Pulse oximetry correction for smoking exposure

Abstract

Pulse oximetry oxygen saturation (SpO2) does not distinguish carboxyhemoglobin (COHb) from oxyhemoglobin (O2Hb), giving a false impression of the apparent degree of oxyhemoglobin saturation in smokers who have elevated levels of COHb. We questioned whether accounting for smoking exposure history could improve description of pulse oximetry by correcting for COHb levels. We evaluated smoking history and %SpO2 as predictors of %O2Hb and %COHb by CO-oximetry of arterial blood in 18 actively smoking and 18 age-matched nonsmoking patients in a clinical pilot study. The difference between %SpO2 and %O2Hb was significantly greater (p < 0.001) in the smokers (5.6 ± 3.1) than the nonsmokers (2.1 ± 2.1). This difference correlated with %COHb (r(p) = 0.789; p < 0.001) and the smoking exposure score (SES, r(p) = 0.621; p < 0.001), a six-point index we developed based on whether patients were active smokers, refrained from smoking prior to testing, or were exposed to passive smoking in the home or workplace. The following formula summarizes the correction: %O2Hb = 0.882[%SpO2] - 0.968[SES] + 9.245 (r(p) = 0.841; SES = 2.478; p < 0.001). This pilot study suggests that smoking exposure history correlates with COHb levels and that correction for smoking exposure improves the accuracy of pulse oximetry.