The forced oscillation technique in intubated, mechanically-ventilated patients.

In a recent discussion concerning the use of the forced oscillation technique (FOT), PRIDE [ 1] stated that, although anaesthesia and intensive care are among the obvious areas of application of the method, there is very little information on these applications. The paper by PEsuN et al. [2] in the present issue of the Journal describes one of the first attempts to apply the FOT to mechanicallyventilated patients. FOT, first described by DuBozs et al. [3] in 1956, is a technique in which externally produced pressure oscillations are applied to the respiratory system. The resulting oscillations in airflow, related to the corresponding pressure oscillations, allow a direct measurement of the impedance, i.e. the pressure-flow relationships of the system. Generally, the oscillations are produced by a loudspeaker, and are applied at the mouth of the subject The impedance is calculated from the signals of a pressure transducer and a pneumotachograph placed close to the subject's mouth. Because of the complex mechanical properties of the respiratory system, consisting of various structures with capacitive (compliant), resistive and inertive properties, pressure and flow signals are not in phase (except at given frequencies, called resonant frequencies): recorded in X-Y coordinates, the pressure-flow relationships describe a loop. Therefore, the impedance is characterized by a modulus, IZI, the ratio of pressure versus airflow, and a phase, <p, expressing the shift in time of these two variables. Another, more usual, way of dealing with this time shift is to divide the impedance into a real part or resistance (R), and an imaginary part or reactance (X), in which R is IZI cos <p, and X is lZI sin <p. In a simple circuit, consisting of a capacity, a resistance and an inertance, coiUlected in series, R corresponds to the actual resistance of the circuit and X to a combination of capacity and inertance. Generally, in a complex system, Rand X vary with oscillatory frequency. Therefore, to define the mechanical properties of the respiratory system, R and X are measured at various frequencies. In the original device of DuBms et al. [3], the response of the respiratory system was investigated by applying a succession of oscillations varying in frequency, during voluntary apnoea. This took time, and made the technique very impractical. This is why the FOT did not gain widespread acceptance, until it was modified so that sev-