In keeping with the role of providing organ support, such as management of a ventilator for pulmonary support or administration of pressors and inotropes 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
The filtration of blood and regulation of solute concentrations, such as sodium and potassium
The elimination of fluid and waste products such as urea
The focus of this chapter will be a description of how to appropriately assist or replace these last three functions of the kidneys, namely, maintenance of normal acid–base status, solute clearance, and volume and waste product removal.
Depending on the population studied, the incidence of acute renal failure (ARF) in intensive care unit (ICU) patients has been reported to be as high as 25%.1,2 There is disparity, however, in how ARF 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 in daily practice. Furthermore, what was previously known as ARF is now described along a spectrum of renal dysfunction, more appropriately termed acute kidney injury (AKI).
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 proposed a now commonly used classification scheme for diagnosing ARF.4 Commonly referred to as the RIFLE criteria, the acronym itself describes the level of renal dysfunction:
R—Risk of renal dysfunction
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 changes in the glomerular filtration rate (GFR) and/or serum creatinine or reduction in urine output (UO).
The GFR is generally considered a better measure of renal function/failure, although it is typically measured only via surrogates such as creatinine clearance. Interpreting a change in GFR requires knowledge of the baseline creatinine that is not always available. Assuming normal baseline renal function, however, the ADQI group has developed a theoretical baseline serum creatinine value for a given patient normalized to body surface area and based on age, race, and sex.5 In the event that a normal baseline creatinine cannot be assumed, UO can also be used to define ...