Objective: Few data are reported in the field of noninvasive ventilation (NIV) regarding humidification devices. It was previously suggested that standard heat and moisture exchangers (HME) had adverse gasometrical and clinical outcomes in acute respiratory failure (ARF) of chronic respiratory failure patients (CRF). This study was performed to evaluate respiratory parameters and arterial blood gases (ABG) of patients during NIV with small dead space HME compared to heated humidifier (HH).