EFFECT OF EXTERNAL CHEST-WALL OSCILLATION ON GAS-EXCHANGE IN HEALTHY-SUBJECTS

Effective gas exchange can be maintained in animals without the need f or endotracheal intubation using external chest wall oscillation (ECWO ). The clinical application of this technique has been limited by equi pment which was either impractical or uncomfortable. We evaluated a pr ototype of a new oscillator in which an oscillatory profile of negativ e and positive pressure was imposed on a negative baseline pressure wi thin a cuirass. In seven healthy subjects, we identified an oscillator y cuirass pressure that could effectively ventilate but would not resu lt in severe hypocapnia over 5 min. We then measured the influence of changing the frequency of oscillation (fo) on PaCO2 and spontaneous ve ntilation. Lastly, we evaluated the capability of this prototype to ac hieve targeted changes in chamber pressure. Subjects were ventilated w ith an inspiratory chamber pressure of -20 +/- 4 cm H2O, an expiratory chamber pressure of 5 cm H2O and an inspiratory-expiratory ratio of 1 :1 at 9 oscillatory frequencies (fo: 1 to 5 Hz at 0.5-Hz increments), Each subject was ventilated for 5 min with consecutive periods of ECWO being separated from each other by 10 min of unassisted breathing, Os cillatory tidal volume (Vo) was sampled and PaCO2 was determined from the expired carbon dioxide concentration (FECO(2)) measured at the mou th. The change in PaCO2 (Delta PaCO2) was the difference in PaCO2 imme diately before and after ECWO. We found that Delta PaCO2 and Vo were i nversely related to fo. At 1 Hz the Delta PaCO2 was -13 +/- 1 mm Hg an d Vo was 344 +/- 34 mt in the absence of spontaneous breathing (fb = 0 ). At 3 Hz and above, at the chamber pressures used, the Delta PaCO2 w as small(-1 to -2 mm Hg) and the Vo was less than the predicted dead s pace. Subjects breathed spontaneously but at a frequency below that of their resting fb. With this prototype, chamber pressure changes up to 30 cm H2O could be accurately achieved at 1, 2.5, and 4 Hz. In conclu sion, ECWO can provide effective ventilation among healthy adults in t he presence or absence of spontaneous breathing, and further studies a re warranted to explore its effectiveness in a variety of clinical cir cumstances.