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ARTICLE

Multiwavelength Pulse Oximetry: Theory for the Future

Takuo Aoyagi, EE, PhD^*, Masayoshi Fuse, EE^*, Naoki Kobayashi, EE^*, Kazuko Machida, MD, PhD, and Katsuyuki Miyasaka, MD, PhD

From the *Ogino Memorial Research Laboratory, Nihon Kohden Corporation; Department of Respirology, National Tokyo Hospital, Tokyo; and Nagano Children’s Hospital, Nagano, Japan.

Address correspondence and reprint requests to: Takuo Aoyagi, PhD, Ogino Memorial Research Laboratory, Nihon Kohden Corporation, 1-31-4 Nishiochiai, Shinjuku-ku, Tokyo 161-8560 Japan. Address e-mail to tyaoyagi{at}s2.ocv.ne.jp.

Abstract

BACKGROUND: As the use of pulse oximeters increases, the needs for higher performance and wider applicability of pulse oximetry have increased. To realize the full potential of pulse oximetry, it is indispensable to increase the number of optical wavelengths. To develop a multiwavelength oximetry system, a physical theory of pulse oximetry must be constructed. In addition, a theory for quantitative measurement of optical absorption in an optical scatterer, such as in living tissue, remains a difficult theoretical and practical aspect of this problem.

METHODS: We adopted Schuster’s theory of radiation through a foggy atmosphere for a basis of theory of pulse oximetry. We considered three factors affecting pulse oximetry: the optics, the tissue, and the venous blood.

RESULTS: We derived a physical theoretical formula of pulse oximetry. The theory was confirmed with a full SO₂ range experiment. Based on the theory, the three-wavelength method eliminated the effect of tissue and improved the accuracy of Spo₂. The five-wavelength method eliminated the effect of venous blood and improved motion artifact elimination.

CONCLUSIONS: Our theory of multiwavelength pulse oximetry can be expected to be useful for solving almost all problems in pulse oximetry such as accuracy, motion artifact, low-pulse amplitude, response delay, and errors using reflection oximetry which will expand the application of pulse oximetry. Our theory is probably a rare case of success in solving the difficult problem of quantifying optical density of a substance embedded in an optically scattering medium.