Chapter 16: Acute Kidney Injury

Acute kidney injury represents a sudden loss of renal function that disrupts the kidneys' ability to maintain fluid and electrolyte homeostasis, with severe cases requiring intervention carrying mortality rates between 50 and 60 percent. The Kidney Disease Improving Global Outcomes classification system divides acute kidney injury into three severity stages determined by changes in serum creatinine levels and urine output patterns. Understanding the underlying cause is essential for treatment, as acute kidney injury falls into three pathophysiologic categories: prerenal causes involving compromised blood flow to the kidneys such as hemorrhage or sepsis, intrarenal causes from direct tissue damage including acute tubular necrosis or medication toxicity, and postrenal causes resulting from obstruction of urine drainage. The condition typically progresses through three clinical phases beginning with the initiating event, followed by the maintenance phase characterized by significantly reduced glomerular filtration rate and sustained kidney dysfunction, and concluding with the recovery phase when renal tissue repair and gradual restoration of function occur. Clinical assessment requires monitoring serum creatinine trends, urine output measurements, and the blood urea nitrogen to creatinine ratio, with fractional excretion of sodium helping distinguish between prerenal and tubular causes of injury. Special populations including older adults with naturally declining kidney function, morbidly obese patients where standard assessment formulas lack validity, and pregnant women at risk for pregnancy-related kidney complications require adapted evaluation approaches. Management priorities focus on maintaining appropriate fluid balance, preventing secondary infections, and carefully adjusting medication dosages to prevent accumulation of toxic metabolites. Life-threatening hyperkalemia requires urgent intervention using agents that protect cardiac function or shift potassium intracellularly. Nutritional support becomes critical given the catabolic state accompanying acute kidney injury. Renal replacement therapy serves as the definitive treatment for severe disease, with options including intermittent hemodialysis for rapid solute removal in stable patients, continuous renal replacement therapy preferred for hemodynamically unstable patients allowing gradual fluid and waste removal, and peritoneal dialysis rarely used due to slower clearance and peritonitis risk.