Oxygen therapy is least useful in

Administration of oxygen-rich gas mixtures is of very limited value in hypoperfusion, anemic, and histotoxic Hypoxia because all that can be accomplished in this way is an increase in the amount of dissolved O2 in the aerial blood. This is also true in hypoxic hypoxia when it is due to shunting of unoxygenated venous blood past the lungs. In other forms of hypoxic hypoxia, O2 is of great benefit. Treatment regimens that deliver less than 100% O2 are of value both acutely and chronically, and administration of O2 24 h/d for 2 y in this fashion has been shown to significantly decrease the moality of chronic obstructive pulmonary disease.Ref: Ganong review of medical physiology 23rd edition page no: 621