Affiliations: University of Florida Health Science Center
Virginia Mason Medical Center
Correspondence to: William P. Bozeman, MD, Department of Emergency Medicine, University of Florida, 655 W 8th St, Jacksonville, FL 32209-6511; e-mail: firstname.lastname@example.org
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.
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
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.
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