What is functional residual capacity and describe how it is measured.
This question requested a definition AND a description of measurement (one method if correctly
discussed could and did generate a pass mark) although additional marks were awarded if
multiple measurement methods were mentioned or described. Detailed descriptions of the
factors effecting FRC and its functions were NOT requested and scored no marks. "Fowlers
method" uses 100% oxygen and nitrogen analysis to calculate anatomical dead space - NOT
FRC - so scored no marks. Both Helium dilution and nitrogen washout (with 100%oxygen)
enable calculation of FRC using C1V1=C2V2 where V2 = V1+FRC. Body plethysmography
requires more complex calculations of P1V1= P2V2 (Boyles Law) applied twice = for the box
and then the lung. Few candidates had a clear understanding of this method.
Most answers did not demonstrate the depth of understanding of the measurement techniques
that was required to score highly.
- 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
- Body plethysmography
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