ACE-CAA Issue 4 Sample Question

Which of the following is MOST likely to cause a pulse oximeter reading of 70% in a patient with a PaO₂ greater than 100 mm Hg?

A. Cyanide toxicity X

B. Carbon monoxide poisoning X

C. Methylene blue administration ✔

Read the discussion below.

Test your knowledge of anesthesia fundamentals with 135 questions just like this one.

Order ACE-CAA Issue 4

Discussion

Pulse oximetry is based on the differential absorption of red and near-infrared light of oxyhemoglobin and deoxyhemoglobin. The wavelengths of red and infrared light penetrate tissues, unlike yellow, blue, green, and far-infrared light, which are absorbed by water and nonvascular tissue. The amount of red and infrared light that is absorbed changes with the alterations in arterial blood volume during systole and diastole, allowing SpO₂ to be determined (Figure 1).

Figure 1. Illustration of the physical principle of pulse oximetry. Most of the tissue in the path of the light beam has a static pattern of absorption, but the volume of arterial blood (and therefore the absorption of light) varies in time with the pulse. © 2018 American Society of Anesthesiologists.

There are certain situations that can lead to unreliable pulse oximeter readings with standard pulse oximeters. Multiwavelength pulse oximeters can be more accurate in these situations. Methemoglobin absorbs red and near-infrared light nearly equally, and the SpO₂ trends toward 80% to 85% with standard pulse oximeters. Methylene blue and other pigmented dyes can alter SpO₂ when injected intravenously. The light absorption of methylene blue is very similar to deoxyhemoglobin and thus can lead to falsely low SpO₂ readings, with a mean nadir of 65% in 1 study (Figure 2).

Figure 2. Absorbance spectra of the 3 intravenously administered dyes. Note the dashed line at 660 nm that corresponds to 1 of the wavelengths emitted and sensed by the Nellcor O₂ saturation monitor. Modified with permission, from Scheller MS, Unger RJ, Kelner MJ. Effects of intravenously administered dyes on pulse oximetry readings. Anesthesiology. 1986;65(5):550-552. doi:10.1097/00000542-198611000-00023

Falsely low SpO₂ readings can also occur with movement, venous pulsations, inherited hemoglobinopathies, and severe anemia and hypoxemia. Some fingernail polishes, especially dark colors, can decrease SpO₂ by up to 10% in older models of pulse oximeters; this is less likely with newer models.

In carbon monoxide poisoning, carboxyhemoglobin absorbs red light similarly to oxyhemoglobin, with little near-infrared light absorption, leading to a falsely elevated SpO₂. Normal or falsely elevated SpO₂ can also occur with sickle cell vaso-occlusive crises. 

Cyanide toxicity is not likely to exhibit a spuriously low SpO₂.

REFERENCES

1. Gropper MA, Cohen NH, Eriksson LI, Fleisher LA, Leslie K, Wiener-Kronish JP, eds. Miller’s Anesthesia. 9th ed. Elsevier; 2020:1304.
2. Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med. 2013;107(6):789-799. doi:10.1016/j.rmed.2013.02.004

Order ACE-CAA Issue 4