The article provides a comprehensive overview of indications, modalities, timing, dose, and practical considerations for dialysis in critical care, highlighting the advantages of continuous renal replacement therapies (CRRT) over intermittent hemodialysis.

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Introduction

Acute renal failure is a common complication in the intensive care unit (ICU). Over the last 25 years, there have been significant technological advances in the delivery of renal replacement therapy, particularly as it pertains to the critically ill patient population. Despite these advances, acute renal failure in critically ill patients continues to carry a poor prognosis.

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Indications for dialysis in critical care

• Diuretic-resistant pulmonary edema
• Hyperkalemia (refractory to medical therapy)
• Metabolic acidosis (refractory to medical therapy)
• Uremic complications (pericarditis, encephalopathy, bleeding)
• Dialyzable intoxications (e.g., lithium, toxic alcohols, and salicylates)

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Modalities of dialysis in critical care

• Peritoneal dialysis (PD)
• Intermittent hemodialysis (IHD)
• Continuous renal replacement therapies (CRRT)

Peritoneal dialysis

1. Peritoneal dialysis is valuable in pediatric critical care, where vascular access is challenging and the peritoneal surface area is relatively larger than in adults.
2. In adult patients, acute peritoneal dialysis is not widely used. The use of peritoneal dialysis is limited to both logistical and practical considerations. Acute peritoneal dialysis requires surgical insertion of a peritoneal dialysis catheter, requiring the additional involvement of a surgical team.
3. Acute PD is frequently complicated by catheter leakage and malfunction. In addition, the use of PD is limited by low solute clearance in hypercatabolic patients, potential pulmonary restriction because of expansion of the peritoneal cavity, and its contraindication in postoperative patients who require abdominal surgery or surgical drains.

Intermittent hemodialysis

Slow efficiency daily dialysis (SLED) is a variant of IHD where the duration of dialysis is extended to between 8 and 12 h, the blood flow is lowered, fluid removal is more gradual and solute clearance slower.

Continuous renal replacement therapies (CRRT)

1. The term CRRT describes a variety of blood purification techniques, which may differ significantly according to the mechanism of solute transport, the type of membrane, the presence or absence of dialysate solution, and the type of vascular access.
2. CRRT provides slower solute clearance per unit time as compared with intermittent therapies, but over 24 hours may even exceed clearances with IHD.
3. Solute removal with CRRT is achieved either by convection (hemofiltration), diffusion (hemodialysis), or a combination of both these methods (hemodiafiltration). Hemodialysis most efficiently removes small molecular weight substances such as urea, creatinine, and potassium. Middle and larger molecular weight substances are more efficiently removed using hemofiltration as compared with dialysis.
4. The choice of modality is dependent on several factors, including availability, cost, physician expertise, hemodynamic stability, and the primary purpose of the procedure (fluid removal vs. solute clearance).
5. The most commonly applied modalities are continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), and continuous venovenous hemodiafiltration (CVVHDF).

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Timing for initiation of dialysis

There is no commonly accepted definition of the timing of initiating renal replacement therapy in ARF. It has been suggested that patient outcomes can be improved by early or more intensive dialysis to keep the BUN under 80–100 mg/dL.

The Acute Dialysis Quality Initiative (ADQI) consensus statement on dialysis treatment makes no recommendations on the timing of initiation of renal replacement therapy beyond those defined by the conventional criteria that apply to chronic renal failure.

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Dialysis dose

1. The notion of dialysis dose quantification in ARF has been controversial, due in part, to the lack of convincing evidence that azotemia control affects outcomes in patients with either acute or chronic renal failure.
2. At present, there is no consensus as to what the minimal dialysis dose should be in patients with ARF.
3. Extrapolating data from the end-stage renal disease (ESRD) population, it seems reasonable to suggest a minimum Kt/V of 1.2 should be delivered at least three times a week in patients with ARF.

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Practical issues regarding dialysis in ICU

Vascular access

1. Vascular access for acute intermittent hemodialysis must be obtained using double-lumen dialysis catheters. These may be inserted in the internal jugular, subclavian, or femoral veins.
2. If possible, subclavian vein cannulation should be avoided because of a high incidence of subsequent venous stenosis, which may significantly complicate venous access if chronic hemodialysis is required.
3. Use of a short 13.5-cm catheter in the femoral vein may result in up to 23% blood flow recirculation and can be avoided by using longer catheters (19–25 cm) when using the femoral vein for vascular access.

Anticoagulation

1. Anticoagulation is an essential component of all blood-based renal replacement therapies, including CRRT. The passage of blood through an extracorporeal circuit causes platelet activation and induces a variety of inflammatory and prothrombotic mediators, resulting in fibrin deposition on filter membranes.
2. Unfractionated heparin is the mainstay of anticoagulation for IHD and CRRT. Low-molecular-weight heparin is excreted renally and should not be used without careful monitoring of factor Xa levels in patients with ARF.
3. Because of the shorter duration and high blood flow used, it is often possible to perform IHD without anticoagulation, particularly when the patient is coagulopathic.

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Advantage of CRRT over intermittent HD

• Good hemodynamic tolerability
• Efficient solute clearance
• Better control of intravascular volume
• Better clearance of middle and large molecular weight substances

Some of the causes for hypertension are:

• Dialysis-specific, such as excessive or rapid volume removal
• Changes in plasma osmolality
• Autonomic dysfunction

In critically ill patients who may be hemodynamically unstable, it would be desirable to minimise this complication, as it may lead to further organ ischemia and injury.

Another advantage of CRRT is the improved efficiency of solute removal. Although the clearance rate of small solutes is slower per unit time with CRRT (17 mL/min vs. more than 160 mL/min with conventional hemodialysis), CRRT is continuously administered; therefore, urea clearance is more efficient after 48 h than with alternate day intermittent hemodialysis.

CRRT may also have an immunomodulatory effect. The rationale for the use of CRRT for the treatment of sepsis arises from the observed association between sepsis severity mortality rate and serum concentrations of various cytokines, including TNF, IL1, IL6, and IL8.

Most of these middle molecular weight molecules are water soluble and are theoretically removable by hemofiltration-based plasma water purification. At present, the immunomodulatory effects of CRRT remain theoretical.

Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

About the author of this article: Dr Bhavin Mandowara is a practising nephrologist at Zydus Hospital, Ahmedabad.