End Tidal C02/ 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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[AdmiralChz]

Surely therefore if more CO2 is being blown off the ETCO2 should be higher

I see where you are getting confused and it's right here. With more ventilation (typically increased RR/Vt) you are blowing off more CO2 but what the end tidal is monitoring is the CO2 in the alveoli. Since you are blowing off more, there will be less there so your ETCO2 will decrease. ETCO2 is a reasonable surrogate for PaCO2 (with a gradient due to dead space ventilation) in healthy, normally ventilated patients. They always change together.

 

[Mman]

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

End tidal CO2 concentration is simply the concentration of CO2 at the end of each breath. It is not the total volume of CO2 being exhaled per minute as that depends on the respiratory rate and tidal volume.

When you hyperventilate, you exhale a larger volume of CO2 per minute which lowers the concentration coming out in each successive breath so ETCO2 goes down. When you hypoventilate, you exhale less CO2 per minute so there is a higher concentration left in the blood/alveoli and so the ETCO2 for each breath will be higher.

Just think of ETCO2 as directly correlated to the concentration of CO2 in the blood. Hyperventilation lowers the concentration so ETCO2 goes down. Hypoventilation raises the concentration so ETCO2 goes up.

 

[medking]

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

I think the confusion is in the terms. Instead of saying "MORE CO2 is being blown off", think of it as CO2 is being blown off at a FASTER RATE.

If CO2 is being blown off at a faster rate, then PaCO2 should drop, which would be reflected as a drop in ETCO2 (in an otherwise healthy patient). Because the rate is constant, the ETCO2 can only drop with hyperventilation (not increase).

A picture you can paint in your head, would be something like a truck carrying x amount of sand at a constant rate. With hyperventilation, that same truck with the same amount of sand just comes by faster. So the amount of sand you see doesn't change until the amount of total sand decreases leading to subsequent decreases in the sand you see until a new equilibrium is reached.

Hope this helps.

 

[Noyac]

You also need to read up on the diffusion of CO2.
CO2 has a greater coefficient.