Mean airway pressure and alveolar pressure during high-frequency ventilation

Studies and applications of high-frequency ventilation (HFV) are often performed under conditions of controlled mean airway pressure (Paw). In the present study we tested the assumption that controlling Paw adequately controls lung volume during HFV by investigating the relationship between a reliably measured Paw and the mean alveolar pressure (Palv) of the lungs during HFV of healthy dogs. We minimized the errors of Paw measurement due to the Bernoulli effect and various technical factors by appropriate choice of transducers, amplifiers, and measurement site. Palv was estimated by clamping the ventilator tube during oscillation and measuring the equilibration pressure of the lung and airways. Paw and Palv were determined as functions of frequency (8-25 Hz), tidal volume (60-90 ml), Paw (-5 to 12 cmH2O), and position of the animal (supine vs. lateral). We found that Paw could significantly underestimate Palv and that the degree of underestimation increased at higher frequencies, larger tidal volumes, and lower Paw. Shifting the animal from the supine to the lateral position greatly accentuated this effect. The elevation of Palv above Paw was seen to be a function of mean flow and largely independent of the frequency-tidal volume combination which produced the flow. A possible explanation of this pressure difference is that it results from differences in inspiratory and expiratory airway impedances, which in turn depend on airway geometry, compliance, lung volume, and expiratory flow limitation.