High-frequency Ventilation Wavelength Is Dependent on Bias Flow, Endotracheal Tube Size, and Frequency

Background: High-frequency ventilation is predicated on the successful transmission of a higher than conventional waveform through an endotracheal tube. There are myriad factors that affect the transmission of the waveform, including tube length and diameter, flow, frequency, and a multitude of patient factors. Logically, these factors may determine the integrity of the wave reaching the patient. The selection of the appropriate bias flow on the oscillator is clinician-dependent. Although guidance from the manufacturer and other sources indicate that a higher bias flow may not be necessary or indicated for smaller patients or correspondingly small endotracheal tubes, the relationship of flow and endotracheal tube size to wavelength and propagation of the waveform has not been well defined.

Methods: Simulated oscillator settings were used for this model (SensorMedic 3100-A, Vyaire Medical, Inc., Yorba Linda, CA). Bias flows of 10 LPM, 15 LPM, 20 LPM, 25 LPM, and 30 LPM were analyzed for endotracheal tube sizes 2.5 mm, 3.0 mm, 3.5 mm., 4.0 mm, 4.5 mm, 5.0 mm, 5.5 mm, and 6.0 mm at frequencies of 5 Hz, 10 Hz, and 15 Hz. The flow velocity used in this calculation is: v = Q / (π · (ø / 2)²), where v = Q / A, A = π · r², and r = ø / 2. Q = Volume flow rate, v = Flow velocity, A = Cross-sectional area, r = Radius, and ø = Diameter. Wavelength (λ) was calculated using the relationship λ=v/f, where f represents the oscillator frequency. For this analysis, it was assumed that there were uniform flow conditions within the entire cross-sectional area of the endotracheal tube, without any loss of speed due to friction at the inner aspect of the ETT wall or the presence of water vapor in the circuit.

Results: The results are presented graphically. For wavelength calculations, Increasing frequency is inversely proportional to decreasing flow rate.

Conclusion: Waveform propagation is dependent on the uniformity of media and position (e.g., ETT tube position). Sinusoidal waves in time propagating through a non-uniform media such as the lung may transmit with a velocity that deviates from the ideal generated by the model. The waveform velocity decreases with a corresponding decrease in wavelength and what would seem to be a paradoxical increase in amplitude. However, the shortened wavelength is associated with an increased loss of waveform integrity and eventual extinction. Bias flow selection, ETT size, and frequency are interdependent and critically important in determining the wavelength. Wavelength dynamics may be critically important to the propagation of high-frequency ventilation.