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13
Masimo
Chapter 1: Technology
The principle of operation for the O3
®
Module System (O3
®
System) is as follows:
Overview
The O3
®
Regional Oximeter System (O3
®
System) operating principle is based on
multi-distance diffuse reflectance spectroscopy. The O3
®
System uses light to examine a
cross-section tissue microvasculature (a mixed bed of arterioles, capillaries, and venules) and
analyzes the light returned after having passed through the tissues.
Principles of Beer-Lambert Law and Regional Oximetry
The Beer-Lambert Law describes the attenuation of light through a medium as a function of
the path length (or distance) and the absorption coefficient of the medium. The
Beer-Lambert Law may be written as:
In the above equation,
I
o
is light intensity at the source, and
I
is light intensity after having
traveled a distance of
Δ
in a medium with an absorption coefficient of
µ
a.
The human body is opaque to most visible light frequencies, yet it is more transparent to red
and infrared light. Moreover, light absorptions of oxygen-related chromophores, such as
oxygenated and deoxygenated hemoglobin, vary as a function of wavelength in the near
infrared spectrum, as shown in Figure 1. Therefore, if the level of oxygenation in the tissue
changes, optical characteristics of the tissue also change according to the concentrations of
oxygenated and deoxygenated hemoglobin. This absorption of light by the chromophore
concentrations forms the basis of measurement of oxygen saturation, defined as the ratio of
oxygenated hemoglobin to total hemoglobin.
Figure 1: Absorption coefficients of tissue chromophores in the NIR spectrum*