Kinematics of spontaneous breathing: the ventilatory system as a non-linear oscillator

The kinematics of spontaneous breathing at rest and during moderate exercise is described exactly by a non-linear differential equation, the parameters of which are determined by observation with a pneumotachograph. Analogue circuits are used for the determination of the coefficients and for the comparison by superimposition of the actual spirogram with its simulation. The ventilatory system, taken as a whole and without any assumption concerning its structure, works as a non-linear oscillator. If the classical distinction between a passive and an active ventilatory system is accepted, the concept of a linear equivalent of a non-linear oscillator is valid for the description of the properties of the passive system. It affords some of the advantages of linear mechanics and indicates the restrictions put upon the use of a linear hypothesis. The role of the different terms in determining the pattern of breathing is displayed and the correlation of scale factors with body size is shown. The physiological meaning of the components of muscle action is discussed.