BACKGROUND High frequency oscillation is a safe and effective treatment for patients with ARDS, but poses a patient and caregiver risk when the circuit is disconnected. We modified the circuit to include a heated expiratory filter, eliminating the need for daily filter changes due to buildup of condensate. The purpose of the study was to determine if substitution of the filter resulted in a clinically important change in delivered tidal volume or amplitude. We additionally compared expiratory resistance and measured efficacy for the substituted filter. METHODS This bench study measured tidal volume and amplitude using 5 of each filter type across 6 patient setting scenarios. Filter efficacy was tested through an independent laboratory, and expiratory resistance measurements were taken after prolonged use with humidification. RESULTS The clinically important threshold value for tidal volume (defined as 5% difference) was excluded by the limits of agreement, confirming that use of the modified circuit does not result in alterations in tidal volume. The clinically important threshold for amplitude (defined as 10% difference) was the same as the lower confidence interval on the lower limit of agreement, indicating it is possible for amplitude values to be different between the 2 filters. Filter efficacy for the substituted filter was not affected. Expiratory resistance was unchanged in the substituted filter, but nearly doubled for the manufacturer's filter after 48 hours. CONCLUSIONS Modifying the circuit to include a heated expiratory filter does not affect tidal volume, and the filter material remains efficacious during oscillation. Amplitude varies under some conditions. Preventing the need for daily filter changes reduces the risk of alveolar de-recruitment. This does not completely eliminate exposure to expired gases, but provides an additional layer of protection against occupational exposure and nosocomial spread of respiratory pathogens. Further testing in a clinical environment is necessary.