Problem

Pulse oximeters are simple bedside devices that measure Hb saturation by way of a noninvasive probe taped to the patient’s finger or forehead. Although oximeters measure Hb saturation percentage, blood oxygenation still tends to be quantified according to arterial PO₂. Is there a simple way to relate these two measures without carrying around an oxyhemoglobin dissociation curve? 

Discussion 

First, although extremely useful, pulse oximeters are not very accurate (4%), and they measure only normal hemoglobin saturation. For this reason, their use should be limited to following trends or warning of significant changes in hemoglobin saturation with oxygen.

            Even so, RT’s will often need to estimate arterial PO₂ from oximeter readings. The following simple rule should be helpful. It is called the 40-50-60/70-80-90 rule. Assuming normal pH, PCO₂, and Hb values, saturations of 70%, 80%, and 90% are roughly equivalent to PO₂ values of 40, 50, and 60 mm Hg, respectively:

Thus a patient with a pulse oximeter reading of 90% has a PaO₂ of approximately 60 mm Hg. Should the saturation drop to 80%, the PaO₂ will fall to approximately 50 mm Hg. Note that this rule works only in the middle range of PO₂ values, where the curve is most linear; it should not be applied with saturations higher than 90%. For example, a saturation of 100% may represent a PaO₂ of 200 mm Hg. 

Copyright RT Corner 2008