Background: This study examined the effect of I:E ratio on differences between mean alveolar pressure and mean airway pressure during high frequency oscillation (HFO).

Subjects: Observations were made in an artificial lung model and in 4 euthanased rabbits.

Measurements: Mean pressure was measured at the patient airway(Paw), in the model lung (Pl) and in the alveolar space(Palv) of the rabbit by the alveolar capsule technique during HFO with the sensorMedics 3100. Pressure difference (Pdiff) between Paw and Pl or Palv was measured at various frequencies(7.5·15hz), amplitudes (10·90 cmH2O) and I:E ratios(1:1·1:2) using a range of ETT sizes (2.5·3.5mm) and model lung compliances (0.2·2ml/cmH2O).

Results: The magnitude of Pdiff in the lung model and rabbit were very similar. Pdiff was negligible at an I:E of 1:1 but reached substantial levels at an I:E of 1:2 (Palv and P1 up to 6 cmH20< Paw). In the model lung Pdiff increased as tidal volume (Vt) increased. At constant Vt, Pdiff increased as frequency increased and as% inspiratory time or ETT size decreased. In pooled observations from the model lung, the magnitude of Pdiff was linearly related (r = 0.98) to the difference between the calculated squared inspiratory (Vi2) and expiratory flow velocities (Ve2 in the ETT.

Conclusions: Palv may fall well below Paw at an I:E of 1:2. This effect depends on asymmetry between inspiratory and expiratory flow velocities and its magnitude can be predicted from Vi2·Ve2.