Estimation of respiratory impedance and source pressure using a Thévenin equivalent circuit model

The objective of this paper is to present a new technique which can provide both active respiration source pressure and lung impedance in a single noninvasive test. The method is based upon a Thévenin equivalent circuit model of respiratory mechanics. Using this model, the equivalent source pressure and source impedance can be computed from the measured changes of respiratory pressures and flows in two consecutive cycles before and after addition of purely resistive loads to the mouth. In maximal breathing the source parameters were reproducible in six normal human subjects. The total respiratory resistance during maximal breathing had an average value of 3.46 cmH2O l-1 s-1, and the total dynamic compliance had an average value of 0.078 l cmH2O-1. The airway resistances measured using a plethysmographic method were within the range of 45-65% of the estimated total respiratory resistances. These two resistances were related with a correlation coefficient of 0.98. An average value of the magnitudes of the fundamental components of the source pressure was 6.73 cmH2O during maximal breathing and 2.09 cmH2O during spontaneous breathing.