# etCO2 not going to zero when patient stops breathing

**URL:** https://discourse.kitware.com/t/etco2-not-going-to-zero-when-patient-stops-breathing/598
**Category:** Pulse Physiology Engine
**Created:** [January 21, 2021, 7:00pm UTC](https://discourse.kitware.com/t/etco2-not-going-to-zero-when-patient-stops-breathing/598 "2021-01-21T19:00:32Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![mac](https://discourse.kitware.com/letter_avatar_proxy/v4/letter/m/cc9497/32.png) [@mac](https://discourse.kitware.com/u/mac)
#### Post date: [January 21, 2021, 7:00pm UTC](https://discourse.kitware.com/t/etco2-not-going-to-zero-when-patient-stops-breathing/598/1 "2021-01-21T19:00:32Z")

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Hi all,

This one is pretty straightforward and the title speaks for itself. There are certain medications that cause respiration rate to go to zero and despite this working properly, etCO2 will remain at 35. I don’t know much of anything at all in regards to physiology but the physicians I’ve been consulting with have led me to believe that etCO2 should go to zero if the patient isn’t breathing.

I’m working with Pulse in Unity 2019.3.14f1.

Thanks!  
Mac

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### Author: ![coolwebb](https://discourse.kitware.com/user_avatar/discourse.kitware.com/coolwebb/32/25_2.png) [@coolwebb](https://discourse.kitware.com/u/coolwebb)
#### Post date: [January 21, 2021, 7:16pm UTC](https://discourse.kitware.com/t/etco2-not-going-to-zero-when-patient-stops-breathing/598/2 "2021-01-21T19:16:27Z")

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etCO2 is really the value determined at the end of the previous breath. It is overwritten and remains constant until the end of the next breath cycle. Therefore, if the patient isn’t breathing, the value isn’t updated and will just remain the value of the last breath. If there is no breath, there is no end to it, and therefore can’t have any specific values associated with it (sounds kind of philosophical 🙂). Real-world sensors may read zero when respiration rate also goes to zero, but there’s no way all CO2 magically disappears.

Logic could be added to just set the etCO2 display to 0 when RR is 0, but I wouldn’t want to implement it that way in the engine itself.

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### Author: ![mac](https://discourse.kitware.com/letter_avatar_proxy/v4/letter/m/cc9497/32.png) [@mac](https://discourse.kitware.com/u/mac)
#### Post date: [January 21, 2021, 7:19pm UTC](https://discourse.kitware.com/t/etco2-not-going-to-zero-when-patient-stops-breathing/598/3 "2021-01-21T19:19:18Z")

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Ok that makes perfect sense! Thank you for the quick and comprehensive explanation!