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Physiology

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Respiration

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All of the following are related to oxygen toxicity except. (AIPG 2007)



A Retinal blindness
B Pulmonary edema

C Decreased cerebral blood flow
D Convulsions

Ans. C Decreased cerebral blood flow

Oxygen toxicity refers to high P02 at cellular level which can damage intracellular enzymes and may form highly reactive ionic species. The tissues most susceptible to oxygen toxicity are:

a. CNS (as nerve cells-. are most sensitive to hypoxia): Most prominent neurological symptom is convulsions followed by unconsciousness.

b. Lungs (they are first to be exposed): The lungs have a very large area in contact with the breathing gas and contain thin membranes with limited antioxidant defenses, making them particularly susceptible to damage by oxygen. Pulmonary toxicity occurs with prolonged exposure of 16–24 hours or more to elevated concentrations of oxygen greater than 50%. Prolonged exposure produces pulmonary fibrosis leading to diffusion defect & Pulmonay Odema.

c. Eyes: can lead to retrolental fibroplasia or retinopathy of prematurity (ROP) in infants

The effect of hyperbaric oxygenation on the cerebral blood flow have been well studied. It has been demonstrated that in response to hyperbaric oxygenation cerebral blood flow is decreased because of cerebral vasoconstriction 13,19,28,49 . Debate exists as to whether the vasoconstriction is a result of blood oxygen per se or a secondary change in carbon dioxide48 . Even in the presence of cerebral vasoconstriction, the total amount of the oxygen available to the brain is believed to be increased4,12,14,29,46,50,51. However it can be argued that the relief of cerebral anoxia may be completely negated by the decrease in blood flow5,20. In quantitative terms ,Lembertsen31 and Jacobson19 have shown that blood flow is decreased by approximately 25%. At three atmosphere absolute the additional oxygen is dissolved in the plasma is increased by 100% in terms of normal arteriovenous oxygen extractions, thereby increasing available oxygen25,26

So, there is decreased cerebral blood flow in oxygen toxicity but it is protective in nature and not a part of disorders seen in O2 toxicity.

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