Reason for False High Sp02 Readings

[AnthonyM83]

This has come up in oximetry discussions before, but for clarity's sake I want to specifically ask:

Other than CO poisoning and hypovolemia, what would provide a false high spO2 reading?

We commonly remind people not to rely on the reading and to provide O2 if you see other signs of hypoxia or that you might get a false low (ex: hypothermia), but not much on HIGH READINGS. I know you can get a highlighter pen to get a sat in the 80s...but if your patient were lower than the 80s 1) You be seeing other obvious signs of hypoxia and 2)With a sat in the 80's, they'd be getting O2 anyways.

So, what I'm looking for are cases were a patient would have a sat in the 90s range high enough to to receive O2, but in reality be hypoxic.

[Scaramedic]

Anemia off the top of my head, low hemoglobin but SpO2 reads RBC's not hemoglobin.

Peace,

Marty

[MeekoBB]

my class recently had a brief lecture on caponography....wouldn't that help in decisions too?

[AnthonyM83]

Anemia off the top of my head, low hemoglobin but SpO2 reads RBC's not hemoglobin.

If it reads RBCs, not Hg, how does it know when you're low on O2? Hypoxia doesn't mean low RBC count... I think the light wavelength/intensity change is due to O2-bound Hg. I'm honestly not sure.

So, no other takers to original post?

The only things I can think of are problems with air exchange at the cellular level...getting O2 from blood vessel to the tissues, thus cells are still hypoxic but blood O2 sat is good...but not sure what would cause that...

So, is EMTs blindly following the O2 sat THAT big of a worry if it's really just anemia, hypovolemia, and CO poisoning...all of which can semi-confidently be ruled out...your opinions?

[VentMedic]

Hope this is helpful.

http://www.oximetry.org/pulseox/principles.htm

Pulse Oximetry Revisited: "But His O2 Sat Was Normal!"

http://www.medscape.com/viewarticle/550665_print

Factors affecting:

Skin color

Perfusion

Anemia

Finger thickness

Nail polish - especially the metalic flecks

External lights

Movement

Skin pigmentation

Carboxyhemoglobin (CO both extrinsic and intrinsic)

Methemoglobin (can be caused by the 'caines - topical analgesics for endoscopy and bronoscopy, nitrates, nitric oxide)

Intravascular or intradermal dyes

Movement

Irregular heart rate

Sickle Cell

For respiratory distress the value SpO2 100% will be not mean much because if you are delivering O2 at 100%, the ABG value will be of most use to see how much O2 is actually in the arteries vs what is delivered. We call this an A-a gradient - Alveolar to arterial gradient.

The hemoglobin may be well saturated but the lungs (Alveoli) were only able to pass X amount to the blood which is measured in mmHg (pressure tension) in the blood.

Good site

PaO2, SaO2 and Oxygen Content

http://www.lakesidepress.com/pulmonary/ABG/PO2.htm

[JPINFV]

I'll bite for an obscure biochem reason. Low levels of 2,3bisphosphoglycerate could cause a "false high" reading by shifting the oxygen binding curve to the left and making it harder for oxyhemoglobin to release oxygen.*

[*note: 1. This is an educated guess based off of theory only.

2. I imagine that, based off of the effect, that this would be fatal in most cases where it would matter, hence becoming essentially a non-issue].

[VentMedic]

2,3-DPG

Oxyhemoglobin Dissociation Curve

http://www.ventworld.com/resources/oxydisso/dissoc.html

[captainstandup]

Add cold extremities and associated vasoconstriction to the list although an earlier post mentioned "things affecting skin perfusion". If the patient is intubated then its rather moot since the gold standard is continuous EtCO2 if its available.

[JPINFV]

Link to a lecture slide showing curves for no DPG, normal levels, and high altitude levels.

[VentMedic]

How do cold extremities and vasoconstriction lead to a false high reading?

Decreased perfusion, less blood flow, inaccurate count of Hb

The Pulse Ox will only give you a percentage of the saturated vs unsaturated Hb it sees. This may not be an accurate representation of the total. It just calls it as it sees it.

If the body temperature is low, hypothermia causes a leftward shift in the Oxyhemoglobin curve. A leftward shift increases the affinity, making the oxygen easier for the hemoglobin to pick up but harder to release.