Dialytic Therapy in the Intensive Care Setting
Deep Adhikari
Dagmar Klinger
I. GENERAL PRINCIPLES
A. Background.
1. Dialytic therapy or renal replacement therapy (RRT) is essential for management of patients with end-stage renal disease (ESRD) and can be helpful for managing acute kidney injury (AKI) and toxic ingestions.
2. Different modalities of RRT include intermittent hemodialysis (IHD), continuous renal replacement therapy (CRRT), and peritoneal dialysis (PD). Slow continuous ultrafiltration (SCUF), continuous venovenous hemofiltration (CVVH) or hemodiafiltration (CVVHD), and slow low-efficiency dialysis (SLED) are different modalities of CRRT.
3. In IHD and hemofiltration, solute and water pass from blood into a solution (dialysate) across a semipermeable membrane within a filter (dialyzer). Blood is continuously pumped via a vascular access through an extracorporeal dialyzer circuit. The dialyzer membrane permits diffusion of small molecules and water into the dialysate compartment, while cells and large molecules are retained in the blood compartment. The blood pump generates a pressure gradient across the dialyzer membrane that removes volume through filtration of fluid.
4. In PD, solute and water diffuse from the blood across the peritoneal membrane into dialysate infused into the peritoneal cavity through a peritoneal catheter. After a period of equilibration, the initial dialysate is exchanged for fresh dialysate.
B. Mechanisms of solute, toxins, and fluid removal.
1. Diffusion: Solute or water moves from an area of high concentration (blood) to an area of low concentration (dialysate) through the dialyzer membrane. Diffusion is affected by concentration gradient, solute characteristics (molecular size and charge), dialyzer membrane characteristics (surface area, porosity, and thickness), and flow rate of blood and dialysate.
2. Convection: Solutes and water are forced across the dialyzer membrane by hydrostatic pressure. Filtrate has essentially the same chemical composition as plasma. Convection is most important for volume removal and hemofiltration.
3. Adsorption: Some substances (cytokines, antibiotics) may adhere directly to the dialyzer membrane. This process is limited by the binding capacity of the membrane and is of uncertain clinical significance.
C. Optimal modality, timing for initiation, and dosing of RRT for AKI remain controversial and guidelines are being developed.
II. INDICATIONS
A. Absolute indications.
1. Life-threatening fluid and electrolyte imbalances (hyperkalemia, hypervolemia, and metabolic acidosis) not amenable to other treatments.
2. Severe uremic symptoms (pericarditis, encephalopathy).
3. Life-threatening intoxication with a dialyzable substance.
B. Relative indications.
1. Minor uremic symptoms (nausea, lethargy, bleeding exacerbated by uremic platelet dysfunction).
2. Non-life-threatening chemical imbalances (moderate hypercalcemia).
3. Need for volume removal to allow for maximal medical therapy (enteral or parenteral nutrition).
III. TYPES OF RRT
A. IHD: the preferred method of RRT in hemodynamically stable patients.
1. Typically performed thrice weekly, but done more often if indicated.
2. Treatment duration is usually 3 to 4 hours; extended dialysis may be necessary for toxic ingestion (e.g., methanol).
3. Yields high solute clearance through diffusion and rapid fluid removal through convection.
B. CRRT: preferred for patients with significant hemodynamic instability, severe volume overload, and/or high obligatory daily fluid requirements. Hourly clearance is lower than IHD, but total daily solute and fluid removal typically greater.
1. SCUF: Ultrafiltrate is removed slowly, but not replaced. Used for fluid removal with minimal solute clearance.
2. CVVH: large volumes of ultrafiltrate (>1.5 L/hour) removed via convection and replaced with a physiologic crystalloid (replacement fluid).
3. CVVHD: combination of CVVH and IHD.
4. SLED: similar to IHD, but with much slower blood and dialysate flows conducted over a longer duration.
