Correspondence

Why “Why” Matters

N Engl J Med 2005; 352:1270-1271March 24, 2005

Article

To the Editor:

Janssen et al., in their Clinical Problem-Solving article (Dec. 2 issue),1 present a case of glucose-6-phosphate dehydrogenase (G6PD) deficiency with hemolysis and methemoglobinemia. They report that the patient had an oxygen-saturation value of 85 percent as measured by pulse oximetry that did not improve with the administration of 100 percent oxygen. This would be true if the methemoglobin level were greater than 35 percent, but the patient's level was 8.8 percent. At levels up to 35 percent, oxygen saturation on oximetry decreases by an amount proportional to the concentration of methemoglobin until the latter reaches approximately 35 percent. At higher methemoglobin levels, the oxygen saturation levels out at about 85 percent.2 Thus, the patient's oxygen saturation on pulse oximetry should have been much higher than 85 percent and would have improved slightly with the administration of 100 percent oxygen. Of course, the true oxygen-saturation value could be measured by co-oximetry.

Edwin W. Grimsley, M.D.
Mercer University School of Medicine, Macon, GA 31201
grimsley_e@mercer.edu

2 References

1
Janssen WJ, Dhaliwal G, Collard HR, Saint S. Why “why“ matters. N Engl J Med 2004;351:2429-2434
Full Text | Web of Science | Medline

2
Barker SJ, Tremper KK, Hyatt J. Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry. Anesthesiology 1989;70:112-117
CrossRef | Web of Science | Medline

Author/Editor Response

We appreciate Dr. Grimsley's comments regarding pulse oximetry and methemoglobin. Pulse oximeters estimate oxygen saturation by comparing the absorbance of light at two wavelengths. Infrared light is absorbed by oxyhemoglobin, whereas red light is absorbed by deoxyhemoglobin. The absorbance characteristics of methemoglobin are similar to those of oxyhemoglobin, falsely elevating pulse-oximeter readings.1,2

Co-oximeters measure light absorbance at four or more discrete wavelengths, providing accurate measurement of oxygen saturation, methemoglobin, and carboxyhemoglobin. In our patient, the oxygen saturation as measured by co-oximetry was 88.9 percent, the methemoglobin level was 8.8 percent, and the carboxyhemoglobin level was 1.2 percent.

A methemoglobin level of 8.8 percent would be expected to yield a pulse-oximeter reading of 92 percent,3 not 85 percent. A number of factors can impair the function of a pulse oximeter, including motion by the patient, the presence of or an excess of ambient light, increased skin pigmentation, abnormal body temperature, hyperbilirubinemia, and severe anemia.2,4,5 It is possible that one or more of these variables may have contributed to the discrepancy between the predicted oxygen-saturation value and the one recorded in our patient.

We agree with Dr. Grimsley that although pulse oximetry provides a useful, noninvasive estimate of oxygen saturation, co-oximetry should be performed in all patients in whom methemoglobinemia is suspected.

William J. Janssen, M.D.
Harold R. Collard, M.D.
University of Colorado Health Sciences Center, Denver, CO 80262
william.janssen@uchsc.edu

Gurpreet Dhaliwal, M.D.
University of California, San Francisco, San Francisco, CA 94143

5 References

1
Eisenkraft JB. Pulse oximeter desaturation due to methemoglobinemia. Anesthesiology 1988;68:279-282
CrossRef | Web of Science | Medline

2
Jensen LA, Onyskiw JE, Prasad NG. Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in adults. Heart Lung 1998;27:387-408
CrossRef | Web of Science | Medline

3
Barker SJ, Tremper KK, Hyatt J. Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry. Anesthesiology 1989;70:112-117
CrossRef | Web of Science | Medline