Pulse Oximetry Saturation Values Differ Between Finger, Ear

July 17, 2018

Bob Kronemyer, Anesthesiology News, June 12, 2018 —A study has found that 8.7% of paired pulse oximetry readings varied by 5% or more, when probes were placed on a finger and an earlobe during the administration of general anesthesia or monitored anesthesia care. In addition, 4.6% of paired readings differed by 7% or more.

“Anesthesiologists need to know that differences as large as 7% or more are not uncommon,” said Gabriela Samayoa, MD, the primary author and a research assistant at Montefiore Medical Center, in Bronx, N.Y. “This finding is important because placement of a pulse oximetry probe in a second location may confirm a higher saturation.” The study was presented at the 2017 PostGraduate Assembly in Anesthesiology (abstract P-9011).

In the absence of a second higher reading, “the anesthesiologist might lighten the plane of anesthesia or instrument the airway,” Dr. Samayoa said. “Unnecessarily lightening the anesthetic might result in patient movement that interferes with performing the procedure, patient recall or pain. Unwarranted manipulation of the airway can also result in bleeding, laryngospasm or even unnecessary intubation.”

The study was conducted in 50 adult patients and 566 paired measurements were recorded.

“Anesthesiologists should appreciate that placing a second oximeter probe in a different location might confirm adequate saturation and prevent changes in management that may not only be unnecessary, but may actually cause harm to the patient,” Dr. Samayoa said.

“Anesthesiologists should appreciate that placing a second oximeter probe in a different location might confirm adequate saturation and prevent changes in management that may not only be unnecessary, but may actually cause harm to the patient"

Gabriela Samayoa

Dr. Samayoa noted that differences in pulse oximetry readings are often attributed to patient movement, hypothermia, decreased regional perfusion or vasoconstriction.

Sheldon Goldstein, MD, a co-author and an associate professor of anesthesiology at Montefiore, said if one probe reads 92% and the other reads 97%, “generally, anesthesiologists will presume the higher reading is correct and indicates that the patient is safe. This is not clinically concerning, though, because a saturation of 92% is unlikely to harm a patient. However, if saturation reads 87% in one location, an anesthesiologist might modify the anesthetic, whereas placement of a second probe and confirmation of a saturation of 92% or greater would indicate the patient is not in danger. Therefore, placement of a second probe could prevent unnecessary changes in anesthetic management.”

Dr. Goldstein emphasized the importance of clinical judgment. “Placing a second pulse oximeter probe is acceptable when saturation is somewhat decreased but unchanging. However, if oxygen saturation drops rapidly, therapeutic maneuvers may be the appropriate first step, with application of a second probe occurring afterward.”

He also pointed out that because neither ear nor finger measurements were consistently higher in the study, “the question remains whether each of the probes was accurately measuring saturation at the two locations, due to differences in regional blood flow.”

The authors are considering investigating pulse oximetry readings during carotid endarterectomy, where the surgeon could take a blood gas sample from the carotid artery. “We would compare the reading on the earlobe to the blood gas level, while simultaneously obtaining a blood gas from a radial artery catheter to compare with the pulse oximetry reading from a finger probe,” Dr. Goldstein said.

He said near infrared spectroscopy (NIRS) has shown that changes in regional blood flow may be earlier signs of shock than lactate levels. “Hence, we would be curious when patients have differences in pulse oximetry readings, if this might be a marker for inadequate blood flow or oxygen delivery to the tissues. If this turns out to be the case, perhaps we should be placing two pulse oximeter probes on ill patients and try to optimize hemodynamics until all readings are at or near the higher level.”

The presenters said comparison with NIRS still would need to be performed to determine if differences in pulse oximetry saturation readings do, in fact, correlate with changes in regional blood flow.

Drs. Samayoa and Goldstein reported no relevant financial disclosures.

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