Breathing 100% Oxygen, why not hypoventilate? - USMLE Forums
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Old 06-19-2011
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Lungs Breathing 100% Oxygen, why not hypoventilate?

Can someone explain this question?

If a normal patient breaths from room air to 100% O2. Why the ventilation remains normal, instead of hypoventilation?

My question is: Given 100% O2, will not this decrease the percentage of PCO2? Thereby the medulla gives the order to hypoventilate?
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Old 06-19-2011
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The partial pressure of CO2 in atmospheric air is virtually negligible. 100% O2 is displacing nitrogen, not CO2
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Old 06-19-2011
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Does 100% O2 displace CO2 in our body? If the PaCO2 is 40mmHg, and now we give 100%O2, this will make PaCO2 lower? And lower PaCO2 stimulate medulla chemoreceptor to make our body hypoventilate in order to accumulate more CO2 in our body?
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Old 06-20-2011
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The main determinant of if this will likely cause hypoventilation is the reason for administration. If a patient is being given 100% oxygen, it is likely that they are acutely hypoxic, in which case, the patient would likely be hyperventilating anyways due to the increased pCO2. So any hypoventilation in this case would likely just bring them back to a "normal" breathing rate.

If its being administered to a chronically hypoxic patient (which it shouldn't) like someone who has COPD. They have a fairly normal breathing rate because their peripheral receptors have adjusted to the LOW levels of O2 in their blood. These patients' breathing is mostly controlled by their peripheral O2 sensors. This is because the H+ concentration change caused by the increased PCO2 has been adjusted by kidney compensation, therefore the central receptors are "blinded" to the increased PCO2 after compensation. So administering 100% O2 right away to someone who has been chronically and severely hypoxic for a long time can severely blunt their respiratory drive. The levels of oxygen must be carefully controlled and the breathing rate must be monitored.
If the details aren't exactly right, someone feel free to correct me.

Last edited by spaidak2; 06-20-2011 at 09:19 AM.
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chienpolska (06-20-2011)
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Old 01-07-2013
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Quote:
Originally Posted by chienpolska View Post
Can someone explain this question?

If a normal patient breaths from room air to 100% O2. Why the ventilation remains normal, instead of hypoventilation?

My question is: Given 100% O2, will not this decrease the percentage of PCO2? Thereby the medulla gives the order to hypoventilate?

No! Two pathway regulate VA,CNS(medulla) and Peripheral receptors, CNS monitor PaCO2 and [H], normally in charge, they can adapt. Peripheral monitors PaO2 and little bit of PaCO2. Peripheral only take over when PaO2 falls dramatically <80mmHg or fO2 < 17%, they does not adapt.

When given 100% oxygen, PaCO2= metablism/Ventilation, so PaCO2 is not affected. You may think PaO2 raise, but it only take over when PaO2 drop. So
ventilation not change。
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