Introduction: The use of high frequency oscillatory ventilation is increasing in treatment ofacute respiratory distress syndrome over the past decade. The technique of HFOV of ventilatingthe lungs at volumes less than the anatomical dead space calms the clinical concerns surroundingventilating stiff ARDS lungs with high pressures and volumes. This largely reduces theprobability of barotraumas and/or atelectrauma. Purpose: The study was on an in vitro bench model that answered the following researchquestions: 1. The effect of three inline closed suction adapters on delivered tidal volume duringHFOV with varying lung compliance 2. The effect of varying compliance on the amplitudedelivered by HFOV; and 3. The effect of compliance on tidal volume delivered by HFOV. Method: An in vitro bench model using high fidelity breathing simulator (ASL 5000, IngMarMedical) simulating an adult patient with ARDS was set up with 3100B SensorMedic highfrequency ventilator. The simulation included varying the compliance for each lung at 50, 40, 30and 20cmH2O while maintaining fixed resistance of 15 cmH2O/L/sec. The ventilator was set tothe following parameters: power of 6, frequency (f) of 5, inspiratory time (Ti) of 33%, bias flow(BF) of 30 LPM and oxygen concentration of 50%. The breathing simulator was connected withthe high frequency ventilator using a standard HFOV circuit and a size 8.0mm of endotrachealtube. Fourteen French Kimberly Clark suction catheters (with T and Elbow adapters) and Air-Life suction catheters (Y adapter) were placed in-line with the circuit successively to carry outthe study. Each run lasted for 1 minute after achieving stable state conditions. Thisapproximated to 300 breaths. The data was collected from the stimulator and stored by the hostcomputer. Data Analysis: The data was analyzed using SPSS v.11 to determine the statistical significance.A probability value (P value) of ≤ 0.001 was considered to be statistically significant. Results: The data analysis showed that Air-Life Y-adapter suction catheters caused the least lostin tidal volume when placed in line with HFOV and hence proved to be the most efficient. Thestudy also showed a direct relationship between amplitude and lung compliance i.e. an increasein lung compliance caused an associated increase in amplitude (power setting remainingunaltered). Lastly, the study did not show a statistically significant change in tidal volume withchanges in lung compliance. Future studies may be required to further evaluate the clinicalsignificance of the same. Conclusion:1. Many factors affect delivery of tidal volume during high frequency ventilation and thus it isnot constant. Choice of in-line suction system to be placed in line is one of the determinants ofthe same.2. Lung compliance changes lead to associated changes in amplitude delivery by HFOV. Thisshould be adjusted as patient condition improves by altering the power settings to ensure optimalventilation and to avoid trauma to the lungs.