This chapter is most relevant to Section F4(i) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the measurement of lung volumes and capacities and factors that influence them". This has appeared a couple of times in the primary exam:
Both of these questions asked specifically about the FRC, and how it is measured. Consequently, to maintain some attachment to an exam focus, the measurement of the FRC is discussed in most detail here. The measurement of the other volumes is mentioned in passing wherever it appears relevant.
- Lung volumes are usually measured by first measuring the FRC.
- 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
Once FRC is determined, ERV and IC can be determined by spirometry.
- TLC can be determined by adding FRC and IC
- RV can be determined by subtracting ERV from the FRC
For this topic, the single best resource is probably Wanger et al (2005). Coates et al (1997) do an excellent job of explaining the measurement of lung volumes by plethysmography, and Newth et al (1997) are the go-to source for the nitrogen washout technique.
Measurement of lung volumes by the measurement of nitrogen washout was probably the first clinically relevant method, described by Darling et al (1940).
The technique is as follows:
There are various inaccuracies and caveats involved in this process:
This method also relies on the measurement of an exhaled gas concentration, except this time one administers a known volume of this gas, and measures the exhaled concentration. Though strictly speaking any gas could be used for this, realistically one would prefer to use some non-toxic gas which has minimal blood solubility and which is cheap enough to use clinically. The blood solubility thing is very important, as the concentration of exhaled tracer gas will be lower if any of it managed to get out into the pulmonary circulation. Nunn's mentions helium (it ticks all the boxes), but others have used various weird gases such as sulfur hexafluoride and argon.
The theory is similar to the concept of measuring the volume of distribution. In this diagram, the volume of tracer is comically exaggerated to make the maths easier; in reality one would usually give 50ml of helium, much less that is required to make your voice sound funny.
In short, the process is as follows:
So, in the diagram above, the exhaled gas concentration is 20g/m3, whereas previously it was 100g/m3. in 1L. It is as if the gas was diluted by 5 times. Thus, the intrathoracic "volume of distribution" must have been 5L. The equation for calculating this is:
C1 × V1 = C2 × (V1 + V2)
This obviously has some problems:
This method is also ancient. It was first devised by DuBois et al (1956). The principles of this method are as follows:
This methods of measurement has the distinct advantage of being able to measure all of the gas in the chest, including the gas which is trapped behind a bunch of collapsed airways. The disadvantages are fairly benign; the main risk is that the patient's bowel gas (however much of it there is) ends up being compressed by the expiratory effort, which affects the measurement somewhat. Unless one is performing plethysmography on a particularly gassy patient, this should not be a major influence.