7.2 Renal Replacement Therapy

When Is Renal Replacement Therapy Needed?

When the kidneys can no longer keep pace with waste, fluid, and electrolyte control, renal replacement therapy (RRT) takes over. The classic emergent indications are captured by AEIOU:

• Acidosis - severe refractory metabolic acidosis not corrected by bicarbonate or ventilation.
• Electrolytes - life-threatening hyperkalemia (or other derangements) unresponsive to medical therapy.
• Intoxications - dialyzable toxins such as methanol, ethylene glycol, lithium, and salicylates.
• Overload - fluid overload/pulmonary edema refractory to diuretics.
• Uremia - symptomatic uremia with pericarditis, encephalopathy, or platelet dysfunction/bleeding.

Any single indication can justify starting RRT; the CCRN rewards recognizing that RRT is a supportive bridge, not a cure, while the underlying AKI recovers.

Modalities: IHD vs CRRT vs SLED

Intermittent hemodialysis (IHD) runs 3-4 hours and clears solute and fluid rapidly through diffusion across a dialyzer. It is efficient and cost-effective for hemodynamically stable patients, but the fast fluid and solute shifts often cause hypotension and can trigger dialysis disequilibrium syndrome - a rapid drop in blood urea that pulls water into the brain, causing headache, nausea, and in severe cases cerebral edema and seizures. Because of this, IHD is a poor choice for the unstable, edematous, or brain-injured patient.

Continuous renal replacement therapy (CRRT) runs continuously over 24 hours at low blood-flow rates, removing fluid and solute slowly and gently. This is why CRRT is preferred for the hemodynamically unstable patient: the gradual ultrafiltration avoids abrupt intravascular volume shifts and the hypotension that accompanies rapid IHD, and it protects patients with raised intracranial pressure or cerebral edema. CRRT sub-modes differ by how they clear solute:

• SCUF (slow continuous ultrafiltration): fluid removal only, no clearance.
• CVVH (continuous venovenous hemofiltration): convection - solute is dragged across the membrane with ultrafiltered plasma water, requiring replacement fluid; good for larger "middle" molecules.
• CVVHD (continuous venovenous hemodialysis): diffusion - dialysate flows countercurrent and small solutes diffuse down their gradient.
• CVVHDF (hemodiafiltration): combines convection and diffusion for the broadest clearance.

Sustained low-efficiency dialysis (SLED) is a hybrid: a 6-12 hour treatment at intermediate blood-flow rates. It is gentler than IHD but frees the patient from the 24-hour circuit of CRRT, a useful middle ground for patients who have partly stabilized.

Anticoagulation, Complications, and Nursing Care

Extracorporeal blood clots unless the circuit is anticoagulated. Regional citrate anticoagulation (RCA) is now first-line for many units: citrate infused into the circuit chelates ionized calcium, blocking clotting only inside the machine. Because citrate lowers ionized calcium, the nurse must infuse calcium back to the patient and monitor systemic ionized calcium closely. Watch for citrate accumulation (especially with hepatic failure), which shows a rising total-to-ionized calcium ratio (>2.5) and a metabolic alkalosis. The alternative, systemic heparin, is titrated to aPTT but risks bleeding and HIT.

Complications to anticipate: hypotension, hypothermia (cool circuit blood - use a blood warmer), electrolyte losses that need active replacement (hypophosphatemia, hypokalemia, hypomagnesemia, hypocalcemia), bleeding, catheter-related infection, air embolism, circuit clotting, and altered drug clearance (many antibiotics need dose adjustment during CRRT). Nursing care centers on strict hourly fluid balance, monitoring circuit pressures (access, return/venous, and transmembrane pressure), assessing the vascular access (typically internal jugular or femoral), tracking the filtration fraction, replacing electrolytes per protocol every 6-12 hours, and troubleshooting alarms before the circuit clots and blood is lost.

Preventing Two High-Yield Renal Insults

Rhabdomyolysis (crush injury, prolonged immobility, statins, seizures) releases myoglobin, which is directly nephrotoxic. Hallmarks are tea-colored urine, CK often >20,000, and rising potassium. Priority management is aggressive IV crystalloid to maintain urine output around 200-300 mL/h, plus vigilant monitoring for hyperkalemia and compartment syndrome; severe cases progress to AKI needing RRT.

Contrast-induced AKI (CI-AKI) is a creatinine rise of >=0.5 mg/dL or 25% within 48-72 hours of iodinated contrast. Prevention is IV isotonic saline hydration before and after the study, using the smallest contrast volume, holding nephrotoxins (NSAIDs, and metformin around the procedure), and avoiding repeat contrast studies. Routine N-acetylcysteine is no longer strongly supported by evidence and should not replace hydration.

Timing, Dosing, and Monitoring Pearls

The optimal timing of RRT initiation in AKI without an urgent AEIOU indication remains debated; recent trials show no benefit to routinely starting early, so unstable patients are watched for a hard indication rather than dialyzed prophylactically. Because CRRT clears drugs continuously, antibiotic and sedative dosing must be reviewed daily - underdosing beta-lactams and vancomycin during CRRT is a real cause of treatment failure, while some agents accumulate. Track the effluent volume and the prescribed dose (commonly around 20-25 mL/kg/h of effluent), reconcile hourly fluid removal against the ordered net balance, and never let the ultrafiltration outpace the patient's hemodynamics. Assess the access catheter for kinking and infection, keep the blood warmed, and treat frequent low-pressure or clotting alarms promptly, because a clotted circuit wastes a full blood volume and interrupts therapy. Replete phosphate, potassium, magnesium, and calcium on a scheduled basis, since continuous clearance steadily strips them. These monitoring habits, more than the specific modality name, are what CCRN scenarios test at the bedside.