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The measurement of TLC is not considered in detail here; the interested reader is referred to the reading list at the end of the chapter.

Figure 3.7 Relative effects on expiratory and inspiratory flow of intra‐ and extrathoracic large airway obstruction. Top: Large airway obstruction within the thorax. (a) Positive intrathoracic (alveolar) pressure generated during expiration acts to compress the airway and further narrow the point of obstruction. (b) Negative intrathoracic pressure during inspiration acts to reduce narrowing at the point of obstruction. Therefore, in large airway obstruction within the thorax, expiratory flow is diminished to a greater degree than inspiratory flow (see Fig. 3.6d). Bottom: Large airway obstruction outside the thorax. (c) Positive pressure within the airway during expiration in relation to atmospheric (‘zero’) pressure outside acts to reduce narrowing at the point of obstruction. (d) Negative pressure within the airway during inspiration acts to compress the airway and further narrow the point of obstruction. Therefore, in large airway obstruction outside the thorax, inspiratory flow is diminished to a greater degree than expiratory flow (see Fig. 3.6e).

Whereas VC and its subdivisions can be measured directly by spirometry, measurement of residual volume and TLC requires the use of helium dilution or plethysmography methods. In the dilution technique, a gas of known helium concentration is breathed through a closed circuit and the volume of gas in the lungs is calculated from a measure of the dilution of the helium, which, being an inert gas, is neither absorbed nor metabolised. This dilution method measures only gas in communication with the airways and tends to underestimate TLC in patients with severe airway obstruction, because of the presence of poorly ventilated bullae.

The body plethysmograph is a large airtight box that allows pressure–volume relationships in the thorax to be determined. When the plethysmograph is sealed, changes in lung volume are reflected by a change in pressure within the box. Plethysmography tends to overestimate TLC, because it measures all intrathoracic gas, including that in the bullae, cysts, stomach and oesophagus. Chest X‐ray can be used to give a very rough estimate of TLC. In airway disease, TLC is increased as a manifestation of hyperinflation and as a result of increased lung compliance in emphysema (see Chapter 1). TLC is reduced in restrictive lung disease (by definition).