Abstract 1859

During high frequency oscillatory ventilation (HFOV), the inspiratory to expiratory (I:E) ratio is a major determinant of mean alveolar pressure, and by extension also lung volume. Measurement of I:E ratio at the airway opening, however, differs from that predicted by the HFO ventilator settings. We asked what ventilator and respiratory system parameters might influence the I:E ratio of flow into and out of the lung at high frequencies. Method: We used a computer model incorporating an inertance (I), both non-linear (Rett=k1+k2V′) and linear resistances (Raw), and an alveolar compartment with a compliance (C). Baseline computations were performed using nominal values that were consistent with severe respiratory disease in a neonate ventilated through a 2.5-mm ETT. The model was driven by a computer-generated square (SQ) or quasi-sinusoidal (Q-SIN) oscillatory pressure waveform. The I:E ratio of the driving pressure waveforms were varied from 1:1 to 1:4. Computations were also made at varying I, Rett, Raw and C. The delivered I:E ratios were analysed from the resultant flow waveforms. Results: Symmetrical pressure waveforms resulted in a delivered I:E ratio of 1:1. When asymmetric pressure waveforms were employed (I:E < 1:1), the difference between the set and delivered I:E ratios were greater for the Q-SIN rather than the SQ wave (see Figure).

Fig 1
figure 1

Relationship between delivered I:E ratio vs (A) set I:E ratio and (B) frequency for a Q-SIN (circel) and SQ (square) oscillatory pressure wave

Full size image

The I:E resulting from the Q-SIN wave was altered by frequency, Rett, Raw and C whereas for the SQ wave, it was affected by frequency, I and C. The direction of the change in I:E ratio with frequency and C depended on the shape of the oscillatory pressure waveform. Conclusion: The I:E ratio during HFOV will vary with the shape of the oscillatory pressure waveform and the mechanical properties of the circuitry and respiratory system. The accurate determination of I:E ratio during HFOV requires the use of an appropriate flow measuring device.

Funded by NHMRC, Australia