Pulse oximetry reading conundrum

[greatgateway523]

Hello. I've always been under the impression that before using a finger pulse oximeter, the patient's hand must be warmed. Too cold digits result in lower spo2.

I have recently had a conundrum with the exact opposite scenario! The warmer the patient's fingers became, the lower his spo2 dropped. When they were cooler, sats stayed stuck at 97-98%, but fluctuated around 91 - 94% with heat.

Another thing I noticed is that when the patient took a deep breath for any reason (yawning from air hunger feeling as an example) sats would climb to around 97% but then rapidly plummet a few seconds later to as low as 87%.

I bet the answer is more simple than I think, so I'm sorry if this is a silly question, but what would cause such things? They seem to both defy logic, yet the reader is properly calibrated and accurate compared to others..!

[Coffee Nurse, BSN, RN]

I'm not positive on this, but I'd imagine that the warmer a patient's hand is, the more *accurate* the SpO2 would be (whether that means higher or lower sats), since you're getting better perfusion and the sensor has more blood flow to "read."

The increase with yawning would seem to make sense. If the patient is air hungry, then surely the point of the yawn is to get additional oxygen into the body.

[malamud69, BSN, RN]

All human beings are different...TREND all vitals...

[Kitiger, RN]

All human beings are different...TREND all vitals...

SpO2 is never an absolute number - you need SAO2 for that. Always watch the trend with SpO2.

The purpose of a yawn is to decrease CO2. Of course, it also increases oxygenation briefly. If the deep breath also moves any mucus that was blocking an airway, then oxygenation can stay up.

The pulse ox probe will warm the finger, especially the wrap-around kind. That's why you rotate the site when you leave a probe on; it can burn the finger, given time,

[KatieMI, BSN, MSN, RN]

Yawn is, essentially, a very deep breath. So, yeah, it increases oxygen saturation for a few seconds.

What is described by OP is named "too warm a finger". Barring subtle deterioration, the most probable reason was overheat and local decrease affinity of Hb to O2 and the right shift of the curve of Hb/O2 dissociation. Patients with underlying acidosis, especially chronic and compensated, are prone to this.

I see decrease of saturation with hands still warm all the time in LTACH. In fact, it is one of my favorite hallmarks of "subtle deterioration" first signs. SpO2 measures % of Hb molecules fully (at all 4 points) "saturated" with O2, or those which bind 4 O2 and so have all 4 active centers occupied. All sensors are calibrated for Hb total content of around 10 mg/dl. Therefore, decrease in SpO2 can mean, most commonly:

- not enough O2

- not enough Hb

- something which doesn't let oxygen and hemoglobin to connect (metabolic acidosis, fever, etc - google "hemoglobin dissociation curve") - very common with warm hands, fever, starting sepsis, etc).

[greatgateway523]

Thanks!

Sorry to sound dumb about this, but do you mean a desaturation occurs with warm hands when the patient already has an underlying factor like acidiosis, or is it likely in a too hot, but otherwise healthy patient without acidiosis too?

[KatieMI, BSN, MSN, RN]

Not likely with completely healthy person (and whoever is experiencing oxygen hunger is far from "healthy" at that particular moment).

The process is not DESATURATION. Desaturation is when O2 leaves Hb molecule, as it supposed to. We're thinking about either primarily low saturation, or loss of oxygen transport's properties.

In any case, do not treat the number, treat your patient.

Think about it as train bringing coal to a factory. Then google "oxygen saturation curve", find source you like and read it. It is quite a mind-binding subject but a fascinating one.