Chest Wall and Pressure Volume

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Elastic Recoil of the Thoracic Cage[edit]

  • Thoracic cage is elastic and tends to spring outwards eg. in pneumothorax
  • Resting volume for the thoracic cage ~ FRC + 600-700mls
  • At FRC, the relaxation pressure of the lung + chest wall is atmospheric - this is the equilibrium volume where elastic recoil of lung is balanced by tendency of chest wall to spring out
  • At volumes above this, the pressure is positive, and at smaller volumes the pressure is subatmospheric

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  • The curve from the lung alone can be derived from isolating a lung in a jar at atmospheric pressure and measuring pressures at various volumes
  • At zero pressure the lung is at its minimal volume - below RV
  • Only at 75% of vital capacity does the relaxation pressure of the chest wall become atmospheric
  • At a given volume: PT = PL + PCW, and as pressure is inversely proportional to compliance, 1/CT = 1/CL + 1/CCW
  • Thoracic cage compliance can be measured using transmural pressure change - δPCW = PIP - PB
  • Thoracic cage compliance is decreased in kyphoscoliosis, scleroderma, muscle spasticity and abdominal distension/obesity