29: Acute Kidney Injury

INTRODUCTION

Acute kidney injury (AKI), formally known as acute renal failure (ARF), is a well-established entity in the critically ill and has long been studied. It is categorized by a sudden decrease in a patient's glomerular filtration rate (GFR), which ultimately leads to the body's inability to maintain fluid, electrolyte, and toxin homeostasis. There are many etiologies, of which the most frequent and applicable to the emergency medicine physician will be described in this chapter. Regardless of the etiology, every diagnosis can be placed among one of three categories, including prerenal, intrinsic renal, or postrenal causes. The frequency of various etiologies is largely dependent on the patient. Hospitalized patients more often have intrinsic renal disease (most commonly acute tubular necrosis) due to sepsis, ischemia, or exposure to nephrotoxic medications or substances.¹ Patients presenting to the emergency department (ED) are more likely to have pre- and postrenal causes in addition to intrinsic glomerular disease etiologies such as nephritic or nephrotic patterns.

AKI carries an associated increase in both acute morbidity and mortality within the critically ill population, but also has impacts on hospital length of stay (LOS), long-term morbidity, and mortality risks from several different etiologies.

DEFINITION

AKI was previously termed ARF in critically ill patients, and had been described using many different definitions. In 2004, Bellomo et al., as a part of the Acute Dialysis Quality Initiative (ADQI) set forward to establish a single definition of AKI in addition to evaluating the literature to offer a cohesive recommendation for the use of fluid management and endpoint goals in AKI. The result of their evaluation was the creation of the RIFLE Criteria, a classification system commonly used today to describe AKI² (Figure 29-1). Each sequential letter of the acronym describes an increased degree of renal impairment based on either GFR or urine output. The term ARF now applies to the most severe degree of AKI, which requires the intervention of renal replacement therapy. Since inception, the RIFLE Criteria have been studied and correlated with prognosis in the intensive care unit (ICU).^3–5 The RIFLE Criteria have been scrutinized and modified by several organizations, including the Acute Kidney Injury Network (AKIN) and the Kidney Disease/Improving Global Outcomes (KDIGO). Overall, however, only small alterations have been made to the original RIFLE Criteria, all of which ultimately still use serum creatinine and urine output as measures of renal injury.^6–7

In most settings, monitoring the GFR and creatinine are more reliable ways of assessing changes in renal function. However, in the ED, baseline creatinine is often unknown; thus, urine output is a more reliable way to establish the diagnosis of renal dysfunction. Urine output can ...