Surgical Critical Care: Renal Disease

Acute Kidney Injury (AKI) – Definition & Staging Systems

RIFLE Classification – Most Commonly Cited System

  • Acute Rise in Cr Over 7 Days
  • Risk of Renal Failure
    • Increase in Serum Cr ≥ 1.5x Baseline
    • Decrease in GFR ≥ 25% Baseline
    • UOP < 0.5 cc/kg/hr for 6 Hours
  • Injury of the Kidney
    • Increase in Serum Cr ≥ 2.0x Baseline
    • Decrease in GFR ≥ 50% Baseline
    • UOP < 0.5 cc/kg/hr for 12 Hours
  • Failure of the Kidney
    • Increase in Serum Cr ≥ 3.0x Baseline or Cr ≥ 0.5 mg/dL if Bassline Cr ≥ 4 mg/dL
    • Decrease in GFR ≥ 75% Baseline
    • UOP < 0.3 cc/kg/hr for 24 Hours or Anuria for 12 Hours
  • Loss of Kidney Function
    • Complete Loss of Function ≥ 4 Weeks
  • End-Stage Renal Disease
    • Complete Loss of Function ≥ 3 Months

Acute Kidney Injury Network (AKIN) Classification

  • Acute Rise in Cr Over 48 Hours
  • Stage I
    • Increase in Serum Cr ≥ 0.3 mg/dL
    • Increase in Serum Cr ≥ 1.5x Baseline
    • UOP < 0.5 cc/kg/hr for 6 Hours
  • Stage II
    • Increase in Serum Cr ≥ 2.0x Baseline
    • UOP < 0.5 cc/kg/hr for 12 Hours
  • Stage III
    • Increase in Serum Cr ≥ 3.0x Baseline or Cr ≥ 0.5 mg/dL if Bassline Cr ≥ 4 mg/dL
    • UOP < 0.3 cc/kg/hr for 24 Hours or Anuria for 12 Hours
    • Initiation of Renal-Replacement Therapy (RRT)

Kidney Disease Improving Global Outcomes (KDIGO) Classification

  • AKI Definition:
    • Increase in Serum Cr ≥ 0.3 mg/dL within 48 Hours
    • Increase in Serum Cr ≥ 1.5x Baseline within 7 Days
    • UOP < 0.5 cc/kg/hr for 6 Hours
  • Staging:
    • Stage I
      • Increase in Serum Cr ≥ 0.3 mg/dL
      • Increase in Serum Cr ≥ 1.5x Baseline
      • UOP < 0.5 cc/kg/hr for 6 Hours
    • Stage II
      • Increase in Serum Cr ≥ 2.0x Baseline
      • UOP < 0.5 cc/kg/hr for 12 Hours
    • Stage III
      • Increase in Serum Cr ≥ 3.0x Baseline or Cr ≥ 4 mg/dL
      • UOP < 0.3 cc/kg/hr for 24 Hours or Anuria for 12 Hours
      • Initiation of Renal-Replacement Therapy (RRT)

Comparison

Stage RIFLE AKIN KDIGO
1 Risk Same as RIFLE Risk
Plus: Increase in Serum Cr ≥ 0.3 mg/dL
Minus: GFR Criteria		 Same as AKIN Stage I
2 Injury Same as RIFLE Injury
Minus: GFR Criteria		 Same as AKIN Stage II
3 Failure Same as RIFLE Failure
Plus: Initiation of RRT
Minus: GFR Criteria		 Same as AKIN Stage III
Change: Cr ≥ 0.5 mg/dL if Bassline Cr ≥ 4 mg/dL to Simply Cr ≥ 4 mg/dL
    • All Have the Same UOP Criteria
    • Only RIFLE Uses GFR Criteria

Acute Kidney Injury (AKI)

Types & Causes

  • Pre-Renal
    • Low Blood Flow
    • Dehydration
    • Bleeding
    • Congestive Heart Failure
    • Sepsis
  • Renal
    • Acute Glomerulonephritis
      • Infection
      • Vasculitis
    • Acute Tubular Necrosis (ATN)
      • Ischemia
      • Prolonged Hypotension
        • Intraoperative Hypotension is the Most Common Cause of Post-Op AKI
      • Toxins
      • Contrast-Induced AKI
    • Acute Interstitial Nephritis (AIN)
      • Infection
      • Drugs
      • Inflammatory Disease
      • Neoplasia
  • Post-Renal
    • Obstruction
    • Urinary Stones
    • Benign Prostatic Hypertrophy (BPH)
    • Neoplasia
    • Retroperitoneal Fibrosis
    • Urethral Stricture

