12.2 Theory of Operation for N-20P
Overview
The N-20P is based on the principles of spectrophotometry and optical plethysmography. Optical
plethsymography 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 forms.
Spectrophotometry uses various wavelengths of light to qualitatively measure light absorption through given
substances. Many times each second, the N-20P passes red and infrared light into the sensor site and
determines absorption. The measurements, which are taken during the arterial pulse, indicate absorption by
arterial blood, non-pulsatile blood, and tissue.
By correcting "during pulse" absorption for "between pulse" absorption, the 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
2
is the ration of correct absorption at each wavelength.
Functional Versus Fractional Saturation
Then 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-20P measurement with those obtained by an instrument that measure
fractional saturation, measurements must be converted as follows:
functional fractional 100_____________________
saturation = saturation x 100-(% carboxyhemo% methemoglobin)
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-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.
Figure 12.1 illustrates the effect that variations
in pH, temperature, partial pressure of carbon
dioxide (PCO
2
), and concentrations of 2,3-DPG
and fetal hemoglobin may have on the
oxyhemoglobin dissociation curve.
12-5
100
50
0
50
100
PH
Temperature
PCO
2,3-DPG
2
PH
Temperature
PCO2
2,3-DPG
(%
) S
a
O
2
PaO (mmHg)
2
Figure 12.1 Oxyhemoglobin dissocation
curve