under the responsibility of the treating clinical team according to routine care.

The study will characterize the physiological response to UFNET using a structured multiparametric monitoring approach. Recorded domains will include conventional macrohemodynamic variables, vasoactive support, selected advanced hemodynamic variables when available, focused cardiac ultrasound, venous congestion assessment, peripheral perfusion parameters, fluid balance variables, and selected biochemical markers. Functional hemodynamic maneuvers may be performed when feasible and clinically safe.

Data will be collected prospectively using a standardized case report form. Variables will be recorded at predefined time points before and during the early phase of UFNET, with additional off-schedule recordings if clinical signs compatible with intolerance occur. Source data will be obtained from the electronic or paper medical record, bedside monitoring systems, CRRT prescription and treatment records, laboratory results, and ultrasound assessments performed as part of clinical evaluation.