Acute Kidney Injury

An abrupt (<72 hours) reduction in kidney function defined as: an absolute increasein serum creatinine by >26.4 µm/l OR an increase in creatinine by >50% OR a reduction in UO

Diagnosti Criteria (KDIGO)

AKI is diagnosed if any of the following are present:
  • Increase in serum creatinine ≥ 0.3 mg/dL within 48 hours
  • Increase in serum creatinine to ≥ 1.5 times baseline within 7 days
  • Urine output < 0.5 mL/kg/hour for ≥ 6 hours

Aetiology

Pre-renal - impaired blood flow to the kidney

  • Reversible volume depletion causes a reduction in perfusion to the kidneys - leads to oliguria (<0.5 mls/kg/hr) and increase in creatinine
  • Hypovolaemia - haemorrhage, volume depletion (e.g. D+V, burns)
  • Hypotension - cardiogenic shock, distributive shock (e.g. sepsis, anaphylaxis)
  • Renal hypoperfusion - NSAIDs/COX-2, ACEi/ARBs, hepatorenal syndrome
    • ACEi result in loss of loss of efferent arteriolar vasoconstriction which is a compensatory mechanism for volume depletion → major fall in GFR
  • Untreated pre-renal AKI leads to acute tubular necrosis (see below)

Renal

  • Diseases causing inflammation or damage to cells causing AKI
  • Vascular - vasculitis, renovascular disease
  • Glomerular - glomerulonephritis
  • Interstitial nephritis - drugs, infection (TB), systemic (sarcoid)
  • Tubular injury - ischaemia, drugs (gentamicin), contrast, rhabdomyolysis
Acute tubular necrosis
  • Tubular cell death
  • Commonest form of AKI in hospital
  • Due to a combination of factors leading to decreased renal perfusion
  • Common causes include sepsis and severe dehydration
  • Other important causes include rhabdomyolysis and drug toxicity

Post-renal

  • AKI due to obstruction of urine flow leading to back pressure and thus loss of concentrating ability
  • Causes: stones, cancers, strictures, extrinsic pressure

Risk factors

Patient
  • Older age
  • CKD
  • Diabetes
  • Cardiac failure
  • Liver disease
  • PVD
  • Previous AKI
Exposure
  • Hypotension
  • Hypovolaemia
  • Sepsis
  • Deteriorating NEWS
  • Recent contrast
  • Exposure to certain medications

Pathophysiology

KDIGO staging classification

notion image

Clinical presentation

Symptoms

  • Constitutional symptoms - anorexia, weight loss, fatigue, lethargy
  • Nausea and vomiting
  • Itch
  • Fluid overload - oedema, SOB

Signs

  • Fluid overload including HTN, oedema, pulmonary oedema, pleural effusion
  • Uraemia including itch, pericarditis
  • Oliguria

Investigations

  • U+Es
  • Bloods - FBC and coagulation screen
    • Abnormal clotting, anaemia
  • Urinalysis - haematoproteinuria
  • USS - obstruction?
  • Immunology - ANA, ANCA, GBM
  • Protein electrophoresis and BJB if considering myeloma (elderly AKI + bone pain + hypokalaemia + anaemia)
Stage
Class
Serum Creatinine or GFR Criteria
Urine Output Criteria
1
Risk
Serum creatinine ×1.5 or GFR decrease >25%
<0.5 mL/kg/hour for 6 hours
2
Injury
Serum creatinine ×2 or GFR decrease >50%
<0.5 mL/kg/hour for 12 hours
3
Failure
Serum creatinine ×3 or serum creatinine ≥354 µmol/L (≈4.0 mg/dL)
<0.3 mL/kg/hour for 24 hours or anuria for 12 hours
ㅤ
Loss
Persistent acute kidney failure = complete loss of kidney function for >4 weeks
—
ㅤ
End-Stage Kidney Disease
End-stage kidney disease for >3 months
—

Management

Pre-renal AKI

  • Assess for hydration
    • Clinical observations (BP, HR, UO)
    • JVP, cap refill, oedema
    • Pulmonary oedema
  • Fluid challenge for hypovolaemia
    • Crystalloid (0.9% NaCl) or colloid (Gelofusin) - do NOT use 5% dextrose
    • Give bolus of fluid then reassess and repeat as necessary
    • Ig >1000mls IN and no improvement, seek help

Further treatment (pre-renal and renal)

  • Ensure good perfusion pressure
    • Fluid resuscitate
    • Once fluid resuscitated, if still not achieving an adequate BP → inotropes/vasopressors
  • Treat underlying cause e.g. antibiotics if sepsis
  • Stop nephrotoxic
  • Dialysis if remains anuric and uraemia

Post-renal AKI

  • Relieve obstruction - catheter, nephrostomy
  • Refer urology if ureteric stenting required

Urgent indications for haemodialysis

  • Hyperkalaemia >7 or >6.5 unresponsive to medical therapy
  • Severe acidosis - pH <7.15
  • Fluid overload e.g. pulmonary oedema
  • Urea >40, pericardial rub/effusion

Complications

  • Hyperkalaemia
  • Fluid overload (pulmonary oedema)
  • Severe acidosis (pH <7.15)
  • Uraemic pericardial effusion
  • Severe uraemia (Ur >40)