What is the estimated sodium removal rate in an 80 kg patient undergoing Continuous Renal Replacement Therapy (CRRT) with an effluent rate of 1600 mL per hour and a starting sodium level of 145 mEq/L?

Sodium Removal Calculation in CRRT

In an 80 kg patient with an effluent rate of 1600 mL/hour and starting sodium of 145 mEq/L, sodium removal is approximately 232 mEq per hour, assuming the dialysate/replacement fluid contains standard sodium concentration of approximately 140 mEq/L.

Understanding Sodium Removal Mechanics in CRRT

The key principle is that sodium removal occurs only when there is a concentration gradient between the patient's blood and the dialysate/replacement fluid 1. This is fundamentally different from volume removal, which occurs regardless of concentration gradients.

Calculation Framework

For this 80 kg patient:

• Effluent rate: 1600 mL/hour (which equals 20 mL/kg/hour, meeting the minimum recommended CRRT dosing of 20-25 mL/kg/hour) 1, 2
• Patient serum sodium: 145 mEq/L
• Standard dialysate sodium: typically 140 mEq/L

Sodium removal per hour = Effluent rate × (Patient Na - Dialysate Na)

• = 1600 mL/hour × (145 - 140) mEq/L
• = 1600 × 5 = 8,000 mEq/hour or 8 mEq/hour

However, this assumes complete equilibration. In reality, the actual sodium concentration in the effluent will approximate the patient's serum sodium when using standard dialysate, resulting in:

• Total sodium in effluent = 1600 mL/hour × 145 mEq/L = 232 mEq/hour

Critical Clinical Considerations

Sodium Balance vs. Sodium Removal

The net sodium balance depends on:

• Sodium removed in effluent (approximately 232 mEq/hour with patient Na of 145 mEq/L) 1
• Sodium returned via replacement fluid (if using standard 140 mEq/L fluid at equivalent rates)
• Net sodium removal = approximately 8 mEq/hour with a 5 mEq/L gradient 1

Modality-Specific Implications

The effluent rate interpretation varies by CRRT modality 2:

• CVVH: effluent rate = ultrafiltration rate
• CVVHD: effluent rate = dialysate flow rate
• CVVHDF: effluent rate = sum of dialysate + ultrafiltration rates

Managing Sodium Disorders During CRRT

For hyponatremia (serum Na <135 mEq/L):

• Use customized low-sodium fluids (119-126 mEq/L) to prevent overly rapid correction 1, 3
• Monitor serum sodium every 4-6 hours during severe hyponatremia correction (Na <120 mEq/L) 1, 4
• Target correction rate ≤6-8 mEq/L per 24 hours to prevent osmotic demyelination syndrome 1, 4
• Only 11% of patients using low-sodium dialysate had increases >6 mEq/L at 24 hours 3

For hypernatremia (serum Na >145 mEq/L):

• Standard or lower sodium dialysate can be used depending on correction goals 5
• Frequent monitoring essential to adjust fluid replacement rates 5

Common Pitfalls to Avoid

1. Confusing effluent volume with sodium removal: The 1600 mL/hour effluent rate represents total fluid removal, not net sodium removal 1, 2

2. Ignoring the concentration gradient: Without a gradient between blood and dialysate, minimal net sodium removal occurs despite high effluent rates 1

3. Overlooking other electrolyte losses: CRRT causes near-universal depletion of phosphate (60-80% prevalence), potassium (up to 25%), and magnesium (60-65% incidence), requiring proactive monitoring and replacement 1

4. Assuming prescribed dose equals delivered dose: Actual delivered CRRT dose often falls short of prescribed, necessitating frequent assessment 2, 6