Lung volumes

Of the following techniques, which one may be used to measure residual volume?

a) Carbon monoxide dilution

b) Total body plethysmography

c) Bohr’s method

d) Pendelluft analysis

e) Wet spirometry

Answer: B

Explanation

Total body plethysmography involves sitting the subject in a closed box while they

make a series of respiratory efforts against an open and closed valve. Collection of

measurable pressure and volume changes with the application of Boyle’s law allows

the derivation of functional residual capacity and subsequently residual volume. Other

methods include helium dilution (carbon monoxide is involved in determining diffusion

capacity), and single- and multiple-breath nitrogen washout analysis. The former

is Fowler’s method and can be used to determine anatomical dead space and closing

capacity as well. The multiple-breath nitrogen washout technique is also used in

studies of uniformity of ventilation. Bohr’s method is used to measure physiological

dead space. Wet spirometry can measure every volume of gas that may pass the

subject’s lips but cannot deduce what volume remains in the subject’s lungs at the

end of a maximal expiration. Pendelluft is the phenomenon where gross mismatching

of compliance in different lung regions (classically caused by a flail chest but also

occurring in acute respiratory distress syndrome) causes gas transfer between lung

regions during the respiratory cycle, rather than in and out of the trachea.