Chest Wall and Pressure Volume

Elastic Recoil of the Thoracic Cage

• 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

• 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: P_T = P_L + P_CW, and as pressure is inversely proportional to compliance, 1/C_T = 1/C_L + 1/C_CW
• Thoracic cage compliance can be measured using transmural pressure change - δP_CW = P_IP - P_B
• Thoracic cage compliance is decreased in kyphoscoliosis, scleroderma, muscle spasticity and abdominal distension/obesity