Forced oscillation techniques for measuring mechanical properties of the respiratory system.

Airway function is usually assessed by tests of forced expiration or, in more specialised laboratories, by measuring airways resistance by whole body plethysmography. Plethysmography depends on the patients' cooperation and requires expensive and bulky apparatus. An alternative non-invasive technique for measuring airflow resistance, using forced oscillation, was introduced by DuBois and colleagues in the 1950s'-almost at the same time as body plethysmography-but did not become as widely established in clinical physiology. The biomechanical principles of forced oscillation are generally less familiar than those underlying conventional measurements of airways resistance and lung compliance, and the technical requirements for obtaining accurate pressure and flow signals and the subsequent signal processing are more demanding. Recent advances in pressure transducer and microcomputer technology, however, have removed many of the earlier problems with signal collection and analysis. Because most oscillation methods require less cooperation from the subject and less bulky apparatus than body plethysmography the balance of advantage between the two methods has altered and we can expect the oscillation technique to become more widely used in clinical physiology. Methods of applying forced oscillation Forced oscillation techniques deduce the mechanical properties of the respiratory system from the response to small externally produced oscillatory forces. From the responsemeasured as the instantaneous pressure-flow relationship (impedance)-flow resistance and the reactance (the combined effect of elastance and inertance) of the respiratory system can be computed. Because these mechanical properties of the lungs are non-linear it is important that only small external forces are applied. Most commonly, forced oscillation is applied at the airway opening by a loudspeaker and the pressure-flow relation is also measured at the airway opening ("input" impedance-see fig 1). Alternative techniques apply an oscillatory signal to the surface of the chest wall and measure the induced flow response at the airway opening or they oscillate at the airway opening and measure flow at the chest wall ("transfer" impedance).2 Transfer impedance has certain technical advantages but requires the subject to be seated in a head out plethysmograph and will not be further discussed. The simplest form of forced oscillation technique applies a single sinusoidal signal during breath holding (fig 2). The impedance