BACKGROUND: The effectiveness of high-frequency chest wall oscillation (HFCWO) in mucolysis and mucous clearance is thought to be dependant on oscillatory flow rate (Fosc). Therefore, increasing Fosc during HFCWO may have a clinical benefit. OBJECTIVES: To examine effects of continuous positive airway pressure (CPAP) on Fosc at two oscillation frequencies in healthy subjects and patients with airway obstruction. METHODS: Five healthy subjects and six patients with airway obstruction underwent 12 randomized trials of HFCWO (CPAP levels of 0 cmH2O, 2 cmH2O, 4 cmH2O, 6 cmH2O, 8 cmH2O and 10 cmH2O at frequencies of 10 Hz and 15 Hz) within a body plethysmograph, allowing measurements of changes in lung volume. Fosc was measured by reverse plethysmography using a 20 L isothermic chamber near the mouth. At the end of each randomized trial, an inspiratory capacity manoeuvre was used to determine end-expiratory lung volume (EELV). RESULTS: EELV increased significantly (p < 0.05) with each level of CPAP regardless of oscillation frequency. Fosc also significantly increased with CPAP (p < 0.05) and it was correlated with EELV (r = 0.7935, p < 0.05) in obstructed patients but not in healthy subjects (r = 0.125, p = 0.343). There were no significant differences in perceived comfort across the levels of CPAP. CONCLUSIONS: Significant increases in Fosc with CPAP-induced increases in lung volume were observed, suggesting that CPAP may be useful as a therapeutic adjunct in patients who have obstructive airway disease and who require HFCWO.

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