What electrolytes are removed during isolated ultrafiltration (UF) in dialysis?

Electrolytes Removed During Isolated Ultrafiltration in Dialysis

During isolated ultrafiltration in dialysis, primarily sodium, potassium, calcium, magnesium, and phosphate are removed, with the electrolyte concentration in the ultrafiltrate being similar to that of plasma. 1

Mechanism of Electrolyte Removal in Ultrafiltration

Ultrafiltration involves the movement of water and small-to-medium weight solutes across a semipermeable membrane to reduce volume overload. Unlike diffusion-based dialysis, isolated ultrafiltration works primarily through convection:

• Convective transport: During ultrafiltration, electrolytes are carried across the membrane along with water (solvent drag) 2
• Concentration similarity: The ultrafiltrate has an electrolyte concentration similar to plasma, allowing relatively more sodium removal than diuretics 1
• Source of fluid loss: Fluid removal comes almost exclusively from the extracellular fluid space 1

Specific Electrolytes Removed

Sodium

• Primary electrolyte removed during ultrafiltration
• Approximately 10 mmol/L of sodium is removed per deciliter of ultrafiltrate in a 4-hour session 3
• Sodium removal is predominantly convective during isolated ultrafiltration 4
• Excessive sodium removal can lead to hyponatremia if not properly monitored

Potassium

• Removed at concentrations similar to plasma
• During standard hemodialysis (not isolated UF), 40-110 mmol of potassium is removed in a 5-hour session, predominantly by diffusion (72-88%) 4
• In isolated ultrafiltration, potassium removal is proportional to the ultrafiltration volume

Calcium

• Approximately 0.12 mmol/L of calcium is removed per deciliter of ultrafiltrate in a 4-hour session 3
• Calcium removal is convection-dominated during ultrafiltration 3
• Excessive removal can contribute to hypocalcemia

Magnesium

• Removed through convection during ultrafiltration
• Hypomagnesemia is a common complication in patients undergoing prolonged kidney replacement therapy 2
• Magnesium removal is particularly significant when regional citrate anticoagulation is used 2

Phosphate

• Removed during ultrafiltration, though less efficiently than during diffusive dialysis
• Hypophosphatemia is a common complication of intensive kidney replacement therapy 2

Clinical Implications and Management

Electrolyte Disorders

• Common laboratory abnormalities associated with intensive/prolonged kidney replacement therapies include hypophosphatemia, hypokalemia, and hypomagnesemia 2
• Hypophosphatemia has a high reported prevalence (60-80%) in ICU patients and is associated with negative impacts on patient outcomes 2
• Hypokalemia is observed in approximately 25% of patients with kidney failure started on prolonged modalities of kidney replacement therapy 2

Prevention Strategies

• Dialysis solutions: Use of dialysis solutions containing potassium, phosphate, and magnesium to prevent electrolyte disorders during kidney replacement therapy 2
• Monitoring: Regular monitoring of electrolytes during and after ultrafiltration is essential 2
• Replacement fluids: Commercial kidney replacement therapy solutions enriched with phosphate, potassium, and magnesium are available and can be safely used 2

Special Considerations

• Patients with heart failure may benefit from ultrafiltration to reduce neurohormone levels and increase diuretic responsiveness 1
• Isolated ultrafiltration can be beneficial for patients who experience hypotension during combined ultrafiltration and dialysis 1
• Consultation with a nephrologist is recommended before initiating ultrafiltration outside standard dialysis 1

Pitfalls and Caveats

• Electrolyte imbalances: Rapid fluid removal can lead to significant electrolyte disturbances if not properly monitored
• Hemodynamic instability: Ultrafiltration rate should be carefully managed to avoid complications, particularly in patients with cardiovascular instability
• Individualized approach: The ultrafiltration rate should be slowed when patients experience symptoms, particularly for those with diabetes or cardiomyopathy 1
• Monitoring requirements: Regular assessment of pre- and post-procedure electrolytes is essential to prevent complications

By understanding the electrolytes removed during isolated ultrafiltration and implementing appropriate monitoring and prevention strategies, clinicians can optimize the safety and efficacy of this procedure in managing volume overload in dialysis patients.