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Mechanical Fluid Removal
Objectives
This chapter will:
- 1.
Provide an overview of the different types of mechanical fluid removal and their risks and benefits.
- 2.
Summarize the results of clinical trials in this field.
Fluid overload is common during critical illness. There are a number of potential reasons why patients may be fluid overloaded. They may be fluid overloaded on admission to hospital, as in decompensated heart failure, or they may develop fluid overload later as a result of excessive fluid administration, reduced urine output, or a combination of both. Fluid overload is associated with serious complications, including an increased risk of acute kidney injury (AKI), the development of respiratory failure, a longer duration on mechanical ventilation, and increased mortality. It is often difficult to manage, especially in the context of hemodynamic instability. According to data from large national registries, approximately 40% of hospitalized heart failure patients are discharged with unresolved congestion, which may contribute to further rehospitalization.
Rationale for Mechanical Fluid Removal
Ultrafiltration (UF) involves the removal of an iso-osmotic solution of plasma water and electrolytes from whole blood across a membrane. During UF, the circulating blood volume is maintained by recruitment of interstitial fluid into the intravascular space (vascular refill). Ideally, both processes should occur at a similar rate to prevent hemodynamic instability. Hypotension is a complication that can occur when the rate of removal of plasma water exceeds the refilling capacity.
Compared with pharmacologic measures, UF has several advantages. First, fluid removal by extracorporeal techniques is fully controllable and adjustable. Second, the fluid removed with extracorporeal techniques is isotonic. It has a different Na + concentration than that of urine produced after diuretic administration, the latter is usually hypotonic. In patients with acute decompensated heart failure, the average urinary Na + concentration after furosemide administration is 60 mmol/L, leaving behind 80 mmol of excess Na + for every liter of urine output. This, combined with neurohormonal activation, explains why the initial weight loss after diuretics is negated rapidly, whereas weight loss after mechanical UF may persist for longer. Finally, in patients with absent kidney function, mechanical fluid removal is the only option.
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