I thought some of the lurkers might find this interesting.
An article on partial pressure of carbon dioxide:
https://www.ncbi.nlm.nih.gov/books/NBK551648/
An article on oxygen saturation:
https://www.ncbi.nlm.nih.gov/books/NBK525974/
A couple of random things about CO2 and O2:
-PO2 is the concentration of O2 in the blood (as PCO2 is a measure of CO2 in the blood) and it can be measured in a few ways. One is by taking an arterial blood sample and running blood gas measurements on it right there (P
aO2, a for arterial). This is done a lot in critical care settings (e.g., an ICU) or in an emergency room.
-SpO2 can be low for a few reasons (not an exhaustive list)
You are not getting enough oxygen
There is something wrong with the SpO2 probe or the placement of the probe--the latter happens A LOT
You are dead
-SpO2 is a surrogate measure of the concentration of O2 in your blood and is used a lot because it is noninvasive--how many people want to try to sleep all night with an arterial line in? Also, ouch.
-CO2 can be measured by arterial blood sample as well. If you do that it's called PaCO2.
-ETCO2 is a measure of CO2 in expired air (the air you breath out). It's a really neat tool because it changes quickly (more quickly than SpO2, imo) during anesthesia and critical care so it lets you know how your patient is doing right now.
-ETCO2 can be low for a few reasons
You are breathing too much (hyperventilation)
There is something wrong with your ETCO2 probe--I don't think this is that common; certainly doesn't happen that often in the OR
You are dead--if you are not alive you do not produce CO2--this is bad.
ETCO2 can be high for a few reasons
You are not breathing enough (hypoventilation)--this is what could happen with central apneas
There isn't enough O2 in the air you are breathing
There is a restriction or blockage in your airway (still called hypoventilation, but we're calling it obstructive apneas and obstructive hypopneas)
You cannot have a high ETCO2 and be dead--you might be about to die but you don't produce CO2 when your body has ceased to perform aerobic respiration
We tend to be more interested in O2 because it keeps our organs, including the squishy one between our ears, functioning, but too much or too little CO2 is a problem as well. For one thing too much CO2 can contribute to metabolic acidosis (here's an article that'll explain it better than I will:
https://www.ncbi.nlm.nih.gov/books/NBK482146/)
Slightly obscure example: surgeons who operate on the brain will often hyperventilate their patients, say, targeting an ETCO2 of 15 mmHg or maybe lower (I usually use 36-44 as a normal for anesthesia but lots of others use 35-45). When you overoxygenate the blood flow to brain slows down and actually shrinks the brain a little (less blood in the brain = smaller brain). I think they do this to control the bleeding a little. It relates to the the chemoreceptors in the body and how they regulate blood flow and respiration (example here on the carotid body:
https://medicine.uiowa.edu/iowaprotocol ... tic%20tone).
There is obviously an interplay between CO2 and O2. By definition if your O2 is low your CO2 is going to be high...unless you're dead. If your O2 and CO2 are both low and you are alive I would check the probes and/or retake measurements.- If your O2 is high (and a working SpO2 probe would not, I would guess, falsely read high) then your CO2 is probably low normal. You need both CO2 and O2 in your bloodstream to breath normally, but you are also constantly producing CO2 from O2 in aerobic respiration in your cells (there is some debate about this latter point if you want to get picky, but only read this if you want to understand the finer points...or confuse yourself--
https://pubs.asahq.org/anesthesiology/a ... Dioxide-or).
Please let me know if I made any errors
