Influence of endotracheal tube bore on tidal volume during high frequency oscillatory ventilation: a model lung study

In adults, the influence of endotracheal tube bore on tidal volume (VT) during high-frequency oscillatory ventilation (HFOV) has been little studied.

Via full-length 6, 7, 8, and 9 mm internal diameter (ID) endotracheal tubes (ETTs), a model lung (TTL, Michigan Instruments, MI) was ventilated using a high-frequency oscillator designed for adult patients (R100; Metran, Japan). Oscillation settings were: frequency, 5, 7, and 9 Hz; mean airway pressure, 15 cmH2O; oxygen inspiratory fraction, 0.21. At each setting: oscillator pressure swing was set at maximum; and each test combination was conducted with the model-lung compliance set at 20 and 50 mL/cmH2O. During the trials, a position sensor attached to the model-lung upper surface provided analog electrical signals for lung movement that were converted to VT evaluation data. Pressure and volume signals were digitized using an analog-to-digital converter and recorded in a computer. At each setting, three waveforms were analyzed and the average value used.

At each setting, measured VT was proportional to ETT internal cross-sectional area (CSA). Regression analysis of VT showed correlation with the CSA, as expressed by formula, y=2.173x where x is CSA and y is VT (R2=0.99, p<0.001) at 5 Hz and compliance of 50 cmH2O.

In an adult-simulating model lung, VT during HFOV varies in proportion to ETT CSA.