ABSTRACT
Lung volumes may be measured in two ways. First, by helium dilution, where a known volume (V1) and concentration (C1) of helium is inhaled into the lungs. When equilibrium is achieved by dilution of the original helium throughout the lungs, the new concentration of exhaled helium is measured (C2). By the formula V1 x C1 = V2 x C2, because C1 and C2 are measured and because V1, the original volume of helium is known, V2 can be calculated. Thus, the lung volumes can be calculated. V2 being the original volume of inhaled gas (V1) plus the volume of the lungs which helium has reached. Hence, V2 - V1 = lung volume (TLC).
