iii. Pulm Gas Volumes – Measurement

Master answer

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

Measurement of FRC

• Cannot be directly measured by spirometry
• Body plethysmography
  • Subject contained within glass tight box
  • Breath against occluded airway
  • Changes in alveolar pressure measured at mouth
  • Boyle’s law used to calculate FRC
• Nitrogen wash-out technique
  C₁V₁=C₂V₂
  • Initial alveolar concentration of nitrogen = 79% (if breathing room air)
  • Subject breathes 100% O2
  • Volume of total nitrogen exhaled measured (e.g 4L)
  • FRC = 4L/79% = approx 5L
• Helium wash-in technique
  C₁V₁=C₂V₂
  • Known volume and concentration of helium inhaled and allowed to
    equlibrate but not diffuse into blood
  • Concentration and volume of exhaled helium measured and used to
    calculate FRC, where

Sakurai / Gladwin / JC 2020

Answer by Gladwin, JC, Sakurai