|
During high frequency ventilation (HFV), mean alveolar pressure has been measured to increase with mean airway opening pressure controlled at a constant level in both humans and experimental animals. Since this phenomenon could potentiate barotrauma limiting advantages of HFV, the present study theoretically predicted the difference between menu alveolar and airway opening pressures (MP4,v). In a Weibel’’’’s trumpet airway model, approximated formula for MPalv was derived based on momentum conservation assuming a uniform velocity profile. The prediction, equation was a func pion of gas density(p), mean flow rate(Q), and diameter of the airway opening where the pressure measurement was made(Do) : MPa,v :4p(Q/Do). This was a result of the difference in crosssectional area between the alveoli and the airway opening. A simple aireway model experiment was performed and the results well fitted to the prediction, which demonstrated the validity of the present analysis. Previously reported Mpalv data from anesthetized dogs in supine position were comparable to the predicted values, indicating that the observed dissociation between mean alveolar and airway opening pressures during HFV can be explained by this innate geometric (or cross-sectional area) asymmetry of the airways. In lateral position, however, the prediction substantially underestimated the measurements suggesting involvement of other important physiological mechanisms.
|