Saturday, 1 June 2013

Oxygen Toxicity

Oxygen therapy is like a two-edged sword, at one edge oxygen is essential for human survival, while at the other edge it may become toxic at an elevated partial pressure. This is a hazard, especially in intensive care units, where oxygen therapy may be administered over a period of days. Oxygen toxicity usually manifests in one of several forms including central nervous system mainfestations, pulmonary manifestations, and ocular manifestations, especially in premature neonates. The major factors affecting the onset and the severity of the toxicity are the concentration of the gas used, the duration of the exposure, and the susceptibility of the individual person. Because the treatment is being purely symptomatic, prevention and early recognition of the toxicity is of prime importance

O2 toxicity is caused by free radicals. These affect the nervous system resulting in anxiety, nausea and seizures when the partial pressure exceeds 200kPa. The alveolar capillary membrane undergoes lipid peroxidation and regions of lung may collapse.

Neonates may develop Retinopathy of Prematurity (retrolental fibroplasias).
Vascularisation of the fetal retina is not complete until term. Retinopathy of prematurity is a condition of premature babies in whom the peripheral retina is avascular. Those vessels present are very sensitive to oxygen and the terminal branches may constrict if levels are too high. In the early stages, the changes are reversible and normal development may ensue. Later, however, new vessels begin to proliferate from the constriction sites into the vitreous. They become covered in fibrous tissue so forming a fibrovascular membrane behind the lens. A tractional retinal detachment may occur. In severe cases, there may be impaired vision, large refractive errors particularly myopia, strabismus and even blindness. The condition is usually bilateral.





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