Pneumonia with Fick's Law Components

• Hypoxaemia in Pneumonia - Gas solubility:

  • Small amounts of O2 dissolved in fluid
  • But O2 is poorly soluble so theres a decrease in gas exchange
• Hypoxaemia in pneumonia - Thickness of alveolar membrane:

  • Presence of inflammatory fluid in interstitium and presence of exudate in the alveoli increases the distance for diffusion
  • With disease progression alveolar membrane can become fibrotic and thickened
• Hypoxaemia in pneumonia - Surface area of alveolar membrane:

  • Areas of consolidated lung are “blocked” and unable to participate in gas exchange
  • Decrease in area of alveolar membrane
  • In reality makes very small contribution to hypoxaemia
• Hypoxaemia in pneumonia - Ventilation perfusion coupling (Main cause of hypoxaemia in pneumonia)

  • Pneumonic areas are consolidated and unventilated
  • Would expect perfusion to also be decreased due to Hypoxic pulmonary vasoconstriction (HPVC)
  • However, presence of inflammatory cytokines reverses HPVC
  • Continued perfusion of nonventilated airspaces produces V/Q mismatch termed “Shunt”