AKI

Acute Kidney Injury (AKI) – Joseph Quintana, Mengyao Tang

 

Background

  • Rise in Creatinine (Cr) > 0.3 mg/dL within 48 hours, or increase > 1.5 x baseline in 7 days
    • Oliguria = < 0.3 mL/kg/hr or < 500 mL/day
    • Anuria = < 50 mL/day
  • ACE-i’s and ARB’s alter the renal autoregulation and exacerbate pre-renal AKI
  • Acute Tubular Necrosis (ATN) and pre-renal disease comprise 66% of inpatient AKI
    • Pre-renal:
      • Fluid losses: GI, blood, Respiratory or metabolic losses [fevers, tachypnea, sepsis]
      • Decreased effective intravascular volume [HFrEF, Hepatorenal]
      • Afferent arteriole constriction [NSAIDs, Iodinated contrast]
    • Intra-renal (Tubular and interstitial disease):
      • ATN = Most common; Ischemia or nephrotoxin
      • ACE-i + NSAIDs, ACE-i + contrast, prerenal + ACE-i, prerenal + NSAIDs
      • Acute Interstitial Nephritis (AIN) usually drug-induced
      • Other causes:
        • Crystalline nephropathy: IV acyclovir, tumor lysis
        • Small vessel disease: MAHA, TTP, HUS
        • Large vessel problems: Aortic dissection, renal artery stenosis
        • Glomerular disease: Nephritic pattern
    • Post-renal (Obstruction):
      • Ureteral: obstructing stones (B/L or one with single kidney), external compression
      • Bladder: neurogenic bladder, malignancy, obstructing blood clot
      • Urethra: BPH
      • Misplaced Foley catheter

 

Evaluation

  • Urinalysis, Strict I/O’s
  • Fluid challenge:
    • Improve w/ diuresis CHF
    • Improve w/ resuscitation hypovolemia
  • Protein/Cr urine spot ratio: quantify tubular vs glomerular range proteinuria
  • Post Void Residual bladder scan, great test, low cost, high reward
    • Elevated PVRs = 250 cc plus
  • Who needs imaging (Renal U/S)?
    • No obvious cause of AKI, unremarkable history and bland urine
    • Abrupt oliguria or anuria
    • High suspicion for bladder outlet obstruction (PVRs might give you same data)
    • Add doppler to evaluate for renal artery stenosis (or if treating resistant hypertension)
  • Urine Electrolytes
    • FE Na & FE Urea (if on diuretics): Can suggest pre-renal vs ATN
      • Faults: validated in “fresh” oliguric patients (interpretation becomes more difficult after intervention with fluids, diuretics, etc.); 1% cutoff more accurate in severe AKI; Na+ intake affects impression; one measurement may be inaccurate
      • Urine sodium can be used to assess tubular avidity (typically urine Na > 40 in ATN and < 20 in pre-renal; note these are not absolute numbers, and there can be overlap)

 

Management

  • Hold ACE-i/ARB (remember to determine plan to resume at/after discharge)
  • Avoid unnecessary nephrotoxins: antibiotics, amphotericin, tenofovir, chemotherapy
  • Dose-adjust renally cleared medications: gabapentin, cefepime, morphine, metformin
  • Relieve Obstruction: I/O cath, foley, UrostomyUrology, Percutaneous nephrostomy IR
  • When to consider Nephrology – AKI consult:
    • Urgent indication for dialysis (see “Renal Replacement Therapy” below)
    • Cr worsening over 48-72 hrs w/o clear cause
    • Evaluate for need for dialysis in the near future
    • Concern for acute glomerulonephritis as cause

 

Additional Information

  • Rhabdomyolysis: UA positive for blood but no RBCs on microscopy
    • Serologic markers of muscle injury:  AST/ALT elevation with normal ALK Phos
        • Fluids: Goal of 6-12 L resuscitation in first 24 hrs if pt can tolerate (not well studied)
        • Traditional thought = Urine goal of 200-300 mL/hr, until CK <5000
        • Avoid calcium repletion for hypocalcemia unless symptomatic
  • Post-obstructive diuresis
    • Monitor pt after relief of obstruction due to possibility of severe polyuria (>4-5 L/day)
    • Replete fluids to prevent volume contraction and hypokalemia
    • ½ NS to replace about 50-75% of urinary losses
    • If pt develops hypo/hypernatremia, consider an alternative resuscitation fluid
    • Monitor calcium, phosphorus and magnesium in severe post-obstructive diuresis
  • Iodinated Contrast:
    • Does not need to be dialyzed; thought to be due to afferent arterial spasm which occurs immediately with contrast
    • Avoid giving if you are trying to preserve kidney function in CKD or AKI pts
    • No absolute cut-off in CKD, although GFR <30 is commonly used as a strong contraindication and GFR 30-45 as a relative contraindication
    • If there is time pre contrast you can ensure the patient is volume replete with isotonic fluid to decrease the risk of contrast nephropathy
    • If it is an emergency, you should not delay the contrasted procedure (i.e. STEMI)
  • Avoid gadolinium contrast from MRI in any AKI or if GFR <30 in CKD given risk of nephrogenic systemic fibrosis as an absolute contraindication; GFR >30 is fine
  • eGFR equations are based on stable kidney Fx, acute changes in Cr make it unreliable