Abstract 1860

Evaluation of the severity of respiratory disease in infants treated with high frequency ventilatory oscillation (HFOV) has been hampered by the lack of appropriate measurements of lung mechanics suitable for use during HFOV. We aimed to evaluate the practicalities of employing the forced oscillation technique (FOT) to measure respiratory impedance in intubated fetal lambs ventilated with HFOV. Methods: Four 116 d fetal lambs were studied. A 4-mm ID wave tube connected to a loudspeaker-in-box was inserted between the circuit and the ETT. For each measurement, the respiratory system was isolated from the HFOV circuit above the wavetube. At 20 min of age, a pseudorandom noise (16 components, 0.5 -20 Hz) was applied to the airway opening for 6 s. The resistance (Rrs) and reactance (Xrs) were calculated at each frequency and a model comprising an airways resistance (Raw), inertance (Iaw) and a constant-phase tissue compartment containing resistive (Grs) and elastic (Hrs) parameters was fitted. Results: Impedance was successfully measured in all four fetal lambs; a representative spectrum is illustrated in Figure 1. The mean (SD) impedance parameters were: Raw 5.6 (1.4) cmH2O.s/L, Iaw 0.18 (.002) cmH2O.s2/L, Grs 854 (126) cmH2O/L, and Hrs 4159 (759) cmH2O/L. Conclusion: Measurement of lung mechanics using the FOT is a technically simple procedure during HFOV. The use of the FOT to measure respiratory impedance at constant mean airway pressure may provide a tool for the objective assessment of HFOV management.

Figure 1
figure 1

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Supported by: NHMRC Australia