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9 TECHNICAL SUPPLEMENT
9.1 Overview
9.2 Functional versus Fractional Saturation
9.3 Measured versus Calculated Saturation
9.4 Circuit Analysis
9.5 Functional Overview
9.6 Definition of Terms
9.7 Overall Block Diagram
9.8 SpO
Analog Circuit
2
9.9 Digital Circuitry
9.10 Circuit Illustrations
9.1
Overview
The N-20/N-20P is based on the principles of spectrophotometry and optical plethysmography.
Optical plethysmography uses light absorption technology to reproduce wave forms 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 form.
Spectrophotometry uses various wavelengths of light to qualitatively measure light absorption through
given substances. Many times each second, the N-20/N-20P passes red and infrared light into the
sensor site and determines absorption. The measurements, which 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 N-20/N-20P 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: SpO
9.2
Functional versus Fractional Saturation
The N-20/N-20P 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 N-20/N-20P measurements with those obtained by an instrument that
measures fractional saturation, measurements must be converted as follows:
functional saturation =
9.3
Measured versus Calculated Saturation
When saturation is calculated from a blood gas measurement of the partial pressure of arterial oxygen
(PaO 2 ), the calculated value may differ from the N-20/N-20P SpO 2 measurement. This is because the
calculated saturation may not have been corrected for the effects of variables that can shift the
relationship between PaO 2 and saturation.
is the ratio of corrected absorption at each wavelength.
2
fractional saturation
100 - (% carboxyhemoglobin + % methemoglobin)
x100
9-1
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