Determination of lung volume by hydrogen dilution at the time of underwater weighing

A method for measuring the volume of air in the lungs at the time of underwater weighing is described. A low concentration of hydrogen is used as a tracer gas in a closed-circuit rebreathing system. At the end of a normal exhalation the subject is connected to a respiratory bladder containing 2 L of air with a small admixture of hydrogen. After an equilibration period of five breaths the subject submerges completely, together with the bladder, and underwater weight is measured. Lung volume, at the time of weighing, is determined by hydrogen dilution. Using this method, the coefficient of variation for body density in the same individual was 0.23%.