Linear modeling of ventilatory mechanics during spontaneous breathing.

The overall mechanical behavior of the ventilatory system during spontaneous breathing is described by linear models such as the first-order equation P = EV + RV + P0 in healthy and diseased subjects. A graphics console is used to select the data and the model, then to display the measured and calculated dependent variable. Pressure and volume waveforms are reconstructed over each ventilatory cycle using the two first harmonics of their Fourier series. Least-squares estimates of respiratory parameters over the entire cycle, inspiration and expiration, are close to values obtained with raw data or the fundamental harmonic of signals. Our technique, however, provides a visual and numerical estimation of the pertinence of linear models. The first-order equation provides accurate and reproducible values of C and R in about one-third of a 31-subject sample. For the other patients, addition of a V or P term improves the quality of the fit, but yields poorly reproducible estimates of parameters.