End Tidal CO2/ Capnography

[applehead]

Hey

I have a basic question that I've never been able to get my head around.

I know that in hyperventilation there is a decrease in ETCO2, however the physiology behind it gets me really confused.

So in hyperventilation I get that the arterial paC02 decreases as the is increased removal of CO2 as the CO2 is blown off and a resp alkalosis may develop. Surely therefore if more CO2 is being blown off the ETCO2 should be higher (as there will be more CO2 in the expired air?)

Again I know in hypoventilation the ETCO2 rises. Again my brain tells me that with hypoventilation leads to decreased gas exchange and pCO2 rises in the systemic circulation (hence acidosis on an ABG) - if there is decreased gas exchange - why will the end tidal CO2 increase?


I know I am missing something basic but would love some clarity!

Cheers
Adam

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[Gither]

partial pressure is component of xyz gas in ratio to summation of all gases. If it builds up systemically , expired gas will demonstrate the change in ratio of co2 to all other gases.


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[tux4]

This is my take on it, correct me if I am wrong! Etco2 decreases when you have inadequate cardiac output(perfusion), inadequate ventilation, or inadequate gas exchange. Hyperventilation is affecting ventilation. People tend to have a narrower TV during hyperventilation which will reflect a smaller volume of exhaled gas on capnography, thus smaller waves.


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[alpinism]

partial pressure is component of xyz gas in ratio to summation of all gases. If it builds up systemically , expired gas will demonstrate the change in ratio of co2 to all other gases.


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Exactly.

Remember capnography measures the partial psi of CO2 not the amount of Co2 in exhaled gases.

Essentially when you start to hyperventilate you breath off more CO2 because of the increase in breathing rate but the partial psi stays the same.

Then with continued hyperventilation more CO2 is removed from the body than is produced and this causes the concentration of CO2 in the blood to decrease.

This then results in a decrease in partial psi of CO2 in exhaled gases.

 

[Arch Guillotti]

Closing. Duplicate thread in anesthesiology forum.