HFCWO compresses and relaxes the chest wall to generate an oscillated volume from the lungs, mimicking a "mini-cough" and producing shear stress at the air-mucus interface which changes the sputum rheology, thereby improving ventilation distribution, gas mixing, and forced expired volume in one second. Most studies that have reported no significant effects have focused on mortality, hospital stay, lung function or BODE (a multidimensional 10-point scale of body mass index, severity of airflow obstruction, dyspnea rated with the modified Medical Research Council, and exercise capacity evaluated with the Six-Minute Walk Distance) score. However, these outcome measurements are not associated with the immediate effects of chest physical therapy and may be affected by other factors such as disease severity.