S-1
TECHNICAL SUPPLEMENT
S1 Introduction
S2 Pulse Oximetry Principles of Operation
S3 Circuit Analysis
S4 Schematic Diagrams
S1 INTRODUCTION
This Technical Supplement provides a description of the principles of pulse
oximetry, a block diagram level theory of operation discussion, and a
schematic level theory of operation discussion. Part locator diagrams and
schematic diagrams are located at the end of this section as fold-out
drawings. These diagrams can be folded out for review while reading the
theory of operation.
S2 PULSE OXIMETRY PRINCIPLES OF OPERATION
S2.1 Overview
The NPB-40 is based on the principles of spectrophotometry and optical
plethysmography. Optical plethysmography uses light absorption technology
to reproduce waveforms produced by pulsatile blood. The changes that occur
in the absorption of light due to vascular bed changes are reproduced by the
pulse oximeter as plethysmographic wave forms.
Spectrophotometry uses various wavelengths of light to qualitatively measure
light absorption through given substances. Many times each second, the
NPB-40 passes red and infrared light into the sensor site and determines
absorption. The measurements that are taken during the arterial pulse,
reflect absorption by arterial blood, nonpulsatile blood, and tissue. The
measurements that are obtained between arterial pulses reflect absorption by
nonpulsatile blood and tissue.
By correcting "during pulse" absorption for "between pulse" absorption, the
NPB-40 determines red and infrared absorption by pulsatile arterial blood.
Because oxyhemoglobin and deoxyhemoglobin differ in red and infrared
absorption, this corrected measurement can be used to determine the percent
of oxyhemoglobin in arterial blood: SpO2 is the ratio of corrected absorption
at each wavelength.
S2.2 Functional Versus Fractional Saturation
The NPB-40 measures functional saturation, that is, oxygenated hemoglobin
expressed as a percentage of the hemoglobin that is capable of transporting
oxygen. It does not detect significant levels of dyshemoglobins. In contrast,
some instruments such as the IL282 Co-oximeter measure fractional
saturation, that is, oxygenated hemoglobin expressed as a percentage of all
measured hemoglobin, including dyshemoglobins.
Consequently, before comparing NPB-40 measurements with those obtained
by an instrument that measures fractional saturation, measurements must
be converted as follows:
functional
saturation
=
fractional
saturation
x
100
100-(% carboxyhemoglobin +%methemoglobin)
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