Question 24

College Answer

Most candidates could state 2 methods of measuring FRC. Some candidates (especially for
nitrogen wash out) failed to provide enough information e.g. statements such as "if the amount
of nitrogen is measured then FRC can be derived" were insufficient to score many marks.

Discussion

• The FRC is:
  • The volume of gas present in the lung at end expiration during tidal breathing
  • It is composed of ERV and RV
  • This is usually 30-35 ml/kg, or 2100-2400ml in a normal sized person
  • It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure. 
• FRC is usually measured by one of three methods:
  • Body plethysmography
    • The subject and the equipment are all confined in a rigid box which contains a known gas volume.
    • As the subject exhales:
      • Intrathoracic volume decreases, which means the volume of the box increases (as the walls are rigid and there is a finite volume shared by the chest and the box).
      • Intrathoracic pressure increases, and therefore box pressure decreases proportionally.
    • Though the amount of the gas in the chest is unknown, we know that (according to Boyle's law) the product of pressure and volume in the chest should be the same as the product of volume and pressure in the box.
    • The volume in the box, the pressure in the box and the pressure in the chest are all known variables at this point, leaving the volume of intrathoracic gas as the last unknown
  • Inert gas dilution
    • A subject is given a known volume (V₁) of an inert tracer gas (eg. helium) which has a known concentration (C₁)
    • The inert tracer gas is inhaled and mixes with intrathoracic as, whcih dilutes the racer.
    • The patient then exhales this gas mixture, and the exhaled tracer concentration (C₂) can be measured
    • From this, the intrathoracic gas volume (V₂) can be calculated from the equation:
      
      C₁  × V₁ = C₂  × (V₁ + V₂)
  • Nitrogen washout
    • The subject is made to breathe 100% FiO₂.
    • The nitrogen concentration of  exhaled gas is measured
    • As the intrathoracic nitrogen content approaches zero, the total exhaled nitrogen voume can be calculated from its concentration in the exhaled gas
    • The intrathoracic gas volume can then be calculated from the total volume of exhaled nitrogen gas and the nitrogen concentration of the first breath

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