In keeping with the role of providing organ support, such as managing a ventilator for pulmonary support, or pressors for cardiovascular support, it is often the intensivist's role to provide support for failing kidneys. This chapter will focus on when and how to provide that support.
Normally functioning kidneys are important in several homeostatic mechanisms:
The production of hormones such as erythropoietin and renin
Partial conjugation required to activate vitamin D that is necessary to absorb enteral calcium
The regulation of acid–base status along with the lungs
The filtration of the blood and regulation of concentrations of solutes such as sodium and potassium
The elimination of fluid and waste products such as urea
It is assisting or replacing these last three functions—solute clearance and volume regulation—that will be the focus of this chapter.
Depending on the population studied, the incidence of renal failure (RF) in ICU patients has been reported to be as high as 25%.1,2 There is disparity, however, in how RF is defined in clinical practice as well as in the literature. This has led to initiation of renal support at different levels of renal function, which makes it difficult to compare studies, construct studies, or extrapolate findings to one's own patient population.
The Acute Dialysis Quality Initiative Group (ADQI), a group formed in 2000 “to provide an objective, dispassionate distillation of the literature description of the current state of practice of dialysis and related therapies,”3 recently proposed a classification scheme for diagnosing acute renal failure (ARF).4 Commonly referred to as the RIFLE criteria, the acronym itself describes the level of renal dysfunction:
- R—Risk of renal dysfunction
- I—Injury to the kidney
- F—Failure of kidney function
- L—Loss of kidney function
- E—End-stage kidney disease
Each level (R-I-F-L-E) of renal dysfunction can be classified or diagnosed by either the glomerular filtration rate (GFR) or urine output (UO).
The GFR is generally considered a better measure of renal function/failure, although it is typically only measured via surrogates such as creatinine clearance. Interpreting a change in GFR requires knowledge of the baseline creatinine that is not always available. For instance, a previously healthy trauma patient who is now acutely ill may have never had his or her baseline serum creatinine measured, while a patient whose primary doctor sends a basic metabolic panel including blood urea nitrogen (BUN) and creatinine every year, or an elective surgery candidate who had preoperative labs drawn, will have a known baseline creatinine.
In the event that a baseline creatinine is not known, or for providers who are more comfortable, UO can also be used. The advantage of UO is that it can be used on all patients except for patients anuric prior to their acute illness (e.g., end-stage renal patients already on dialysis) or in whom UO cannot be measured (e.g., patients with ureteral ...