The Pulse Oximetry Gap in Carbon Monoxide Intoxication

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.].