An in vitro assessment of gas trapping during high frequency oscillation

Our aim was to assess the influence of oscillatory frequency, inspiratory-to-expiratory (I:E) ratio and airway pressure on gas trapping during high frequency oscillatory ventilation (HFOV). A lung model was used, which had a high compliance (4.0 ml (cmH(2)O)(-1)) and resistance (160 cmH(2)O l(-1) s(-1)) resulting in a long time constant (0.64 s). To assess whether gas trapping occurred, the mean pressure within the lung model (equivalent to alveolar pressure) was compared to the mean airway pressure (MAP) measured at the manifold (manifold MAP) of the two oscillators used, the SLE 5000 (I:E ratio 1:1) and the Sensor Medics 3100A (tested at l:E ratios of 1:2 and 1:1). The effects were assessed of raising the MAP from 15 to 35 cmH(2)O, the oscillatory amplitude from 30 to 70 cmH(2)O and the frequency from 5 to 15 Hz (5 to 20 Hz SLE only). There were no significant trends for differences between the pressure within the lung model and the 'manifold' MAP to increase (i.e. no evidence of gas trapping), as MAP, amplitude or frequency was increased, regardless of which oscillator or LE ratio was used. Increasing the pressure amplitude led to a progressive fall in the pressure within the lung model when an I:E ratio of 1:2 was used (p < 0.05). Our results suggest that significant gas trapping does not occur during HFOV even if there is high compliance and resistance.