Hypoxia, Hypoxemia, and Ischemia Mechanisms

By SWA

Detailing mechanisms of oxygen deprivation, oxygen-hemoglobin dissociation, the alveolar gas equation, and ventilation-perfusion mismatch.

Mechanisms of Oxygen Deprivation

Hypoxia

  • Definition: Reduced oxygen delivery to tissues.
  • Causes:
    • Decreased cardiac output
    • Hypoxemia
    • Anemia
    • Carbon monoxide poisoning

Hypoxemia

  • Definition: Reduced partial pressure of oxygen in the blood.
  • Mechanisms:
    • Normal A-a gradient:
      • Low FiO2 (e.g., high altitudes)
      • Hypoventilation
    • Increased A-a gradient:
      • V/Q mismatch
      • Right-to-left shunt
      • Diffusion limitation

Ischemia

  • Definition: Reduced blood flow to tissues.
  • Causes:
    • Impaired arterial flow
    • Decreased venous drainage

Oxygen-Hemoglobin Dissociation Curve

  • Rightward Shift:
    • Decreased affinity of hemoglobin (Hb) for oxygen
    • Facilitates oxygen unloading to tissues
    • Causes: ↓pH (↑H+), ↑CO2, exercise, ↑temperature, ↑2,3-BPG, altitude
  • Oxygen Content of Blood:
    • Anemia: ↓[Hb], reduced O2 carrying capacity
    • Carbon monoxide poisoning: CO binds Hb, shifts curve left, reduces O2 unloading
    • Hypoxemia: ↓PaO2, less dissolved O2 for binding Hb

Alveolar Gas Equation

  • Equation: PAO2 = PIO2 - (PaCO2 / R)
    • PAO2: Alveolar PO2
    • PaO2: Arterial PO2
    • PIO2: PO2 of inspired air
    • Pb: Barometric pressure
    • R: Respiratory quotient (0.8)
  • Example Calculation:
    • At sea level: PIO2 = 0.21 × 760 mm Hg = ~150 mm Hg
    • PAO2 = 150 mm Hg - (PaCO2 / 0.8)

A-a Gradient

  • Normal A-a gradient: Increases with age (age/4 + 4)

V/Q Mismatch

  • V/Q = 0: Perfusion without ventilation (shunt)
    • Example: Alveolar obstruction
  • V/Q = ∞: Ventilation without perfusion
    • Example: Pulmonary embolism

 

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