Mathematical Modeling of Ventilatory Mechanics during Spontaneous Breathing

The overall mechanical behavior of the ventilatory system during spontaneous breathing can be described using linear models such as the first order equation P = V/C + R.V, where V is the “thoracic flow” measured with a total body plethysmograph, V is its time integral and P is the difference between mouth pressure and intraesophageal pressure recorded from a latex ballon. Compliance C is related to the elastic properties of lung tissues, resistance R to the nonelastic properties of both pulmonary tissues and airways. Here the operator can interact with the computer to select the data, the filter and the model. The quality of the fit and the reproducibility of C and R over the ventilatory cycle are variable from one patient to another, sometimes from one data set to another in the same patient. The fit is always better on the shorter phase of the cycle, whether inspiration or expiration. When replacing air by helium plus oxygen, the goodness of the fit is not noticeably modified, while the value of R is significantly decreased in all subjects.