PaO2 and intratracheal pressure in oscillatory ventilation in experimental respiratory failure

In order to study the effect of high frequency ventilation on the pulmonary gas exchange in respiratory failure, we measured the PaO2 and other pulmonary gas exchange parameters for ventilatory rates of 0.5, 1.2, 4.8, and 16 Hz in dogs, to which acute respiratory failure was created by intravenous infusion of oleic acid. Either the mean intratracheal pressure or the end-tidal intratracheal pressure was controlled. A loud-speaker ventilator of our own design was modified so as to enable variation of the intratracheal pressure. Ventilation was measured using an ultrasonic instrument which counts the number of turbulent eddies. The tidal volume was set slightly higher than that was determined for healthy animals, but the resulting PaCO2 values were consistently higher than normal when PaO2 values were low. With the mean intratracheal pressure kept constant at 5, 10, and 15 cm H2O, PaO2 values with the FIO2 of 1 were between 41 and 46 at mPit of 5, 65 and 82 at 10 cm H2O, 278 and 423 at 15 cm H2O. No increase in PaO2 was observed with the increase in respiratory frequencies. If anything, a slight reduction in PaO2 at 8 and 16 Hz was observed, though not statistically significant. With the end-tidal intratracheal pressure constant, PaO2 varied but again correlated well with the mPit.