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

Dyspnea and acute respiratory failure are among the top 5 reasons for patients to present to the emergency department (ED)(1).Supplemental oxygen therapy forms the cornerstone of resuscitation of the acutely unwell patient.Under normal circumstances, tissue hypoxia occurs within 4 min of inadequate tissue oxygenation (2). Oxygen delivery to the peripheral tissues depends upon the product of cardiac output and the arterial oxygen content (3), which is partly dependent on FIO2. In intensive care unit (ICU) patients with acute respiratory failure, non-invasive strategies of oxygenation are mainly used to avoid invasive mechanical ventilation (4). In such situations, the goal of treatment should not only be survival, but also improved respiratory condition and avoidance of inappropriate discomforts. Since the 90's, noninvasive ventilation (NIV) has been largely used with strong level of evidence in cardiogenic pulmonary edema and chronic obstructive pulmonary disease (COPD) exacerbation. NIV improves gas exchange and reduces inspiratory effort through positive pressure. However, good tolerance to NIV is sometimes difficult to achieve due to frequent leaks around the mask, possibly leading to patientventilator asynchrony and even to intubation. It may have other deleterious effects such as delayed intubation by masking signs of respiratory distress, or barotrauma by the high tidal volume potentially generated under positive pressure (5). High-velocity nasal insufflation (Hi-VNI), a viable alternative to NiPPV in treating adults in undifferentiated respiratory distress. It provides both oxygenation and ventilatory support to reduce hypercapnia(6).It focuses on optimum efficiency of the deadspace purge to augment ventilation (7,8), in addition to delivering up to 100% oxygen by nasal cannula(9,10) This is accomplished by use of small-bore nasal cannulae (typically 2.7-mm internal diameter for adult patients) to produce high velocity flow that is approximately 360% greater than that of the larger bore cannulae. It requires a flow of 25 to 35 L/min in adults to accomplish a complete purge of the extrathoracic anatomic reservoir between breaths (8,11).

Inclusion Criteria:Participants admitted to the RICU with acute hypercapnic respiratory failure requiring NIV support with the following criteria:

1. RR> 25 breath/minute.

2. Use of accessory muscles of respiration, paradoxical breathing, thoracoabdominal asynchrony.

3. Hypoxemia evidenced by the following items:

   • In patient with hypercapnic ARF:

I.Need for oxygen therapy at FIO2> 40% to maintain PaO2 > 60 mmHg or SaO2> 90% II.Blood gas analysis shows pH <7.35 and paCO2> 45 mmHg

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Exclusion Criteria:Patients with ARF who have any of the following:

I.Indication for emergency endotracheal intubation. II.HR < 50 beat\minute with decreased level of consciousness III.Persistent hemodynamic instability with

• Systolic blood pressure <90 mmHg after infusing a bolus of crystalloid solution at a dose of 30 ml / kg.
• life-threatening arrhythmia. IV.Undrained pneumothorax or Pneumothorax with persistent air leak. V.Extensive facial trauma or burnVI.Refusal to participate. VII.Usual long-term treatment with NIV for chronic disease