We’ve all experienced it. There’s an intubated patient who is stable, saturating >98% on pulse ox, and then all of a sudden you hear the monitor start alarming and the SpO2 is plummeting. Lets assume that all other vitals are unchanged and no vent alarms are going off. Nothing on exam has changed except for that pulse ox number. Then 30 seconds later, the pulse ox goes back up to 99%. 5 minutes later it happens again…five minutes later, again…and again. What is going on?
Troubleshooting the pulse os becomes much easier if you understand how it works. The pulse oximeter determines the SpO2 by accomplishing two things: 1) picking up on the arterial pulse and 2) measuring the absorption spectrum at 2 wavelengths corresponding to reduced and oxidated forms of hemoglobin at the arterial pulse. Both of these need to happen in order to get an accurate SpO2 reading.
The machine then calculates the ratio which correlates to a SpO2 reading. This correlation was discovered with trials of healthy patients who plotted on a ratio vs ABG O2 Sat curve. The range of ABG O2 Sats were only within saturations compatible with life since medical equipment companies considered it bad form to code volunteers when developing products. Basically, the pulse ox has a pre-programmed table that has an SpO2 value for each ratio based on healthy patients within an SpO2 that was compatible with life. Outside of the range is purely extrapolated. These means low SpO2 on pulse ox are not accurate. This also means if your patient has less than ideal circulation or a dyshemoglobinemia, it may no longer correspond to the actual SpO2 of the patient.
In the example above, the patient has a BP cuff on the same arm as the pulse ox. Every time the monitor cycles the BP, the pulse ox losses the arterial pulse as seen on the SpO2 tracing, and the SpO2 reading drops. This is easily solved by moving either the cuff or the pulse ox to different extremities.
So…things to keep in mind when placing a placing a pulse ox:
1) There needs to be a good wave form on the pulse ox monitor.
2) The emitter diode and receiver of the pulse ox needs to be directly opposing each other on the digit. Only certain pulse ox attachments can be placed on the forehead where the emitter is not opposing the receiver.
3) There shouldn’t be anything between the emitter diode and the receiver of the pulse oximeter besides the finger (or toe or earlobe). Most common example is nail polish.
The following table shows some other sources of error. Source: Mardirossian G, Schneider RE. Limitations of pulse oximetry. Anesth Prog. 1992;39(6):194-6.