First make sure the PH is in normal range. If the Ph is normal, then there's nothing major to really do (except some fine tuning). It's compensated. When starting out, nurses have an obsession with oxygen levels. As time goes on, you will see that the CO2 is more important. If a pt has a normal co2 level, but a reduced O2 level, they get a nasal cannula. If the o2 is normal to low, but the co2 is high, the pt just bought a Bipap or vent. The body is much more sensitive to co2 as a driving force to breath. When you hold your breath, the crave to breath is from your CO2 level rising, not the O2 dropping. If you hyperventilate, you will get dizzy...not because anything is happening to the O2, but because your Co2 significantly drops. So, make sure the co2 is properly corrected. Kind in mind that not everyone needs a CO2 of 34-45. COPDers can live with a CO2 above 60 and be fully compensated.
Look at the bicarb to see if they are respiratory or metabolic. Chronic CO2 retainers should have higher bicarb levels to buffer the acidic CO2. That's why they are compensated. fyi...If a COPDer goes into renal failure, their ph will bottom out. That's b/c your kidneys produce bicarbonate. Without that, that's not much else to help buffer....except tachypnea or a bicarb gtt.
So, to answer your question on O2. The oxygen saturation is actually pretty accurate. There are times when the O2 sat can be false, but that is from other things going on, like carbon monoxide poisoning (house fire inhalations or car emissions) and other conditions. The O2 sat won't really be saturated with different stuff unless you already know about it (usually). You can go by the O2 sat most of the time. If a pt has a 100% sat, but their PO2 is very low...something else is going on...and it's an emergency. When you are titrating Fi02 on a vent, first note the PO2 level on the blood gas. Many resources say 75-100 for the PO2, but you can be above 60 or 70 and be ok. If the PO2 level is like 150 and you are on 100% Fio2, you can knock it down to 60% (maybe 70% if you want to be cautious). Let them ride for a little bit (1-3 hrs). Then, go by increment of 10-15 %. Watch their o2. Go with each change for a couple hrs. You don't have to have the o2 sat be 100%. 94% is fine. For bad COPDers, you can keep them above 90% (make sure you have a doc's order for the O2 sat range). And, it's ok to even titrate over a couple of days if the lungs are bad. As you start titrating below 45%, go by increments of 5-10%. 40 to 35 to 30. A titration from 90% to 80% isn't a big deal b/c they are already in the plus, but a titration of 40 to 30 can be a big deal b/c you are fine tuning on the lower end of the spectrum. Make sure their o2 sat is staying within range. Some pt's just can't tolerate being below 40%. That's ok. Maybe they just need some time.
Don't feel bad if you need to go back up on the FiO2. It's not a race. Many times, you have to go up and down, up and down. It's not your fault if you can't get them to the good 'ol 30% by the end of your shift.
The other thing is to look at the pt. If their resp rate goes from 16 to 31, then they are not getting what they need. See if they are air hungry. If you see forced inhalation, nasal flaring, and the head bobbing back with each breath (along with the mouth opening wide with each breath) then the pt is air hungry. If all is quiet, then you have a good idea of their respiratory status. I have seen pts with 93% O2 sat, and relatively normal blood gas, but they look like a fish out of water. The vent needs to be adjusted.
Remember your side of the clinical aspect. Is the pt anemic? Do they need blood? Does the PEEP need to be increased b/c of atelectasis? Do they have pneumonia or pulmonary edema? Do they need lasix? Are you mobilizing the pt as much as possible. Could they have a mucus plug or do they need not a bronchoscopy? Is the albuterol treatment cutting it or do they need atrovent and mucomyst added? Are they just way to snowed with sedatives? Maybe they need extensive pulmonary toileting. Is there a lung injury or do you suspect ARDS? Hope this helps