Should External Outputs Be Included in CRRT Fluid Removal Goals?
No, external outputs such as chest tube, nasogastric tube, or other drain outputs should NOT be directly included in the CRRT net ultrafiltration goal, but they MUST be accounted for separately in your overall fluid balance calculation to determine the appropriate CRRT prescription.
Understanding the Distinction
The key is understanding that CRRT fluid management involves two separate but related concepts:
- CRRT net ultrafiltration (UFNET): The fluid removed by the CRRT machine itself
- Total patient fluid balance: All inputs minus all outputs (including CRRT, drains, urine, etc.)
How to Approach Fluid Management with CRRT
Step 1: Calculate Total Fluid Balance
You need to account for ALL sources:
Inputs:
- IV fluids
- Medications
- Nutrition
- Blood products
- Oral intake
Outputs:
- CRRT ultrafiltration
- Urine output
- Chest tube drainage
- NG tube output
- Surgical drain output
- Insensible losses
Step 2: Determine Your Net Fluid Removal Goal
Based on the patient's fluid status, determine how much net negative balance you want to achieve over 24 hours. The evidence suggests avoiding volume overload is critical 1, particularly in patients with acute lung injury 1.
Step 3: Set CRRT Prescription Accordingly
Your CRRT UFNET prescription = Desired total negative fluid balance + All other outputs - All inputs
For example:
- If you want -1000 mL net fluid balance over 24 hours
- Patient has 500 mL chest tube output expected
- Patient has 200 mL urine output expected
- Patient receiving 2000 mL IV fluids/nutrition
Then: CRRT UFNET needed = 1000 mL (goal) + 2000 mL (inputs) - 500 mL (chest tube) - 200 mL (urine) = 2300 mL over 24 hours
Critical Considerations for UFNET Rates
Avoid extremes in ultrafiltration rates. Recent evidence demonstrates a U-shaped relationship between UFNET rates and mortality 2, 3. Both excessively slow and excessively fast rates are associated with worse outcomes 3.
Target moderate UFNET rates:
- In pediatric populations: optimal NFB appears to be between -4.46 and -0.305 mL/kg/hr 4
- Fast UFNET rates are associated with cardiac arrhythmias and hemodynamic instability 3
- The impact of fluid balance on mortality is more dependent on cumulative output than input 5
Common Pitfalls to Avoid
Don't set CRRT to remove fluid that's already being removed by drains - This leads to excessive intravascular volume depletion and hemodynamic instability
Don't ignore preserved urine output - Patients maintaining UOP >0.3 mL/kg/hr have better outcomes and more CRRT-free days 4. Factor this into your prescription.
Monitor achieved vs. prescribed fluid balance - There is often a significant gap between what you prescribe and what is actually achieved, which is associated with mortality 2
Use integrated fluid balancing systems - Adaptive use of standard IV pumps for CRRT has been shown to risk significant errors in fluid balance 1
Monitoring and Adjustment
Reassess frequently - The 2020 KDIGO consensus emphasizes that delivery of RRT must reach the goals of electrolyte, acid-base, solute, and fluid balance specific to each patient 6.
Watch for signs of inadequate plasma refill:
- Hypotension requiring increased vasopressors 7
- Worsening hemodynamics
- Rising lactate
Adjust your prescription - If external outputs increase (e.g., chest tube drainage increases), you may need to decrease your CRRT UFNET rate to maintain the same overall fluid balance goal.
The Bottom Line
External outputs are part of your total fluid balance equation but should not be added to your CRRT ultrafiltration goal. Set your CRRT prescription to achieve your desired net fluid balance after accounting for all other inputs and outputs. This approach prevents excessive volume depletion while still achieving appropriate fluid management goals.