What is the typical composition of dialysate and replacement fluid for patients undergoing dialysis, particularly in terms of electrolyte levels?

Dialysate and Replacement Fluid Composition

Standard Electrolyte Composition

Dialysate and replacement fluids used during continuous renal replacement therapy (CRRT) and hemodialysis should contain physiologic concentrations of electrolytes to restore plasma composition toward normal values. 1

Core Electrolyte Targets

• Sodium: Approximately 140 mmol/L to match physiologic plasma concentrations 2, 3
• Potassium: Typically 2-4 mmol/L, though this requires consideration of individual patient needs 4, 5
• Calcium: 1.25-1.5 mmol/L (2.5-3.0 mEq/L) for hemodynamic stability and prevention of cardiac arrhythmias 5, 3
• Magnesium: 0.5-0.75 mmol/L (approximately 1.2-1.8 mg/dL), with higher concentrations (≥0.70 mmol/L) recommended to prevent hypomagnesemia 1, 6, 5
• Chloride: Adjusted based on buffer composition to maintain acid-base balance 7

Buffer Systems

Either lactate or bicarbonate can be used as buffer in most CRRT patients, but bicarbonate is strongly preferred in patients with lactic acidosis and/or liver failure. 1

• Bicarbonate: 22-35 mmol/L is the preferred buffer for most patients, particularly those with liver failure or lactic acidosis 1
• Lactate: Can be used in stable patients without hepatic dysfunction, but worsening acidosis has been documented when used in patients with lactic acidosis or liver failure 1
• Bicarbonate is mandatory for high-volume hemofiltration to prevent acid-base disturbances 1

Critical Composition Considerations

Glucose Content

Supra-physiologic glucose concentrations found in some dialysis or substitution fluids should be avoided as they frequently result in excessive glucose intake and hyperglycemia. 1

• Standard dialysate should contain minimal or no glucose to prevent metabolic complications 1

Fluid Sterility Requirements

• Replacement fluid must be sterile 1
• Dialysate should be sterile in high-flux dialysis due to back-filtration risk 1
• Ultra-pure dialysis fluid is increasingly recommended to prevent chronic inflammation from bacterial fragments 4

Special Populations and Adjustments

Patients with Extreme Electrolyte Imbalances

Dialysate composition should be adjusted from physiologic concentrations only in patients with extreme imbalances. 1

• Patients with severe hyperkalemia may require potassium-free dialysate temporarily 5, 3
• Those with severe hypocalcemia or hypercalcemia require calcium concentration adjustments 5, 3

Patients on Continuous Renal Replacement Therapy (CRRT)

For patients requiring CRRT, use commercial solutions enriched with magnesium, potassium, and phosphate rather than relying on exogenous intravenous supplementation. 8, 6

• Hypomagnesemia occurs in 60-65% of critically ill patients on CRRT, particularly with regional citrate anticoagulation 6, 9
• Citrate chelates ionized magnesium, dramatically increasing losses through removal as magnesium-citrate complexes 6, 9
• Exogenous intravenous electrolyte supplementation during CRRT should be avoided as it carries severe clinical risks 8, 6

High-Volume Hemofiltration

When performing high-volume hemofiltration (6-8 L/hour exchanges), standard commercially available replacement solutions may cause significant acid-base and electrolyte imbalances. 7

• Fluids with higher lactate concentrations and lower chloride predispose to hypochloremic alkalosis 7
• Fluids with lower lactate and higher chloride can lead to hyperchloremic acidosis 7
• Bicarbonate-based solutions are preferred for high-volume exchanges 1

Pre-Dilution vs. Post-Dilution Considerations

Fluids administered before the hemofilter (pre-dilution) enhance achievable ultrafiltration rates and may be useful in patients with frequent filter clotting. 1

• Pre-dilution is especially important in high-volume CVVH 1
• Combination of pre- and post-dilution may be used when extracorporeal clearance is limited by achievable blood flow 1
• Sodium balance differs between replacement solution and dialysate use, with greater sodium balance in post-dilutional fluid replacement due to lower sieving coefficient 7

Critical Pitfalls to Avoid

• Never use peritoneal dialysis fluids designed for low-volume exchanges in high-volume CRRT, as they can cause severe electrolyte and acid-base disturbances 7
• Avoid lactate-buffered solutions in patients with liver failure or lactic acidosis, as this worsens acidosis 1
• Do not overlook magnesium supplementation in CRRT patients, especially those receiving citrate anticoagulation 8, 6
• Prevent hyperglycemia by avoiding glucose-containing dialysate unless specifically indicated 1
• Maintain negative fluid balance in patients with acute lung injury to decrease ICU length of stay, but avoid temperature reductions below 35°C 1