Laboratory Evaluation

  • Fractional Excretion of Sodium (FENa)
    • Best Test for Azotemia
    • FENa = 100 x (UNa/PNa) / (UCr/PCr)
  Pre-Renal Renal Post-Renal
BUN:Cr > 20 < 15 Varies
FENa < 1% 1-4% ≥ 5%
Urine Na < 20    
  • Urinary Casts – Seen from Renal Causes
    • Acute Glomerulonephritis – Protein & Red Blood Cell (RBC) Casts
    • Acute Tubular Necrosis (ATN) – Granular & Muddy Brown Casts
    • Acute Interstitial Nephritis (AIN) – Eosinophils & WBC Casts

Electrolyte Disturbances of Kidney Disease

  • Increased
    • Potassium
    • Magnesium
    • Phosphorus
  • Decreased:
    • Vitamin D & Calcium
    • Erythropoietin & Hemoglobin

Treatment

  • Management is Primarily Supportive
  • Treat Underlying Cause
  • Avoid Nephrotoxic Medications
  • Volume Management:
    • Crystalloid Fluid Resuscitation if Hypovolemic
    • High-Dose Loop Diuretics May Be Required for Volume Overload if Not Anuric
  • Manage Electrolyte & Acid-Base Disturbances
    • Hyperkalemia is Generally the Most Immediate Threat in AKI
  • Manage Nutrition
  • Assess for Uremia
  • Renal Replacement Therapy (RRT) if Indicated

Renal Replacement Therapy (RRT)

Indications Mn

  • Acidosis (pH < 7.1)
  • Electrolyte Disturbances
    • Potassium > 6.5 mEq/L
    • Potassium > 5.5 mEq/L with Tissue Breakdown (Rhabdomyolysis) or Ongoing Potassium Absorption (GI Bleed)
    • Symptomatic Hyperkalemia (Cardiac Conduction Abnormality)
  • Intoxicants/Poisoning
  • Fluid Overload with Pulmonary Edema
  • Symptomatic Uremia (Encephalopathy, Coagulopathy or Pericarditis)
  • Renal Function Indications:
    • Symptomatic & GFR < 15
    • Asymptomatic & GFR < 5

Modalities

  • Intermittent Hemodialysis (IHD)
    • Rapid & Large Volume Shift
      • Blood Flow: 300-400 cc/min
      • Dialysate Flow: ≥ 500 cc/min
      • Duration: 3-4 Hours – Often 3 Times Per Week
    • Large Shifts Can Cause Hemodynamic Instability
    • Does Not Require Anticoagulation
  • Continuous Renal Replacement Therapy (CRRT)
    • Slow & Continuous Venovenous Hemodialysis (CVVHD)
      • Blood Flow: 150-200 cc/min
      • Dialysate Flow: 17-34 cc/min
      • Duration: Continuous (24 Hours)
    • Smaller Shifts Generally Do Not Cause Hemodynamic Instability – Best for Critically Ill Patients Who are Unable to Tolerate Hemodynamic Changes of IHD
    • Not as Effective for Treatment of Severe Hyperkalemia as IHD
    • Requires Anticoagulation – Can Lose 150 cc Blood if Filter Clots
  • Sustained Low-Efficiency Daily Dialysis (SLEDD)
    • Considered a Hybrid of IHD & CRRT
    • Slow Intermittent Flow Done More Frequently than IHD
      • Blood Flow: 100-150 cc/min
      • Dialysate Flow: 300 cc/min
      • Duration: 6-12 Hours Every Day
    • Smaller Shifts Generally Do Not Cause Hemodynamic Instability
    • Usually Does Not Require Anticoagulation
  • Early Initiation of RRT in AKI May Improve Survival (Debated)

Mnemonics

Basic Indications for Dialysis

  • “A-E-I-O-U”
    • Acidosis
    • Electrolyte Disturbances
    • Intoxicants/Poisoning
    • Overloaded Fluid
    • Uremia