R. Peslin et al., RESPIRATORY MECHANICS STUDIED BY FORCED-OSCILLATIONS DURING ARTIFICIAL-VENTILATION, The European respiratory journal, 6(6), 1993, pp. 772-784
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.