Pulse oximetry in methemoglobinemia
M. F. Watcha, M. T. Connor and A. V. Hing
Department of Anesthesiology, Washington University School of Medicine, St Louis, MO.
Pulse oximetry is a major improvement in the assessment of oxygenation. The
device uses plethysmography and light absorbance measurements at two
wavelengths to estimate oxygen saturation. It is inaccurate, however, when
more than two types of hemoglobin are present. This article describes two
infants with methemoglobinemia in whom pulse oximetry overestimated oxygen
saturation. We discuss the mechanism of this systematic error and emphasize
that pulse oximetry should not be used to estimate true oxygen saturation
in the presence of methemoglobin. However, a disparity between oxygen
saturation estimates by pulse oximetry and by calculations based on the
arterial partial pressure of oxygen and the oxygen-hemoglobin dissociation
curve can provide an important clue to the presence of such abnormal types
of hemoglobins. Therapy should be based on direct measurements of
oxyhemoglobin by cooximetry and not on measurements of oxygen saturation by
pulse oximetry or on saturations calculated from the Pao2 and the
oxyhemoglobin dissociation curve.
