Effects of Ventilatory Settings on Oropharyngeal Gas Conditioning in a Bench Model of Nasal High-Frequency Oscillatory Ventilation

Background: Nasal high-frequency oscillatory ventilation (nHFOV) is a novel mode of non-invasive ventilation used in neonates. However, upper airway obstructions due to viscous secretions have been described as characteristic side effects of nHFOV. Hypothetically, high frequency oscillations may impair oropharyngeal gas conditioning and cause upper airway dryout.

Objective: To investigate the effects of nHFOV ventilatory settings on oropharyngeal temperature (T) and humidity in a neonatal bench model.

Methods: Nasal continuous positive airway pressure (nCPAP) or nHFOV were applied by a neonatal ventilator (Leoni Plus, Heinen & Löwenstein) at a mean pressure of 8 cmH₂O in conjunction with a heated humidifier (MR850, Fisher & Paykel). T and humidity were measured by a thermohygrometer inside a model oropharynx using a previously established bench model. In a first set of measurements, nCPAP and different nHFOV frequencies (7, 10, 13 Hz) and amplitudes (10, 20, 30 cmH₂O) were tested. In a second set, different inspiratory to expiratory (I:E) ratios (25:75, 33:66 and 50:50) were tested at fixed amplitude and frequency. Measurements were repeated ten times for each parameter setting.

Results: The breathing gas in the model oropharynx was always fully saturated (relative humidity 99.9%). NHFOV resulted in lower mean T and absolute humidity (AH) in comparison to nCPAP (T 33.8±0.7°C, AH 37.3±1.4 g∙m^-3 vs T 34.8±0.6°C, AH 39.3±1.3 g∙m^-3, p<0.001). Decreasing nHFOV frequency and increasing amplitude resulted in an additional decline of T and AH (p<0.001, respectively). For frequencies over 10 Hz, the effect was no longer dependent on frequency, but on amplitude alone (p=0.022). Increasing the I:E ratio led to a further decrease in T and AH (p=0.03, respectively).

Conclusion: NHFOV resulted in impaired oropharyngeal gas conditions in comparison with nCPAP. Intensification of nHFOV frequency, amplitude and I:E settings caused further deterioration. Future studies should explore strategies to improve heated humidification during nHFOV.