Exploring respiratory mechanics by forced oscillations: principles and pitfalls.

Submitting a physical system to forced oscillations is a very general approach to investigation of its structure and/or properties. Its application to respiratory mechanics was first proposed by DuBors et al. in 1956 [1], but it has also been used in other fields of respiratory physiology, e.g. to investigate the control of breathing and the response to exercise. As used in respiratory mechanics, the method consists of applying sinusoidal pressure variations to the respiratory system (or one of its parts) with an external generator, and in studying the relationship between the pressure applied and the resulting respiratory flow. The pressure-flow relationship at a given frequency is termed "impedance" (Z) and may be expressed by the amplitude ratio of the variables (modulus of impedance (Z)) and by their phase angle ((1.1), or by two related parameters: the effective resistance or real part of the impedance (Re = (Z)·cos (1.1) and the reactance or imaginary part of the impedance (Im = (Z)·sin (1.1). Although the method may already be helpful when used at a single frequency, it is especially fruitful to make the measurements at a number of different frequencies, i.e. to obtain the frequency response of the system. Then, using an appropriate model, several properties of the system may be computed from the impedance data. The extent to which a specific property influences the impedance depends very much upon the frequency. By properly choosing the frequency range, it is therefore possible to study selectively different aspects of respiratory mechanics. The method is increasingly used since digital computers are available for data processing. Computers also made it possible to explore many frequencies simultaneously by using non-sinusoidal inputs with a large frequency content. Then, the analysis in terms of elementary sine-waves is made using Fast Fourier Transforms [2]. When oscillating the respiratory system at frequencies above a few Hz, substantial differences are seen between instantaneous flow at the mouth and at the chest wall in relation to alveolar gas compression. It follows that several types of respiratory impedance may be obtained according to where pressure oscillations are applied and which flow is considered [3]. The most commonly measured is termed "input impedance" (Zin) and is obtained by varying the pressure at the airway opening and