The investigators conducted this comparative prospective randomized controlled single-blinded study at a university hospital. Adult subjects with pneumonia complicated with acute respiratory failure requiring endotracheal intubation and mechanical ventilation were consecutively recruited from a medical intensive care unit (ICU) (20-bed capacity). Pneumonia was defined as the presence of new or progressive pulmonary infiltrates and two of the following: body temperature > 38.3C or < 36C; white blood cell count > 12,000/mL or < 4,000/mL; purulent tracheal secretions without other signs of infection requiring antimicrobial treatment. Acute respiratory failure was defined as a sudden decrease in PaO2 < 60 mm Hg (or arterial oxyhemoglobin saturation < 90%) with or without PaCO2 > 45 mm Hg.17-19 All of the patients had sufficient sputum production to require the physician to order airway secretion clearance. Disease severity was assessed by Acute Physiology and Chronic Health Evaluation (APACHE) II score. Adverse events were evaluated by the investigators and reported to the institutional review board. The exclusion criteria were pregnancy, pneumothorax, manifest hemoptysis, unstable hemodynamics, increased intracranial pressure, and those undergoing major cardiac, thoracic or abdominal surgery.