Does O2sat lower in anemics?

allnurses Guide · Mar 18, 2009

O2 saturation only measures how well the oxygen is binding to ("saturating") the heme in the red cells that are there. O2 sat would be normal (assuming no other problems) in an anemic person, even though they may be hypoxic at the tissue level due to reduced O2 delivery to tissues from reduced red cells.

There is a recent thread about this very thing with some great posts: https://allnurses.com/general-nursing-discussion/o2-sat-100-a-375712.html

Jolie, BSN · Mar 18, 2009

my question is: does being anemic cause the o2sat to lower?
i know that o2sat is measuring the amount of oxygen in the blood
o2 sat doesn't measure the amount of oxygen in the blood. it measures the percentage of the hemoglobin that is saturated with oxygen, or in other words, the percentage of hemoglobin that is carrying its full capacity of oxygen. from the patient's oxygen saturation, we can sometimes make reasonable assumptions about the level of oxygen in the patient's blood. in order for an oxygen saturation monitor to provide useful information about the approximate amount of oxygen in a patients blood, the patient must have a normal hemoglobin count. pao2, the partial pressure of o2 in the arterial blood measures the amount of oxygen in the blood.
i had a patient who was severely iron deficiency anemic. she had ulcerative colitis and had been losing blood in the stool for 5-6 months before she was admitted. her nail beds and palms were white and she complained of swollen ankles, headaches upon walking up stairs, and blurred vision like a strobe light in the final days. her hct was 15 and hemoglobin was 4. however, her o2sat was normal.....

this is a common scenario. your patient is severely anemic, but the hemoglobin that remains in her blood is fully saturated. that means that her blood is carrying all of the oxygen that it can carry. it does not mean that it is carrying a sufficient amount of oxygen to meet the patient's demands. this patient's pao2 is likely well below normal.

please google "oxygen-hemoglobin dissociation curve" for further information.

AOx1 · Mar 18, 2009

PaO2 is the partial pressure of oxygen, as measured in the plasma of your arteries. It's how much oxygen is dissolved in there. It's mainly up to the structure of lungs and not dependent on Hgb (unless there's also a shunting process going on, like a big right to left shunt seen in some congenital heart disorders, and that's because poorly oxygenated blood is shunting to reduce the overall mix of O2). Your sats (or SaO2) are based on your PaO2 and other things like pH, temp, etc.

Each little hemoglobin has four places for oxygen to bind (they're called heme sites). The more hemoglobin you have, the more sites there are to bind. But SaO2 is a percentage of sites taken up by an O2 molecule. So for example, if my sats are 94%, that means that out of every hundred heme sites available for binding, I have 94 taken up by O2. The other ones are either empty or bound to something other than O2. The best example I can give is this- if you and a classmate take two different test, and you score a 4/5 and they score an 80/100, you have both made an 80% even though the number of test questions varied.

You WILL see a low oxygen content overall, though; it does include the Hgb. It's the amount of oxygen in arterial blood, both the O2 that is bound, and that that isn't. This is important to look at because, as I mention, PaO2 and SaO2 can be totally normal, even when there's hypoxemia. I hope this helps some.

cooper25 · Mar 18, 2009

thanks for the help. so if i understand correctly, the o2 sat is telling us how well hgb is doing its job - whether we have 1 drop of blood or 100% of our blood, the o2 sat is saying how well the hgb is carrying oxygen molecules.

in which case, giving this person oxygen would not help at all, since there arent any more "cars"[hgb] to drive the oxygen around. right? what they need - is BLOOD.

if thats correct, then is it only things that physicallly prevent oxygen from getting into our bodies [such as COPD, asthma attck or decreased RR from anesthesia for example], things that would lower the o2sat reading? or are there situations, besides CO poisoning, that would prevent hemoglobin from picking up oxygen?

thanks for posting that other thread as well, it was helpful.

AOx1 · Mar 18, 2009

It could also depend on anything that can impact PaO2, which could also be almost anything that could affect your alveoli (since they are the thing that adds tons of surface area) or structure of lungs, plus all that good stuff on the dissociation curve like temp and pH.

Of course, there are times we will give people O2 even with good sats- some things that come to mind are applying O2 to a mom whose fetus is experiencing fetal distress (it's actually helping the baby), r/o MI (to help cardiac muscle), sickle cell crisis (prevents further sickling), etc, etc.

allnurses Guide · Mar 19, 2009

if thats correct, then is it only things that physicallly prevent oxygen from getting into our bodies [such as COPD, asthma attck or decreased RR from anesthesia for example], things that would lower the o2sat reading? or are there situations, besides CO poisoning, that would prevent hemoglobin from picking up oxygen?

No - anything which affects O2 utilization can affect the O2 sat. This could be:

- O2 delivery issue: asthma, altitude, mechanical injury affecting inspiration (eg. broken ribs), anemia, other hematological problems

- O2 uptake issue, where there is a problem with O2 uptake across the alveolar-capillary membrane (ARDS, emphysema, infection, mucus etc)

- O2 binding: smoke inhalation, carbon monoxide inhalation

- Metabolic issues which affect O2-Hb binding: pH, temp etc

- Other: brain injury, anesthesia, pain affecting respiratory rate/depth

Probably more I am not thinking of right now