4.2 Acute Renal Replacement Therapies & Professional Issues

Continuous Renal Replacement Therapy Modalities

In patients with acute kidney injury (AKI) who are hemodynamically unstable, continuous renal replacement therapy (CRRT) is the treatment of choice. Unlike intermittent hemodialysis, CRRT provides slow, continuous fluid and solute clearance over 24 hours, minimizing rapid shifts in blood pressure and osmolarity.

CRRT modalities are classified based on the physical transport principles they utilize:

• Continuous Venovenous Hemofiltration (CVVH) relies on convection. Hydrostatic pressure drives water across a semipermeable membrane, pulling solutes along with it—a process known as solvent drag. CVVH is highly effective at clearing middle and larger molecules (such as inflammatory cytokines). To replace the large volume of fluid removed, a replacement fluid is infused into the blood circuit either before the filter (pre-dilution) or after the filter (post-dilution). Pre-dilution decreases blood viscosity and reduces clotting risk inside the dialyzer, but dilutes solute concentration, lowering clearance efficiency by approximately fifteen to twenty percent. Post-dilution provides maximum solute clearance but increases hemoconcentration and clotting risk within the filter fibers.
• Continuous Venovenous Hemodialysis (CVVHD) utilizes diffusion. Solutes move down their concentration gradient from the blood side to the dialysate side across the membrane. A sterile dialysate solution flows counter-current to blood flow. CVVHD is highly efficient at removing small molecules (such as urea, creatinine, potassium) but does not clear middle-to-large molecules well.
• Continuous Venovenous Hemodiafiltration (CVVHDF) combines both convection and diffusion. It utilizes both dialysate and replacement fluid, providing maximum clearance of both small and middle-sized molecules.
• Slow Continuous Ultrafiltration (SCUF) is a simplified modality designed solely for fluid removal in patients with volume overload but without significant uremia or electrolyte disturbances. It uses ultrafiltration without dialysate or replacement fluid.

Circuit Anticoagulation Methods

Maintaining circuit patency in CRRT requires effective anticoagulation. The two primary methods are:

• Regional Citrate Anticoagulation (RCA): Citrate is infused into the pre-filter blood line. It acts by chelating ionized calcium, which is cofactor IV in the coagulation cascade. By reducing ionized calcium levels within the circuit (target circuit ionized calcium is zero point twenty-five to zero point thirty-five mmol/L), clotting is prevented. Before the blood is returned to the patient, calcium chloride or calcium gluconate is infused centrally post-filter to restore normal systemic ionized calcium levels (target zero point nine to one point two mmol/L). The main risk is citrate toxicity (citrate lock), where the liver or muscles fail to metabolize citrate. This results in systemic accumulation of calcium-citrate complexes, raising systemic total calcium while dropping ionized calcium. Citrate toxicity is recognized by a systemic total-to-ionized calcium ratio greater than two point five, accompanied by metabolic alkalosis (from citrate metabolism to bicarbonate) or metabolic acidosis (if unmetabolized).
• Heparin Anticoagulation: Systemic unfractionated heparin is infused pre-filter. It is monitored via activated clotting time (ACT) or partial thromboplastin time (PTT). It carries a high risk of systemic bleeding and heparin-induced thrombocytopenia (HIT).

Circuit Pressures and Alarm Troubleshooting

CRRT pressures are monitored at various points in the extracorporeal circuit. Understanding these pressures is critical for troubleshooting alarms:

• Access Pressure: The pressure required to pull blood from the patient. It is always negative (usually -50 to -150 mmHg). Excessively negative access pressure indicates suctioning against the vessel wall, catheter kinking, catheter clotting, or patient dehydration.
• Return Pressure: The pressure required to return blood to the patient. It is always positive. High return pressure indicates a kinked return line, clotting in the venous drip chamber, venous catheter occlusion, or patient movement.
• Filter Pressure Drop (Pressure Drop): The difference between pre-filter and post-filter pressures. A steady increase in pressure drop indicates progressive clotting of the hollow fibers within the dialyzer.
• Transmembrane Pressure (TMP): The pressure gradient across the membrane (average blood path pressure minus effluent path pressure). A rising TMP with a stable pressure drop indicates membrane clogging (adsorption of proteins coating the membrane pores), which reduces filtration capacity.

Professional, Quality, and Ethical Standards

Nephrology nursing practice is guided by strict regulatory and quality frameworks:

• CMS Conditions for Coverage: Centers for Medicare & Medicaid Services (CMS) mandates standards for safety, infection control, water quality, and patient care planning in dialysis facilities. Interdisciplinary team (IDT) collaboration—comprising the nephrologist, nephrology nurse, social worker, and dietitian—is required to develop and update individualized patient plans of care.
• QAPI Programs: Quality Assessment and Performance Improvement (QAPI) programs are data-driven initiatives designed to track and improve clinical indicators such as dialysis adequacy, vascular access infection rates, anemia management, and patient satisfaction. Facilities utilize Plan-Do-Study-Act (PDSA) cycles to address quality deficits.
• Ethical Issues: Core ethical principles in nephrology include autonomy (supporting a patient's right to refuse or withdraw from renal replacement therapy), beneficence (acting in the patient's best interest), and non-maleficence (avoiding harm, such as continuing futile therapy). Nurses facilitate discussions regarding advance directives, surrogate decision-making, and transition to palliative care.