Updated: 9/11/2020

Oxygen Deprivation

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Hypoxemia

Alveolar Gas Equation

A-a Gradient

  • Alveolar gas equation
    • predicts the change in PAO2 that will occur for a given change in PACO2
    • PAO2 = alveolar PO2
    • PIO2 = PO2 in inspired air
      • normally, PIO2 = 150 mm Hg
    • PACO2 = alveolar PCO2
    • R = respiratory exchange ratio = (CO2 production) / (O2 consumption)
      • normally, R = 0.80
  • A-a Gradient
    • alveolar PO2 (PAO2) - systemic arterial PO2 (PaO2)
    • normal A-a gradient = 15 mm Hg (~Age/4 +4)
      • equilibration of O2 across alveolar gas and pulmonary capillary blood
        • pulmonary capillary blood becomes systemic arterial blood
      • increases with age
    • hypoxemia↑ A-a gradient
      • e.g., shunt, V/Q mismatch, pulmonary fibrosis (diffusion defect)
  • Causes of Hypoxemia (↓ PaO2)
    • High Altitude (normal A-a gradient) 
      • a decrease in barometric pressure (PB) decreases the PIO2 of inspired air
        • FiO2 doesn't change (21%)
        • ↓ PB → ↓ PIO2  ↓ PAO2 → ↓ PaO2
    • Hypoventilation (normal A-a gradient)  
      • a decrease in minute ventilation (respiratory rate x tidal volume) decreases PAO2
        • ↓ ventilation → PACO→ ↓ PAO2
        • FiO2 and PB don't change in alveolar gas equation
    • V/Q Mismatch (↑ A-a gradient)
    • Diffusion Limitation (↑ A-a gradient)
      • e.g., pulmonary fibrosis
        • ↑ diffusion distance for diffusion
      • e.g., pulmonary edema
        • ↓ surface area for diffusion, ↑ diffusion distance
    • Right-to-Left Shunt (↑ A-a gradient)
      • low O2 shunted blood mixes with and dilutes high O2 non-shunted blood
Hypoxia
  • Decreased Oxygen Delivery to Tissues
    • O2 content of blood = (O2 binding capacity) x (% saturation) + dissolved O2
      • (O2 binding capacity) x (% saturation) = O2 bound to hemoglobin
    • O2 delivery = (cardiac output) x (O2 content of blood)
  • Causes of Hypoxia (↓ O2 Delivery to Tissues)
    • circulatory (↓ Cardiac Output)
      • ↓ blood flow
    • hypoxemic
      • ↓ PaO2 → ↓ % saturation → ↓ O2 content of blood
    • anemic (see CO poisoning below)
      • ↓ concentration of hemoglobin→ ↓ O2 content of blood
    • histotoxic (example: cyanide poisoning)
      • ↓ O2 utilization by tissues (mitochondrial poison)
    • CO Poisoning
      • ↓ O2 bound to hemoglobin → ↓ O2 content of blood, ↓ O2 delivery to tissues
        • CO binds to hemoglobin with 200x greater affinity than O2
          • oxygen-hemoglobin dissociation curve left shift
      • early sign of exposure is headache; significant exposure causes coma, death
      • classical presentation at autopsy is cherry-red appearance of skin
        • not common in living patients
Ischemia
  • Causes of Ischemia (↓ Blood Flow)
    • ↓ arterial blood flow
      • e.g., atherosclerosis
    • ↓ venous return
      • e.g., Budd-Chiari syndrome

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