The Pulse Oximetry Gap in Carbon Monoxide Intoxication☆,☆☆,★,★★

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Abstract

See related editorial, "Should the Pressure Be Off or On in the Use of Oxygen in the Treatment of Carbon Monoxide-Poisoned Patients?"

Study objective: Pulse oximetry has been reported to be falsely elevated in the presence of carbon monoxide (CO). However, the degree to which pulse oximetry overestimates measured oxyhemoglobin saturation (O2 Hb) has not been investigated in patients with CO exposure. This study quantifies the effect of CO on pulse oximetry and O2 Hb in a series of patients with elevated carboxyhemoglobin (COHb) levels.

Methods: A prospective case series of 25 pulse oximetry measurements, with concurrent arterial blood gas sampling, were obtained on 16 adults with CO exposure. Results: COHb levels (mean, 16.1%; SD, 11.6%; range, 2.2% to 44%) did not significantly correlate with pulse oximetry saturation (mean, 97.7%; SD, 1.5%; range 96% to 100%) (r =.45; P =.1 [NS]). Compared with COHb, a pulse oximetry gap (mean, 17.5%; SD, 1.5%; range, 2.3% to 42%), defined as pulse oximetry saturation minus O2 Hb, yielded a linear regression model: pulse oximetry gap=1.82+0.94×COHb (SEM=0.07; F=204; R 2 =.90; P<.0001). Conclusion: Oxygen saturation as measured by pulse oximetry failed to decrease to less than 96% despite COHb levels as high as 44%. Regression between the pulse oximetry gap and COHb suggests that pulse oximetry overestimates O2 Hb by the amount of COHb present. Pulse oximetry is unreliable in estimating O2 Hb saturation in CO-exposed patients and should be interpreted with caution when used to estimate oxygen saturation in smokers. [Buckley RG, Aks SE, Eshom JL, Rhydman R, Schaider J, Shayne P: The pulse oximetry gap in carbon monoxide intoxication. Ann Emerg Med August 1994;24:252-255.]

Section snippets

INTRODUCTION

Pulse oximetry, originally used for intraoperative monitoring during anesthesia,1, 2 is now used in most emergency departments and ICUs and in some prehospital settings.3, 4, 5 Despite its widespread use, many physicians remain unaware of its clinical usefulness as well as its limitations.6, 7 One of the potential drawbacks of pulse oximetry is carbon monoxide (CO) poisoning.

A pulse oximeter measures light absorbance from pulsed and nonpulsed signals at two different wavelengths of light: 660

MATERIALS AND METHODS

Patients over the age of 17 years presenting to the emergency department with a suspicion of CO intoxication were eligible for entry in a prospective case series. Pulse oximetry was obtained using a finger probe (Nellcor N-200, Hayward, California) simultaneous with arterial blood gas sampling and co-oximetry using standard spectrophotometric techniques (ABL-520 Radiometer, Copenhagen, Denmark). Four patients had multiple sampling performed as the decrease in CO was monitored over time.

RESULTS

Arterial blood gas and pulse oximetry measurements on 25 samples from 16 patients are shown (Table). Pulse oximetry was never less than 96% (mean, 97.8%; median, 98.0%; SD, 1.4%, range, 96% to 100%) despite COHb levels as high as 44% (mean, 17.4%; median, 16.2%; SD, 11.6%; range, 2.2% to 44.0%). Pulse oximetry gap values paralled those of COHb.

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. Arterial blood gas and pulse oximetry measurements of 25 samples from 16 patients with suspected carbon monoxide

DISCUSSION

Pulse oximetry was consistently found to overestimate O2 Hb saturation in the presence of COHb. Furthermore, pulse oximetry quantitatively overestimated the percentage O2 Hb saturation by the amount of COHb present, thus characterizing a pulse oximetry gap in human beings.

Barker and Tremper reported a similar correlation in mongrel dogs exposed to increasing levels of CO. Our linear regression model showed that the pulse oximetry gap slightly overestimated COHb. Within the confidence intervals

CONCLUSION

This relationship has two practical implications for emergency and critical care medicine. First, in patients with known or suspected CO intoxication, pulse oximetry cannot be relied on to reflect true O2 Hb saturation. Second, even with low levels of COHb saturation, the pulse oximeter will overestimate the true O2 Hb saturation by the amount of COHb present. For this reason, the pulse oximeter saturation should be interpreted with caution when used to estimate oxygen saturation in smokers.

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    Citation Excerpt :

    Both carboxyhemoglobin (COHb) and methemoglobin (MetHb) absorb light within the red and near-infrared wavelength ranges used in pulse oximetry; standard pulse oximeters are unable to distinguish COHb and MetHb from normal oxyhemoglobin (O2Hb). Hence, COHb will falsely absorb red light, and the pulse oximeter will display a falsely high saturation reading.18 With MetHb, standard oximeters falsely detect a greater degree of absorption of both Hb and O2Hb, increasing the absorbance ratio.

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From the Department of Emergency Medicine* and the Toxikon Consortium,† Cook County Hospital, Chicago, Illinois.

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The views expressed in this article are those of the authors and do not reflect the official policy or postion of the Department of the Navy, Department of Defense, or the United States Government.

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Address for reprints: Robert G Buckley, MD, Department of Emergency Medicine, Naval Medical Center San Diego, San Diego, California 92134-5000, 619-532-8276, Fax 619-532-9853

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Reprint no. 47/1/56936

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