When the partial pressure of oxygen in the circulation is abnormally high for a longer period of time, oxygen poisoning can develop. The lungs and the central nervous system are main sites where toxic effects are exerted. Manifestations range from mild symptoms to severe and life-threatening acute respiratory distress syndrome. Identifying exposure to high amounts of oxygen, clinical findings, arterial blood gas analysis, and pulmonary function tests are mandatory during workup.
Despite its essential role in sustaining human life, oxygen can be highly toxic to humans under the circumstances of prolonged exposure to increased amounts of oxygen, which may be seen during hyperbaric oxygen therapy or in divers, who must inhale very large amounts of oxygen to sustain vital functions    . Although many tissues have been reported as targets of oxygen toxicity, the two most important are the lungs and the central nervous system (CNS)   . The pathogenesis of tissue damage stems from accelerated formation of reactive oxygen species (ROS) in the presence of higher partial pressure of oxygen in the circulation. The CNS is particularly sensitive to these changes  . Symptoms and signs of toxicity appear within few hours, most common being headaches, loss of consciousness, convulsions, and visual disturbances. The loss of peripheral vision and myopia are reversible, while cataracts are a delayed complication  . On the other hand, respiratory manifestations such as carinal irritation, dyspnea, chest pain, an uncontrolled cough, tachypnea, and a sensation of choking are main indicators of lung-related oxygen poisoning   . Without cessation of exposure to high oxygen concentrations, pulmonary fibrosis, atelectasis, and acute respiratory distress syndrome (ARDS) may ensue  . Neonates who suffered from oxygen poisoning could present with retinopathies, intraventricular hemorrhages, and chronic diseases of the lungs .
Timely cessation of exposure will result in complete resolution of symptoms  , but in the absence of an early diagnosis, consequences could be severe. For this reason, it is important to identify oxygen poisoning early on and establish the mode, as well as source of exposure in order to facilitate treatment. The physician must obtain a detailed patient history that will assess the presence of risk factors (eg. diving, hyperbaric oxygen therapy) and note the course and progression of symptoms, whereas physical examination, particularly lung auscultation, can detect crackles, rhonchi, or wheezing . Laboratory investigation comprised of a complete blood count (CBC), serum electrolytes, but most importantly, arterial blood gas (ABG) analysis, should be performed as soon as possible, together with a chest X-ray that frequently shows findings consistent with bilateral pulmonary edema . Spirometry is also a vital constituent of the diagnostic workup and many patients exhibit a reduction in the vital capacity (VC). The diffusing capacity for carbon monoxide and dynamic lung compliance are both reduced in certain cases .