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University of Florida Health Science Center Jacksonville, FL
Correspondence to: William P. Bozeman, MD, Department of Emergency Medicine, University of Florida, 655 W 8th St, Jacksonville, FL 32209-6511; e-mail: wbozeman{at}ems.umc.ufl.edu
To the Editor:
I read with interest Dr. Hampson’s retrospective review (October 1998)1 of a decade of experience with severe carbon monoxide (CO) poisoning. His series revealed 15 patients who had concurrent pulse oximetry and spectrophotometrically measured arterial oxygen saturation readings. The pulse oximetery gap, which is produced by carboxyhemoglobin and represents the difference between the pulse oximetry reading and the actual oxyhemoglobin saturation, is demonstrated to be a real and clinically important phenomenon. The report is well written and contributes to an area that historically has had a dearth of experimental data.
Unfortunately, there was a significant omission in the "extensive English-language literature search" performed. This omission led the author to incorrectly assess that only seven humans previously had been demonstrated to have a pulse oximetry gap with high levels of carboxyhemoglobin. This search did not reveal our existing report, published the year before, of 124 patients with CO exposure and a determination of the pulse oximetry gap by essentially the same methodology.2 Twenty-two of these patients had carboxyhemoglobin levels from 20 to > 50%. With a greater number of data points, we were able to show a very clear linear relationship between CO levels and the pulse oximetry gap. Our series included mild, moderate, and severe cases of CO poisoning. Dr. Hampson’s report confirms this finding in severe cases and adds valuable additional data to our collective experience with this condition.
Physicians who may treat CO intoxication must be aware of the profound limitations of pulse oximetry in this setting. High-flow oxygen should be administered to all patients suspected of significant CO exposure until direct measurement of CO levels can be performed, regardless of pulse oximetry readings.
References
Virginia Mason Medical Center Seattle, WA
Correspondence to: Neil B. Hampson, MD, FCCP, Virginia Mason Clinic, Pulmonary and Critical Care Medicine, 1100 Ninth Ave., Seattle, WA 98111; e-mail: cidnbh{at}vmmc.org
To the Editor:
Dr. Bozeman expresses concern that his study,1 published in November 1997, was not noted in my article (October 1998).2 As indicated on the first page of my article, my manuscript was submitted in its original form on October 2, 1997. It is only possible to reference material that is published at the time of manuscript preparation. I would like to assure Dr. Bozeman that I would have referred to his excellent study had it been available.
I am, however, concerned that Dr. Bozeman’s interpretation of his own data continues the perpetuation of the myth that my investigation clearly disproved. It often has been stated that pulse oximeters overestimate true arterial hemoglobin oxygen saturation by the amount of carboxyhemoglobin (COHb) present. Examining 30 patients with extreme elevations of COHb (> 25%), I demonstrated that this difference (the "pulse oximetry gap") was less than the COHb concentration in 19 patients (73%). While pulse oximeters overestimate the arterial hemoglobin oxygen saturation, the amount is not equal to the COHb level.
Reviewing Dr. Bozeman’s graphed data in Figure 2 of his article, 13 individuals appear to have COHb levels > 25%. Among these patients, nine (69%) appear to have pulse oximetry gap values that are lower than the COHb level. In my article, I used as an example an individual with a COHb level of 50% and indicated that the pulse oximetry gap would be approximately 5% less than the COHb level (ie, 45%). Using the regression equation published in Dr. Bozeman’s paper, he would predict a pulse oximetry gap value of 45.85%.
Thus, while pulse oximeters do overestimate true arterial hemoglobin oxygen saturation, it is by an amount that is less than the COHb level. Dr. Bozeman’s data confirm my finding in this regard. It should not be surprising that the two values are not equal. Pulse oximeters rely on the differential absorption of light to make their measurement. The absorption spectra of carboxyhemoglobin and oxyhemoglobin are not identical at the wavelengths utilized.
Erratum
In the July 1999 issue of CHEST, the article "Lymphocyte Glutathione Levels in Children With Cystic Fibrosis" by Lands et al contained lymphocyte glutathione levels that were reported using the incorrect unit µmol/106 cells. The correct unit is nmol/106 cells, which is equivalent to µmol/L.
References
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