RESPIRATORY MECHANICS STUDIED BY FORCED-OSCILLATIONS DURING ARTIFICIAL-VENTILATION

Potential advantages of the forced oscillation technique over other me thods for monitoring total respiratory mechanics during artificial ven tilation are that it does not require patient relaxation, and that add itional information may be derived from the frequency dependence of th e real (Re) and imaginary (Im) parts of respiratory impedance. We want ed to assess feasibility and usefulness of the forced oscillation tech nique in this setting and therefore used the approach in 17 intubated patients, mechanically ventilated for acute respiratory failure. Sinus oidal pressure oscillations at 5, 10 and 20 Hz were applied at the air way opening, using a specially devised loudspeaker-type generator plac ed in parallel with the ventilator. Real and imaginary parts were corr ected for the flow-dependent impedance of the endotracheal tube; they usually exhibited large variations during the respiratory cycle, and w ere computed separately for the inspiratory and expiratory phases. In many instances the real part was larger during inspiration, probably d ue to the larger respiratory flow, and decreased with increasing frequ ency. The imaginary part of respiratory impedance usually increased wi th increasing frequency dur mg expiration, as expected for a predomina tely elastic system, but often varied little, or even decreased, with increasing frequency during inspiration. In most patients, the data we re inconsistent with the usual resistance-inertance-compliance model. A much better fit was obtained with a model featuring central airways and a peripheral pathway in parallel with bronchial compliance. The re sults obtained with the latter model suggest that dynamic airway compr ession occurred during passive expiration in a number of patients. We conclude that the use of forced oscillation is relatively easy to impl ement during mechanical ventilation, that it allows the study of respi ratory mechanics at various points in the respiratory cycle, and may h elp in detecting expiratory flow limitation.