Renal Replacement Therapy and Rhabdomyolysis

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Chapter 43 Renal Replacement Therapy and Rhabdomyolysis

Brad W. Butcher, MD , Kathleen D. Liu, MD, PhD, MAS

Renal Replacement Therapy

1 What are the indications for renal replacement therapy (RRT)?

Indications can be grouped by using the AEIOU mnemonic:

A: (Metabolic) Acidosis refractory to bicarbonate administration.

E: Electrolyte imbalances, of which hyperkalemia is the most life threatening.

I: Ingestions. Some drugs and toxins (and their toxic metabolites) can be cleared with dialysis, including aspirin, lithium, methanol, or ethylene glycol.

O: Overload. Ultrafiltration with dialysis can relieve hypoxemia resulting from volume overload, which may be particularly problematic in the setting of oliguria or anuria.

U: Uremia. Symptoms and signs of uremia can range from mild (anorexia, nausea, pruritus) to severe (encephalopathy, asterixis, pericarditis); patients may also have clinical platelet dysfunction (bleeding) due to uremia.

3 What are hybrid therapies?

SLED, SLEDF, EDD, and SCD collectively refer to recently developed hybrid modes of dialysis. Dialysis can be delivered through a variety of conventional IHD machines (an advantage over CRRT), usually with some minor modifications to allow for slower dialysate flow rates compared with IHD. Therapy is delivered intermittently but over a longer time period (6-12 hours per session) than conventional IHD (3-4 hours per session) and often on a daily basis. Thus hybrid therapies have many of the benefits of CRRT without some of the disadvantages (see question 5).

4 In whom should CRRT or hybrid therapy be considered?

CRRT or hybrid therapy should be considered in any critically ill patient with an indication for dialysis. CRRT or hybrid modalities tend to be better tolerated hemodynamically than intermittent dialysis because of slower rates of solute flux and fluid removal. CRRT may also allow for increased net daily ultrafiltration compared with IHD because volume removal occurs continuously, albeit at a slower rate. Furthermore, in highly catabolic, critically ill patients, increased clearance with CRRT or hybrid modalities compared with IHD may allow for better control of azotemia, acidosis, and electrolyte abnormalities, including hyperphosphatemia. Last, patients with elevated intracranial pressure or fulminant hepatic failure may not tolerate IHD because of rapid shifts in solute concentrations. Importantly, IHD is preferable to CRRT in patients with severe, life-threatening hyperkalemia and some types of ingestions because clearance per unit time is slower with CRRT than with IHD.

5 What are some disadvantages of CRRT?

Because of its continuous nature, CRRT requires long-term relative immobilization of the patient, which can increase the risk for venous thromboembolism, pressure ulcers, and persistent deconditioning. Continuous anticoagulation is often necessary to prevent filter clotting and subsequent blood loss, and this may increase the bleeding risk. CRRT frequently results in hypothermia as blood is cooled during transit through the extracorporeal circuit; importantly, this can mask the development of a fever. Last, CRRT is highly labor intensive, typically requiring 1:1 nursing, and therefore costly.

6 Define hemofiltration, hemodialysis, and hemodiafiltration

Hemofiltration: Plasma is forced from the blood space into the effluent via the application of pressure across a highly permeable membrane. This results in convective clearance of small and middle-sized molecules through the physical property of solvent drag. This modality does not significantly change the concentration of serum electrolytes and waste products unless a replacement fluid is infused into the blood, effectively diluting out those solutes the physician wishes to remove (e.g., urea nitrogen and potassium) and increasing the concentration of those solutes in which the patient might be deficient (e.g., bicarbonate in a patient with acidemia).

Hemodialysis: Blood flows on one side of a semipermeable membrane, and the dialysate, which contains various electrolytes and glucose, flows along the other side, usually in the opposite (countercurrent) direction. A concentration gradient drives electrolytes and water-soluble waste products from the plasma compartment into the dialysate. The dialysis machine generates a pressure across the membrane to drive plasma water from the blood side to the dialysate side. Dialysis results in diffusive clearance, preferentially of small molecules.

