Acute Kidney Injury (AKI)

DEFINITION ₍₁₎

• Increase in creatinine by 1.5 x baseline OR
• Increase in creatinine by 0.3 mg/dL from baseline OR
• Urine output < 0.5 mL/kg/hour for 12+ hours

CATEGORIES

Pre-Renal

• Etiologies
  • Cardiorenal Syndrome
  • Hepatorenal Syndrome
  • Hypovolemia (Dehydration, Diarrhea, Diuresis, Vomiting)
  • Arterial insufficiency (Renal Artery Stenosis, Fibromuscular Dysplasia)
• Work-Up
  • History and Physical Exam: Determine volume status
  • Urinalysis: Increased specific gravity and hyaline casts
  • FENa (Fractional Excretion of Sodium): <1%
  • FEUrea (Fractional Excretion of Urea) <35% (if patient is on diuretics)
    • Tubules are still working, trying to absorb as much solute as possible, so less of a fraction of solute is excreted through the urine
  • BUN/Cr ratio > 20
    • Since the tubules are still working, urea may still be reabsorbed and a small amount of creatinine may still be secreted
• Management
  • Cardiorenal Syndrome: Diuresis (to move leftward on the Frank-Starling curve)
  • Hepatorenal Syndrome: Octreotide, midodrine, norepinephrine, and eventually a liver transplant is usually necessary
  • Hypovolemia (Dehydration, Diarrhea, Diuresis, Vomiting): IV fluid resuscitation
  • Arterial insufficiency (Renal Artery Stenosis, Fibromuscular Dysplasia): Vascular Surgery intervention may be necessary

Intra-Renal

• BUN/Creatinine <20
  • When the tubules are injured, they are unable to reabsorb urea, so a higher percentage is excreted
• FENa >1% and FEUrea >35%
  • When the renal tubules are injured, they are unable to reabsorb solute, so a larger fraction of solute is excreted through the urine
• Etiologies
  • Nephrotic Syndrome
    • 3.5+ g/day of proteinuria (or random urine protein:urine creatinine ratio of 3.5+), hypoalbuminemia, edema
    • May require renal biopsy
  • Nephritic Syndrome
    • UA with large hematuria, hypertension
    • May require renal biopsy
  • Acute Interstitial Nephritis
    • Fever, eosinophilia or WBC casts, rash
  • Acute Tubular Necrosis
    • May be secondary to ischemia or toxins (such as myoglobin, aminoglycosides, cisplatin, iodinated contrast, etc)
    • “Muddy brown” AKA “granular” casts AKA amorphous crystals

Post-Renal

• Work-Up
  • History of incontinence, prostate disease, kidney stones, trauma
  • Post-void residual for urinary retention
  • Ultrasound kidneys and bladder for hydronephrosis
  • Consider CT scan for renal stones or neoplasm
• Etiologies
  • BPH, prostate cancer
  • Cervical cancer (less common)
    • History, CT pelvis can diagnose
  • Nephrolithiasis (typically would need to be bilateral to cause AKI)
    • CT scan (without contrast) is most sensitive, may be able to use ultrasound or abdominopelvic radiography to diagnose
  • Neuropathy
    • Spinal cord injury
    • Diabetes Mellitus
  • Medications
    • Anticholinergics are generally the biggest offender
• Management
  • Foley catheter may be necessary (except for in the case of nephrolithiasis), but definitive treatment is to treat the underlying cause

Initial Workup:

• History and physical (volume status, history of heart failure, cirrhosis, spinal trauma, etc)
• Urinalysis + microscopy
• Basic Metabolic Panel
• Urine creatinine, urine sodium, urine urea nitrogen
• Bladder scan
• Consider retroperitoneal ultrasound to assess for hydronephrosis

References

1. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl 2012; 2:8.