Importance of Therapy Fluid in CVVHD
The dialysate fluid composition in CVVHD is critically important for maintaining physiologic electrolyte balance, correcting acid-base disturbances, and preventing metabolic complications that directly impact patient morbidity and mortality. 1
Critical Fluid Composition Requirements
Electrolyte Concentrations
Dialysate must contain physiologic concentrations of electrolytes except in patients with extreme imbalances 1. The fluid composition directly determines what is removed from or added to the patient's circulation through diffusion across the dialysis membrane. Since CVVHD relies primarily on diffusive solute removal (unlike hemofiltration which uses convection), the dialysate composition becomes the primary driver of metabolic correction.
Buffer Selection: A Critical Decision
The choice of buffer in dialysate has profound clinical implications:
Bicarbonate is preferred over lactate in patients with lactic acidosis and/or liver failure 1. This is a grade C recommendation based on evidence showing worsening acidosis when lactate-buffered fluids are used in these populations 1.
Either lactate or bicarbonate can be used as buffer in most CRRT patients 1, though emerging evidence suggests bicarbonate-buffered solutions improve acid-base status and reduce cardiovascular events better than lactate 2.
In high-volume hemofiltration, bicarbonate is preferred 1.
Glucose Content: A Common Pitfall
Avoid dialysate fluids with supra-physiologic glucose concentrations 1. These frequently result in excessive glucose intake and hyperglycemia 1, which worsens outcomes in critically ill patients. This is a grade E recommendation but represents an important safety consideration.
Clinical Impact on Outcomes
The dialysate composition directly affects:
Metabolic stability: Inappropriate fluid composition causes metabolic acidosis, alkalosis, hyperglycemia, and electrolyte disturbances 3
Hemodynamic stability: Studies show that using inappropriate solutions (like peritoneal dialysis fluid) for CVVHD results in significantly higher rates of hypotension (71% vs 30%) compared to proper bicarbonate-buffered solutions 3
Acid-base correction: The buffer type and concentration determine whether acidosis is corrected or worsened 2
Practical Considerations
Sterility Requirements
While sterile replacement fluid is imperative in hemofiltration, the bacteriological requirements for CVVHD dialysate are less stringent, except in high-flux dialysis where back-filtration occurs and dialysate should be sterile 1.
Temperature Management
Although reductions below 35°C should probably be avoided, current evidence doesn't allow firm recommendations on whether dialysate should be warmed 1.
Quality Control
Clinicians must ensure quality control for sterility, physical properties, individualized prescription, and balance control 2. This is particularly important when using machine-generated solutions rather than commercial products 4.
Common Errors to Avoid
The most significant pitfall is using inappropriate substitute fluids (like peritoneal dialysis solutions) for CVVHD, which causes:
- Metabolic acidosis in 57% of patients (vs 30% with proper solution) 3
- Hyperglycemia in 64% of patients (vs 30% with proper solution) 3
- Increased hypotension rates 3
The dialysate composition must be balanced with any replacement fluids being used simultaneously 2, as both contribute to the patient's overall metabolic balance.