The study was conducted on 123 persons grouped as the following: Group I including 25 stable patients with chronic hypoxemia (PO2<60 mmHg) due to a pulmonary disease and within normal pH in arterial blood gases. They were subdivided into two sub-groups; group ІA including 15 patients with chronic obstructive pulmonary disease, Group ІB including 10 patients with interstitial pulmonary fibrosis and Group ІІ including 20 stable patients with chronic pulmonary disease that causes hypoxemia but with no hypoxemia in arterial blood gases (PO2>60 mmHg) and within normal pH. They were subdivided into two sub-groups; Group ІІA including 10 patients with chronic obstructive pulmonary disease, Group ІІB including 10 patients with interstitial pulmonary fibrosis and Group ІІІ including 58 patients with hypoxemia and decompensated ABGs due to a pulmonary cause that required mechanical ventilation. They were subdivided into two sub-groups; Group ІІІA including 28 patients with acute respiratory failure and Group ІІІB including 30 patients with acute on top of chronic respiratory failure.