Correcting for the thermodynamic characteristics of a body plethysmograph

A constant volume body plethysmograph is used to measure changes in total body volume by relating pressure changes measured inside a closed chamber to changes in its volume. The relationship between pressure and volume, however, is complicated by the fact that rapid changes in pressure are accompanied by changes in the temperature of the gas in the plethysmograph. The rate at which the temperature of the gas subsequently approaches that of the environment affects the frequency response of the plethysmograph as a volume measuring instrument. In this paper it is shown that the transient response in plethysmograph pressure to a step change in volume must be a multi-component function, due to the gas near the walls of the plethysmograph approaching thermal equilibrium with the environment at a faster rate than gas near the center. The step response of a 200 L plethysmograph was determined experimentally and found to be well described empirically by a sum of two decaying exponential functions of time. The fitted exponentials were used to construct an algorithm for digitally correcting a measured pressure signal for the effects of the plethysmograph response. When applied to a hypothetical volume waveform over a single breath, an almost perfect correction was obtained.