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Updated: Sep 11 2020

Oxygen Deprivation

  • Hypoxemia
    • 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 → ↑ PACO2 → ↓ 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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