Confirmation of the pulse oximetry gap in carbon monoxide poisoning

Study objectives: To demonstrate the degree to which pulse oximetry overestimates actual oxyhemoglobin (O(2)Hb) saturation in patients with carbon monoxide (GO) poisoning. This phenomenon has been reported in fewer than 20 humans in the English medical literature. Methods: A retrospective chart review of 101 patients evaluated for CO poisoning at a regional hyperbaric center identified 124 patients 10 years of age and older who had had both arterial blood gas and pulse oximetry measurements and who had received either high-flow oxygen through a nonrebreather mask or 100% inspired oxygen through an endotracheal tube. Blood gas measurements, including direct spectrophotometric determination of O(2)Hb and carboxyhemoglobin (COHb) saturation values, were compared with finger-probe pulse oximetry readings. Results: Measured O(2)Hb saturation (mean+/-SD, 88.7+/-10.2%; range, 51.4% to 99.0%) decreased linearly and predictably with rising COHb levels (10.7+/-10.4%; range, .2% to 46.4%). Pulse oximetry saturation (99.2%+/-1.3%; range, 92% to 100%) remained elevated across the range of COHb levels and failed to detect decreased O(2)Hb saturation, The pulse oximetry gap, defined as the difference between pulse oximetry saturation and actual O(2)Hb saturation (10.5%+/-9.7%; range, 0% to 40.6%), approximated the COHb level. Conclusion: There is a linear decline in O(2)Hb saturation as COHb saturation increases, This decline is not detected by pulse oximetry, which therefore overestimates O(2)Hb saturation in patients with increased COHb levels, The pulse oximetry gap increases with higher levels of COHb and approximates the COHb level. In patients with possible CO poisoning, pulse oximetry must be considered unreliable and interpreted with caution until the COHb level has been measured.