Modeling of the relationship between volume variations at the mouth and chest

A model was developed to interpret the difference between volume variations at the mouth and at the chest observed by body plethysmography during inspiration in normal subjects breathing ambient air. The mechanical phenomena were characterized, as usual, by a time constant alpha, the product of airway resistance and alveolar gas compressibility. In contrast to other models, the change in temperature and water vapor pressure of the inspired gas was not considered to be instantaneous but rather a first-order process characterized by a thermal time constant tau. Experimental curves obtained in eight subjects were analyzed by use of parameter estimation techniques. The values of alpha were in agreement with those obtained by conventional plethysmography and tau averaged 63+/-24 ms. The results of additional experiments devised to test the physical meaning of the coefficients and the influence of neglected factors support the validity of the model. It was shown that tau cannot be neglected when breathing air at room temperature.