Hemodiafiltration: This technique makes simultaneous use of hemofiltration and hemodialysis, resulting in both diffusive and convective clearance.

7 List the basic components of a prescription for IHD and for CRRT

Dialysis access: Arteriovenous fistula, arteriovenous graft, tunneled dialysis catheter, or temporary dialysis catheter

Treatment duration: For most patients with end-stage renal disease, this ranges between 3 and 4 hours. When a patient with acute renal failure or acute kidney injury (AKI) starts hemodialysis, initial sessions may be as short as 1 to 1.5 hours to decrease the risk of dialysis disequilibrium syndrome.

Filter size and type: Biocompatible dialysis membranes are now routinely used.

Blood flow rate: Blood flow rates of up to 400 to 450 mL/min can be achieved with an arteriovenous fistula or graft and up to 350 mL/min with a tunneled or temporary catheter. Generally, the faster the flow, the more efficient the dialysis.

Dialysate flow rate: Typical flow rates range from 500 mL/min to 800 mL/min.

Dialysate bath: Concentrations of potassium, sodium, calcium, and bicarbonate can be customized on the basis of the patient’s laboratory studies.

Ultrafiltration goal: This is the amount of fluid to be removed from the patient over the course of the session; determined by clinical assessment of the patient’s volume status.

Anticoagulation: Clotting within the dialysis circuit can result in significant blood loss; heparin is typically used unless the patient has a contraindication.

As in IHD, the prescription includes dialysis access, filter size and type, hourly fluid balance, and anticoagulation. An alternative to heparin anticoagulation often used with CRRT is regional citrate anticoagulation, in which citrate is administered to chelate calcium, a critical cofactor in the clotting cascade. Arteriovenous fistular and grafts are not used for CART.

Blood flow rates are typically slower than in intermittent dialysis (150-200 mL/min).

Mode of therapy: CVVH, CVVHD, or CVVHDF

Dialysate or replacement fluid: The specific fluid is based on the metabolic parameters of the patient, including the patient’s acid-base status and serum potassium concentration.

Dialysate or replacement fluid flow rate: Dosing is weight based and is typically prescribed at a dose ranging from 20 mL/kg/hr to 35 mL/kg/hr, based on the patient’s weight. Studies have shown no mortality difference between patients with renal replacement therapy administered at these two rates.

8 What kinds of laboratory tests should be ordered regularly for patients receiving CRRT?

Sodium, potassium, bicarbonate, calcium, and phosphate levels can change rapidly during CRRT. Hyperphosphatemia frequently occurs in IHD because of inefficient clearance of phosphate, but hypophosphatemia is more common during CRRT given the continuous clearance of phosphate. Hypocalcemia and hypomagnesemia are also seen, especially when these cations are complexed with citrate (e.g., when citrate is used as an anticoagulant) or when a replacement fluid without these cations is infused into the patient (e.g., during CVVH). Patients with impaired lactate metabolism (e.g., because of severe sepsis or hepatic failure) may have high systemic lactate levels if the dialysate or replacement fluid contains lactate as a base equivalent. In these cases, high lactate levels or worsening acidosis should prompt the use of a bicarbonate-based dialysate or replacement fluid. Blood gas levels should be checked to monitor the patient’s acid-base status.

9 What are nutrition considerations for patients with AKI receiving RRT?

Amino acids are lost in both IHD and CRRT. Critically ill patients with AKI are often highly catabolic; many patients receiving CRRT will require at least 1.5 to 2 g/kg/day of protein or amino acids.

Water-soluble vitamins are lost in both IHD and CRRT. Replacement of these vitamins can be achieved with the daily administration of a vitamin complex specifically designed for patients receiving RRT.

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