A high level of oxygen exerts which of the following effects on the lung?
A. Improves oxygen uptake.
B. Increases carbon dioxide levels.
C. Stabilizes carbon dioxide levels.
D. Reduces amount of functional alveolar surface area.
Correct Answer: D. Reduces amount of functional alveolar surface area.
Oxygen toxicity causes direct pulmonary trauma, reducing the amount of alveolar surface area available for gaseous exchange, which results in increased carbon dioxide levels and decreased oxygen uptake. Continued exposure to high concentrations of oxygen results in heightened free radical production. This may damage the pulmonary epithelium, inactivate the surfactant, form intra-alveolar edema, interstitial thickening, fibrosis, and ultimately lead to pulmonary atelectasis.
Option A: Oxygen-derived free radicals have been proposed as being the probable etiological cause in the development of oxygen toxicity. Free radicals are generated due to the mitochondrial oxidoreductive processes and also induced by the function of enzymes such as xanthine/urate oxidase at extra-mitochondrial sites, from auto-oxidative reactions, and by phagocytes during the bacterial killing.
Option B: 100% oxygen can be tolerated at sea level for about 24-48 hours without any severe tissue damage. Lengthy exposures produce definite tissue injury. There is moderate carinal irritation on deep inspiration after 3-6 hours of exposure of 2 ATA, extreme carinal irritation with uncontrolled coughing after 10 hours, and finally, chest pain and dyspnea ensue.
Option C: Extended exposure to above-normal oxygen partial pressures, or shorter exposures to very high partial pressures, can cause oxidative damage to cell membranes leading to the collapse of the alveoli in the lungs. CNS toxicity is expedited by factors such as raised PCO2, stress, fatigue, and cold.
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