Noninvasive Positive Pressure Ventilation Using Helium:Oxygen Versus Air:Oxygen in Acute Respiratory Failure in Chronic Obstructive Pulmonary Disease Patients Exposed to Sulfur Mustard

Inclusion Criteria: * a history of previous exposure to sulfur mustard * worsening dyspnea during the last 10 days * respiratory rate \>25/min * arterial pH\<7.35 * PaCO2 of \>50 mmHg * PaO2 of \<50 mmHg Exclusion Criteria: * recent pneumothorax (\<1 month) * severe respiratory failure or hemodynamic instability with forthcoming intubation * FIO2 of \<0.4 * impaired consciousness or absence of patient cooperation * facial lesions precluding NIV