Acute Renal Failure Overview

Acute Renal Failure (ARF)

• ARF is an acute, severe decline in kidney function, developing within days.
• A key sign is azotemia, characterized by elevated BUN and creatinine levels, often accompanied by oliguria (low urine output).

Prerenal Azotemia

• Results from reduced blood flow to the kidneys, such as in heart failure.
• This reduced blood flow leads to decreased glomerular filtration rate (GFR), azotemia, and oliguria.
• Urine sodium excretion is low (FENa < 1%), and urine osmolality is high (>500 mOsm/kg).

Postrenal Azotemia

• Caused by obstruction of the urinary tract, such as a blockage in the ureters.
• Obstruction leads to reduced GFR, azotemia, and oliguria.
• Early in the obstruction, elevated BUN:Cr ratios (>15) may be seen.
• With prolonged obstruction, BUN:Cr ratios might decrease (<15), sodium reabsorption decreases (FENa > 2%), and urine concentration ability is impaired (urine osmolality < 500 mOsm/kg).

Acute Tubular Necrosis (ATN)

• ATN is a common cause of intrarenal azotemia, often due to tubular epithelial cell injury/necrosis.
• Necrosis results in blockage of tubules, reducing GFR.
• Causes can be ischemic (reduced blood supply) or nephrotoxic (toxins).
• Common nephrotoxins include aminoglycosides, heavy metals, myoglobin, ethylene glycol, radiocontrast, and urates.
• Urine often contains brown, granular casts.

Clinical Features of ARF

• Oliguria (low urine output) and brown granular casts are common findings.
• Elevated BUN and creatinine are observed.
• Hyperkalemia (high potassium levels), and metabolic acidosis are possible, requiring supportive dialysis.
• Oliguria can persist 2- 3 weeks before recovery, as tubular cells regenerate.

Acute Interstitial Nephritis (AIN)

• AIN is a drug-induced hypersensitivity reaction impacting the kidney interstitium and tubules, often causing acute renal failure.
• Common causes are NSAIDs, penicillin, and diuretics.
• Symptoms include oliguria, fever, and rash, days to weeks after starting the drug.
• Eosinophils may be noted in urine.
• Resolved with drug cessation.

Renal Papillary Necrosis

• Renal papillary necrosis involves necrosis of renal papillae, characterized by gross hematuria and flank pain.
• Causes can include chronic analgesic abuse (e.g., phenacetin or aspirin use), diabetes mellitus, sickle cell trait/disease, and severe pyelonephritis.