HVNI vs Noninvasive Ventilation for Acute Hypercapnic Respiratory Failure

1. To study the efficacy and safety of high flow nasal oxygenation using high velocity nasal insfflation technique(HiVNI) in comarison with non invasive positive pressure ventilation in COPD patients with acute hyercapnic respiratory failure 2. To do selection criteria for the indication of this new physiological technique in critically ill COPD patients