What are the principles of renal replacement therapy in patients with impaired renal function?

Principles of Renal Replacement Therapy

Core Indications for Initiating RRT

Renal replacement therapy should be initiated when patients develop life-threatening complications of kidney failure, including diuretic-unresponsive pulmonary edema, severe hyperkalemia, uremic complications (pericarditis, encephalopathy), or severe metabolic acidosis, with intervention typically occurring before these complications fully manifest in critically ill patients. 1

Absolute Indications

• Severe hyperkalemia unresponsive to medical management 1
• Volume overload with pulmonary edema refractory to diuretics 1
• Uremic complications including pericarditis, encephalopathy, or bleeding 1
• Severe metabolic acidosis unresponsive to conservative therapy 1
• Symptomatic uremia regardless of GFR level 1

Timing Considerations

• In chronic kidney disease, initiate when GFR falls below 10-15 mL/min/1.73 m² (Stage 5 CKD) 1
• In acute kidney injury, earlier intervention is justified before development of overt uremic symptoms, particularly in critically ill patients 1, 2
• Prophylactic dialysis may be considered for severe progressive hyperphosphatemia (>6 mg/dL) or symptomatic hypocalcemia in tumor lysis syndrome 1

Patient Selection and Preparation

Pre-Dialysis Education (Stage 4 CKD)

All patients reaching Stage 4 CKD (GFR <30 mL/min/1.73 m²) must receive comprehensive education about treatment options including kidney transplantation, peritoneal dialysis, home hemodialysis, in-center hemodialysis, and conservative management. 1

• Education should begin at Stage 4 CKD to allow adequate time for decision-making, access placement, and potential preemptive transplantation evaluation 1
• Family members and caregivers must be included in education 1
• Conservative therapy without dialysis is appropriate for select patients, utilizing low-protein diets, loop diuretics, and palliative care principles 1

GFR Estimation

• Use validated estimating equations rather than serum creatinine alone to guide decision-making 1
• Measure creatinine and urea clearances when equations may be inaccurate 1

Modality Selection

Continuous Renal Replacement Therapy (CRRT)

CRRT is the preferred modality for hemodynamically unstable patients with acute kidney injury, providing superior fluid balance control and hemodynamic stability compared to intermittent hemodialysis. 1, 3

Specific Indications for CRRT

• Hemodynamic instability with hypotension 1, 3
• Acute respiratory distress syndrome (ARDS) requiring improved gas exchange 1
• Septic shock with need for inflammatory mediator removal 1
• Cerebral edema or risk thereof (burns, severe metabolic derangements) 1
• Continuous volume removal requirements 1
• Pulmonary edema requiring careful fluid balance 1

CRRT Techniques

• Continuous venovenous hemofiltration (CVVH): predominantly convective clearance 3
• Continuous venovenous hemodialysis (CVVHD): predominantly diffusive clearance 3
• Continuous venovenous hemodiafiltration (CVVHDF): combines both mechanisms 3

Intermittent Hemodialysis (IHD)

• Preferred for hemodynamically stable patients 1, 4
• More cost-effective than CRRT 4
• Does not require continuous anticoagulation, reducing bleeding risk 4
• Provides rapid solute removal in acute life-threatening conditions 4
• Uric acid clearance approximately 70-100 mL/min 1

Peritoneal Dialysis

• Reserved for situations where other modalities are unavailable 1
• Lower efficiency in solute and fluid removal limits usefulness in acute settings 1
• Seldom used in tumor lysis syndrome or acute critical illness 1

Hybrid Therapies

• Sustained low-efficiency dialysis (SLEDD) and extended daily dialysis combine hemodynamic stability of CRRT with cost-efficiency of IHD 1, 4
• May be appropriate alternative in centers without CRRT capability for cardiovascularly unstable patients 1

Dialysis Dosing and Frequency

Acute Settings

• Daily dialysis recommended when continuous metabolite release occurs (e.g., tumor lysis syndrome) 1
• Timing and dose should be linked to the generation rate of toxins 1
• More frequent treatments may improve outcomes in acute kidney injury 1

CRRT Optimization

• Ensure adequate blood flow rates 5
• Use dialysate/replacement fluid with appropriate electrolyte composition (low or zero potassium in hyperkalemia) 5
• Increase CRRT dose/intensity if current prescription inadequate 5
• Maintain magnesium ≥0.70 mmol/L through dialysate composition 5

Vascular Access Planning

Vascular access planning must begin early in Stage 4 CKD, as arteriovenous fistula maturation requires months. 1, 6

• Permanent access should be in place at dialysis initiation 1
• Early planning prevents urgent catheter placement and associated complications 1

Metabolic Management During RRT

Phosphate Control

• Phosphate binders (calcium acetate) combine with dietary phosphate to form insoluble calcium phosphate excreted in feces 7
• Target serum phosphorus below 6.0 mg/dL 7
• Calcium acetate is systemically absorbed up to 40% fasting, 30% non-fasting 7

Electrolyte Monitoring

• Monitor magnesium, calcium, and phosphate daily during CRRT 5
• Target magnesium ≥0.70 mmol/L and phosphate >0.81 mmol/L 5

Key Clinical Pitfalls

• Avoid delaying RRT until severe complications develop in critically ill patients—earlier intervention is safer 1, 2
• Do not use serum creatinine alone to guide timing decisions; always calculate GFR 1
• Peritoneal dialysis is inadequate for acute situations requiring rapid solute/fluid removal 1
• CRRT requires proper anticoagulation and continuous monitoring, increasing complexity and bleeding risk compared to IHD 4
• Verify CRRT function if hyperkalemia persists despite therapy—check blood flow, dialysate composition, and prescription adequacy 5