from: https://www.quora.com/How-long-can-a-pat...saturation
By Gary Larson, MD for 35 years - ER and Radiation Oncology - academic and private practice
Updated May 31
"One warning - incorrect administration of supplemental oxygen to patients with severe COPD can be fatal (I don’t mean gradually, I mean dying right then and there). The other answers given are very good, but unless I missed it somewhere, no one mentioned the danger of increasing the rate of supplemental oxygen beyond a safe level.
Patients with COPD live with a CO2 level in their blood much higher than the average person. Normal values for arterial blood gas are given in the table below. (Note - O2 and CO2 levels given are the “partial pressures” of these gases in the blood - not the same thing as the “percent saturation of hemoglobin” - the measure that is used in the question).
Those of us without COPD live with a CO2 level in our blood very close to 40. This is the main thing that drives our respiration. If we’re not breathing deeply or rapidly enough, CO2 builds up in our blood and this stimulates the respiratory center in our medulla to increase our respiratory rate/depth. As long as we’re breathing well enough to keep our CO2 at the right level, we also put enough oxygen in our blood to saturate our hemoglobin to nearly 100% (and keep the partial pressure of oxygen in our blood at around 80 - 100).
People with severe COPD, don’t get rid of CO2 as well as the rest of us, and may run around with (or more likely sit around with) a CO2 of 60 - 100. Their kidneys adapt to the high CO2 levels and maintain their acid/base balance. This happens over a long period of time - and - gradually, their respiratory center gets used to their blood containing high levels of CO2. (Having high levels of CO2 in the blood is called hypercapnia.) Since their respiratory center stops paying attention to their CO2 level, they lose the “hypercapnic drive” to breath. At this point, they depend on their “hypoxic drive” to breath. When their oxygen level decreases their respiratory center causes their respiratory rate/depth to increase.
By this point, most people are oxygen dependent, receiving two or three liters per minute by nasal prongs. They get enough oxygen (their O2 sats are at least in the mid to high eighties) and everything is OK. If, however, their oxygen is turned up from say two liters per minute to five, they lose their hypoxic drive (their respiratory center doesn’t sense the need for more oxygen, so their respiratory rate slows). As respiration slows, their CO2 goes up - sometimes to a very high level, their kidneys can’t compensate acutely, and they develop respiratory acidosis and die.
Certainly people with COPD may need higher levels of O2 from time to time, but this should only be done in a closely monitored setting.
So the take home message is - don’t turn grandma’s O2 up high to make her more comfortable while you go to wash the dishes, or you might come back to find her dead."
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