Pulse oximetry vs. arterial saturation

[Bruno Matos]

How can we explain the difference between the values of the pulse oximetry and the arterial saturation in the same child at the same moment? A lot of times we have a pulse oximetry of 85% with 98% on ABG.

Thanks for your help.

[NotReady4PrimeTime, RN]

You need to understand several things about pulse oximetry for this to make sense. There are some technical limitations to pulse oximetry that include the temperature of the site being monitored (cold = lower sats), the perfusion to that site (shut down = lower sats), how the probe is applied (good contact with the skin and well-approximated sensors = better waveforms and more accurate readings), ambient light interference (messes with the waveforms and causes false readings ... or no readings) and the patient's hemoglobin (probe measures percentage of hemoglobin that passes between the sensors that is fully saturated with O2 ... low hemoglobin can give false measurements as can high hemoglobin ... watch what happens when the patient is transfused!). When you get an ABG sat, the analyser has measured both hemoglobin and O2 content and calculated the sat. That's why we never rely on only one or the other, but look at both and follow trends.

[needdynurse]

Sats will give you only that=the amount of oxygen on the R.B.C

---------------------------------------------------------------------------

A.B.G's will give you a lot more critical info. eg. PaO2-PaCO2-NAHCO3-SAT's

Will tell if there is a compensatory factor between acid and base.

will show you Respiratory ALKALOSIS though rare. :)

[kjschulke]

Here's a link to an article that discusses limitations to oxymetry including abnormal hemoglobins such as carboxyhemoglobin and methemoglobin.

http://www.rcjournal.com/online_resources/cpgs/pulsecpg.html