Functional Residual Capacity — Reduced FRC

Functional residual capacity

• Volume of air in lungs at the end of normal tidal expiration
• Occurs at equilibrium point between the recoil of chest/diaphragm and lungs to collapse inwards
• Sum of residual volume and expiratory reserve volume
• Normal FRC 30ml/kg (approx 2.2l in 70kg male)
• Functions:
  • Oxygen reservoir
  • PaO2 buffer
  • Prevents atelectasis
  • Minimizes work of breathing
  • Lung kept at most compliant segment of compliance curve
  • Decreases pulmonary vascular resistance

Consequence of 1L loss of FRC

• ↑Work of breathing
  • FRC is the volume at the minimum work at rest
  • Compliance curve shifted to a flatter portion (greater change in pressure required for change in volume)
  • FRC < closing capacity → ↑dynamic air closures/↑air trapping → ↑WOB
  • ↑atelectasis (↑with supine position) → ↓ lung compliance → ↑WOB
  • ↑airways resistance 2° ↓caliber of airway → ↑WOB
• ↑V/Q mismatch
  • 2° to dynamic airways closure, gas trapping, atelectasis
  • ↑shunt
  • ↓PaO2 due to inability of zone 1 to compensate for the larger zone 3
• ↑Pulmonary Vascular Resistance (PVR)
  • PVR minimal at FRC
  • ↓FRC → ↓radial traction on extraalveolar vessels → ↓radius of vessels → ↑PVR
    • ↑PVR → ↑ RV Afterload
• ↓FRC → ↑West Zone 4 → ↑pressure on extraalveolar vessels by poorly inflated lung → ↓radius → ↓flow
• ↓O Store and Buffer capacity
  • Greater variability in saturations thoughout the respiratory cycle
  • More rapid desaturation with hypoxia

Sakurai / Gladwin / JC 2020

Answer by Gladwin, JC, Sakurai