Whole-body plethysmography, does it measure tidal volume of small animals?

A whole-body plethysmograph was used for mice. The increase in pressure cau sed by each inhalation was equivalent to the increase that could be calcula ted to result from heating and humidification of the inhaled air. However, comprehending that a drop in temperature and humidity would cause an abrupt pressure decline during exhalation was difficult. Pressure changes in the plethysmograph were also studied with an artificial chest, modeling the res piratory mechanics, but without the "inhaled" air being heated or humidifie d. The "chest" consisted of a metal bellows oscillated by a stepper motor 2 5 to 175 times per minute. Hereby air (0.05 to 0.20 mt) moved in and out of the bellows. The air passed through a polyethylene tube, the length of whi ch was proportional to "airway resistance" and varied from 5 to 35 cm. It w as found that the pressure oscillation was affected not only by "tidal volu me" of the mechanical chest but also by "respiratory rate" and by "airway r esistance." We concur with previous investigators that the plethysmograph p ressure reflects alveolar pressure and that fluctuations cannot be explaine d by changes in temperature and humidity. Accordingly, tidal volume can onl y be qualitatively and not quantitatively assessed.