Intermittent Techniques for Acute Dialysis

Objectives

This chapter will:

1.
Present the specificity to implement intermittent techniques to treat acute renal failure in the intensive care unit.
2.
Review the evidence regarding the effect of the type of dialysis membrane and the dialysis dose on the outcome for the patient with acute renal failure.
3.
Discuss the basic principles of prescribing intermittent hemodialysis for patients with acute renal failure, including treatments using sustained, low-efficiency intermittent hemodialysis.

Intermittent hemodialysis (IHD) was the first method available to treat patients with acute renal failure (ARF) in intensive care units (ICUs). In most countries, the use of IHD required a close collaboration with the nephrology team, and the implementation of the technique relied on nephrology practices. Continuous renal replacement therapies (CRRT), therefore, rapidly were accepted by intensivists to enhance ICU independency in the treatment of ARF. In addition, these new methods were thought to improve hemodynamic tolerance and were used widely in most parts of the world. Despite abundant relevant literature over the last 20 years, IHD seems to offer a similar outcome as CRRT for mortality and probably renal recovery. Intermittent techniques present some advantages, such as the preservation of patient mobility, the lower exposure to anticoagulants, and the capability to treat several patients a day with one machine. IHD remains widely used in some countries either as the first-line treatment or for patients without hemodynamic instability. Nevertheless, practical aspects have been developed specifically to enhance hemodynamic tolerance and dialysis dose to be adapted to ICU patients. Adaptations must be considered related to blood and dialysate flow, dialysate characteristics, net ultrafiltration, and session duration. New developments to improve efficiency and tolerance tend to become more popular with the use of low-efficiency prolonged IHD, usually called sustained low-efficiency dialysis (SLED) or slow efficiency extended hemodialysis. The chapter explains the specific characteristics to implement intermittent techniques of IHD to treat acutely ill patients with ARF.

Physical Principles

In IHD, molecule removal is driven by a concentration gradient between the vascular and dialysate side of the membrane, using a diffusive mechanism of exchange. This method favors small molecule removal, given their high diffusibility across the membrane, and provides a high efficiency (clearance around 200 mL/min). In a standard way, this method is based on a high dialysate flow (500 mL/min) and needs although a high blood flow (250–300 mL/min). In addition to diffusion a certain amount of convection called “net ultrafiltration” is used during each session to remove an excess of fluids. However, its effect on metabolic control or solute removal is insignificant.

Technical Aspects

The implementation of IHD requires specific equipment: a dialysis machine, a water treatment system, and electrolyte concentrates. Some other aspects may be different from CRRT: vascular access, dialysis membrane, and anticoagulation.

Dialysis Machine

The machine is devoted to the production of the dialysate using the online prepared pure water and the electrolyte concentrates, to the control of the ultrafiltration, and to the monitoring of the treatment. Recent improvements have been implemented in most machines, such as the online monitoring of the ionic dialysance (to monitor the delivered dialysis dose) or blood volume. The machine used in the ICU must be robust, compact, and easy to use.

Pure Water

The microbiologic quality of the prepared water is essential to achieve the best tolerance, including the absence of endotoxin, which may pass through the membrane from the dialysate to the vascular side. Inorganic and organic substances are removed from the water supply to obtain pure water. The water treatment system is composed with filters, a charcoal cartridge, and a reverse osmosis system. The water delivery may occur in three different ways: a central distribution from a specific water treatment system such as in chronic hemodialysis unit, a mobile water treatment incorporated in the dialysis machine, or more recently a batch-delivered system.

Dialysate

The electrolytic composition of the dialysate is attempted to achieve a good electrolytic equilibrium and a good uremic control at the end of the session. The choice of the electrolytic concentrate is paramount. Bicarbonate-based buffer is the standard buffer, given the hemodynamic effects provided by the old acetate-based buffer. For the electrolytic solution, particular attention must be paid to the potassium concentration (from 2 to 3 mmol/L) and the calcium concentration (from 1.25 to 1.75 mmol/L) to avoid dysrhythmia and hemodynamic instability. Their final dialysate concentrations depend on the product used and are provided by the manufacturer. The final sodium concentration (from 140 to 150 mmol/L) and bicarbonate concentration (from 30 to 36 mmol/L), however, can be selected on the dialysis machine and may be modified during treatment. The dialysate flow can be modified in almost all machines (from 300 to 750 mL/min).

Vascular Access

For the treatment of ARF in the ICU, double-lumen catheters are used instead of single-lumen catheters. The latter requires a dialysis machine able to deliver dialysis using the mode “single needle” but is associated with higher recirculation, decreasing the delivered dialysis dose. The best insertion site providing the higher blood flow is the right jugular vein, but femoral access still remains the emergency site and is associated with the lower rate of acute complication during insertion. Concerning the rate of nosocomial infection or catheter dysfunction between jugular and femoral access, recent data seem to challenge the usually reported higher rate of infection or catheter dysfunction with femoral access. The subclavian access should be avoided, considering the high rate of venous stenosis after dialysis catheter insertion. Usually the use of an arteriovenous fistula in chronic renal failure patients is discouraged in the ICU, considering the risk of infection, the risk of low cardiac output, and the lack of experience of the ICU nurse. Use of the long-term cuffed catheter may be considered after the acute phase in a stable patient, but the occurrence of systemic infection usually leads to catheter removal. The diameter of the catheter is important to consider to obtain a good blood flow with acceptable pressures. In this setting, 12 Fr seems to be the minimal inner diameter.

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