The goal of this randomised controlled trial is to compare the cumulative fluid balance over the first 72 h following inclusion guided by mVExUS versus standard of care in critically ill patients with acute kidney injury receiving CRKT . It will also compare the proportion of CRRT-related complications-including intradialytic hypotension and arrhythmias-between patients managed with mVExUS-guided fluid management and those receiving standard care.

The main questions it aims to answer are:

Does fluid removal rate guided by mVExUS will reduce cumulative fluid balance over the course of the first 72 h of CRRT in ICU patients compared to standard care

Participants will:

Get fluid assessment by mVExUS protocol or a strandard care every 8 hours for 72 hours

After enrollment, the participants have to be randomized within 1 h. In the intervention arm, mVExUS has to be performed every 24 hours until 72 hours after enrollment. Participants with profile A will have UFNET 0 mL/hr with target fluid balance 0 to +500 mL/day. Patients with profile B will have UFNET 0-20 mL/hr with target fluid balance 0 to -500 mL/day. Patients with profile C will have UFNET 20-40 mL/hr with target fluid balance -500 to -1000 mL/day. Patients with profile D will have UFNET 40-100 mL/hr with target fluid balance neg >1000 mL/day.

This intervention will be combined with serial lactate monitoring in which lactate levels will be measured every 8 h. When the lactate level rises more than 2 or if there is presence of HIRRT (tachycardia, hypotension, mottling or drop in cardiac index), passing leg raising or mini-fluid challenge (crystalloid fluid bolus 200 mL) will be performed to assess fluid responsiveness. If there is fluid responsiveness, UFNET will be stopped. Once lactate is normalized or episode of HIRRT resolved, the UFNET strategy has to be restored.

The CRRT technique (haemodialysis, haemofiltration, haemodiafiltration) will be left at the discretion of the treating team. CRRT dose (i.e. dialysate or reinfusion fluids flow rate) will be prescribed at 25-30 ml/kg/hr according to the KDIGO guideline. CRRT dose and UFNET will be indexed to the patient's body weight, and in patients with a body mass index at 30 kg/m2, to the adjusted body weight (calculated as ideal body weight to which will be added 40% of the difference between the actual body weight and the ideal body weight - Du Bois formula). Choice of extra-corporeal circuit anticoagulation (heparin, citrate or none) and implantation site of CRRT catheters will be left at the discretion of the treating physician.

Vasopressors will be titrated as per unit-based protocols to maintain the mean arterial pressure within the bounds prescribed by the physician in charge (most frequently between 65 and 75 mmHg). Indications and category of vasopressors and inotropes will be left at the discretion of the treating team. The diuretic will be stopped once the patient is enrolled.

Inclusion Criteria:

• Adults (≥ 18 years of age)
• Admitted to ICU
• Acute kidney injury by KDIGO criteria
• Initiated CRRT by at least one of the following indications for RRT initiation:
• Serum potassium ≥ 6.0 mmol/L, or
• pH ≤ 7.20 or serum bicarbonate ≤ 12 mmol/L, or
• Evidence of severe respiratory failure, based on a PaO2/FiO2 ≤ 200 and clinical perception of volume overload, or
• Persistent severe AKI (sCr remains > 50% the value recorded at randomization) for > 72 hours from randomization
• Participants giving informed consent

Exclusion Criteria:

• Refuse to participate
• Previous diagnosis of end-stage kidney disease (ESKD) currently on kidney replacement therapy
• Kidney transplant recipient
• Receive RRT before ICU admission within 90 days
• Structural kidney diseases which will interfere with intrarenal doppler ultrasound e.g. renal artery stenosis, autosomal dominant polycystic kidney disease
• Patients with previously known conditions that interfere with portal doppler assessment, namely liver cirrhosis, severe tricuspid regurgitation with structural heart disease or massive ascites.
• Underlying disease process with a life expectancy less than 90 days
• Pregnancy
• Severe cardiac rhythm disturbances (tachyarrhythmia, supraventricular tachycardia)
• Intra-cardiac shunts; Ventricle septal defect, patent foramen ovale, atrial septal defect
• Aortic aneurysm
• Intra-abdominal hypertension (intraabdominal pressure ≥20 mmHg)
• Expected life expectancy <48 hours
• Receiving extracorporeal membrane oxygenation (ECMO)

The datasets used and/or analysed will be available from the corresponding author on reasonable request.