Barbituate overdose causes CNS depression that can lead to hypoventilation. In Hypoventilation due to CNS depression, there is hypoxemia with a normal A-a gradient because the alveoli themselves are not injured and rather there is simply less oxygen overall reaching the alveoli due (FA2020 p669). A normal A-a gradient is around 10 mmHg (estimated as age/4+4, FA2020 p668). CO2 will also be increased because of retention of more "stale air" in the alveoli and lower respiration which allows less expiration of CO2.
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Barbituate overdose causes CNS depression that can lead to hypoventilation. In Hypoventilation due to CNS depression, there is hypoxemia with a normal A-a gradient because the alveoli themselves are not injured and rather there is simply less oxygen overall reaching the alveoli due (FA2020 p669). A normal A-a gradient is around 10 mmHg (estimated as age/4+4, FA2020 p668). CO2 will also be increased because of retention of more "stale air" in the alveoli and lower respiration which allows less expiration of CO2.