Breathing 100% O2 is a way of slowing breathing rate, decompressing hyperinflated, low VA/Q lung units, reducing the FRC and allowing the patient with COPD in acute respiratory failure to breath at a more comfortable lung volume and with a higher tidal volume. This should also improve the efficiency of breathing, increasing the fractional turnover of alveolar gas. Simultaneously O2 protects the brain, heart and other organs from hypoxic injury. Breathing 100% O2 provides the major therapeutic approach for rapidly reversing the pathophysiological cycle of impaired respiratory mechanics which threatens the life of the patient in acute respiratory failure. It is a therapeutic approach which might be employed in the tachypnoeic patient in acute respiratory distress, before initiating airway intubation and mechanical ventilation. This provides the physician with time to allow other mechanisms of bronchodilating the patient to be started, and for the need of a more aggressive approach to airway management to be evaluated.