One of those things that probably should have been obvious but nobody ever thought of. The brain stem constantly keeps check on the CO2 concentration in the blood and adjusts bodily function accordingly. When levels are low, a situation known as hypocapnia, the individual begins to hyperventilate, as well as undergo vasoconstriction:
But pure oxygen causes rapid breathing, meaning that as it is pumped into the lungs, more carbon dioxide is exhaled, "and that makes the blood vessels much smaller," says Ronald Harper, a neurobiology professor at the University of California, Los Angeles, and senior author of the study published today in PLoS Medicine. The shrunken vessels "cannot deliver as much blood—or the oxygen that's in the blood—to the brain" or the heart.This can cause a cascade of physiological problems. On the plus side, this can be solved by switching standard emergency care from 100% O2 to 95% O2/5% CO2. One caveat though; this study was done only in healthy children. Though I wouldn't expect a difference, it is possible that adult physiology is better capable of handling such a situation. It would be nice to see this same study in adults and some case studies from hospitals with compromised, rather than healthy, individuals.
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