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Pre-Op Screen: What Does A Pulse Oximeter Tell Us?

Routinely utilized in ICUs, operating rooms, and telemetry floors, the pulse oximeter (“pulse ox“) is perhaps the single greatest monitoring advancement in the modern medical era. It relies on the fact that oxyhemoglobin and deoxyhemoglobin absorb light at two different peak wavelengths. Without getting into too much detail, an algorithm is able to compare these ratios, exclude noise, and output an O2 saturation (SpO2).

What I want to clarify with this post is a misconception I’ve occasionally heard when interpreting the SpO2. A pulse oximeter does not measure the amount of oxygen in the blood. It only measures the degree to which a person’s hemoglobin is saturated.

CaO2 = (1.34 * [Hb] * SpO2) + (0.003 * PaO2)

CaO2 is the total arterial oxygen content, [Hb] is the hemoglobin concentration (from an arterial blood gas), SpO2 is the O2 saturation (from a pulse ox, expressed as a decimal), and PaO2 is the partial pressure of arterial oxygen (from an arterial blood gas). Clearly the SpO2 does contribute to the CaO2, but it certainly isn’t the whole story.

Your patient’s pulse ox can still read 100% even if their hemoglobin is 3 g/dL (critically low). In this case, the little hemoglobin they have is all saturated, but based on the CaO2 equation, their overall O2 content is extremely low. Take home message: SpO2 must be interpreted with the [Hb]!

If I know a patient is severely anemic (ie, hemorrhaging from penetrating trauma), they rely more on the undissolved oxygen (PaO2) to supply their overall content (CaO2). I’ll oxygenate these patients on close to 100% oxygen to maximize their PaO2 and ultimately CaO2 by the equation above until I catch up with resuscitation.

As a side note, if I could only have one monitor connected to my patient, it would be a pulse oximeter. With this noninvasive tool, we can ascertain the oxygen saturation of hemoglobin, the presence of distal perfusion due to a waveform, the heart rate, and even a rough idea of the heart rhythm. Pretty cool! 🙂

Pre-Op Screen” is an educational series reviewing basic topics encountered in anesthesiology. Feel free to contact me with topic suggestions.