F7iv: Define diffusing capacity & its measurement

Definition

  • The rate of gas transfer/partial pressure difference for the gas
    • Gases move along their partial pressure gradients (NOT concn gradients)
    • This is how you measure the rate of diffusion of a gas because it’s not possible to determine ‘T’ thickness & ‘A’ SA of Fick’s Equation

Measurement – DLCO

VOL GAS              = DL  x (P1 – P2)

(gas uptake)      = diffusion capacity of lunch x Partial P difference

  • CO is used to measure DL
  • Partial P of CO in capillary blood is negligible

∴VOL GAS = DL x P1

DL

Carbon Monoxide

  • Moves rapidly across membrane
  • Reacts with high affinity to Hb
  • Partial Pressure hardly rises at all
  • ∴gives a constant partial pressure gradient for diffusion
  • ∴said to be DIFFUSION LIMITED → so it’s v. good to determine diffusing capacity!

Single Breath method to assess DLCO

Carbon Monoxide
  • Subject exhales maximally
  • Nose clamped & takes single vital capacity breath with mixture of 0.3% CO & 10% helium (He)
  • Breath held for 10 secs (to overcome maldistribution of expired gas)
  • He does NOT enter blood
  • ∴Ratio of He Fi/FE x Volume drawn into alveoli will indicate total alveolar volume during breath holding
  • VOL of CO UPTAKE measured by multiplying He FI/FE ratio x PCO of inspired gas
  • Expired PCO is measured directly
  • Exhaled gas is analysed for CO by infrared analysis

Values

  • KCO = transfusion co-efficient of CO 
  • ↓KCO & ↓DL­CO
    • Emphysema
    • Fibrosing alveolitis
    • Pulmonary vascular disease 
  • KCO(N) & ↓DLCO: small lung volumes
    • Pleural effusion
    • Muscle weakness
    • Consolidation 
  • ↑DLCO: hmrrg → because more blood available for reaction ∴over-estimated 
  • ↓D­LCO: anaemia → underestimated because ↓[Hb]