Biomedical Engineering Reference
In-Depth Information
FIGURE 10.31
Time dependence of light absorption by a peripheral vascular tissue bed showing the effect of
arterial pulsation.
measure the photodetector output when the corresponding LEDs are turned on. The sensor is
usually attached either to the fingertip or earlobe so the tissue is sandwiched between the
light source and the photodetector.
Pulse oximetry relies on the detection of a photoplethysmographic signal, as shown in
Figure 10.31. This signal is caused by changes in the arterial blood volume associated with
periodic contraction of the heart during systole. The magnitude of this signal depends on
the amount of blood ejected from the heart into the peripheral vascular bed with each car-
diac cycle, the optical absorption of the blood, the composition and color of the skin and
underlying tissues, and the wavelengths used to illuminate the blood. S
a
O
2
is derived by
analyzing the magnitude of the red and infrared photoplethysmograms measured by the
photodetector. Electronic circuits separate the red and infrared photoplethysmograms into
their pulsatile (AC) and nonpulsatile (DC) signal components. An algorithm inside the
pulse oximeter performs a mathematical normalization by which the AC signal at each
wavelength is divided by the corresponding DC component that is mainly due to the light
absorbed by the bloodless tissue, residual arterial blood when the heart is in diastole,
venous blood, and skin pigmentation. Since it is assumed that the AC portion in the photo-
plethysmogram results only from the pulsatile arterial blood component, this scaling pro-
cess provides a normalized red/infrared ratio,
, which is highly dependent on the color
of the arterial blood and is therefore related to S
a
O
2
but is largely independent of the vol-
ume of arterial blood entering the tissue during systole, skin pigmentation, and thickness.
Hence, the instrument does not need to be recalibrated for measurements on different
patients. The mathematical relationship between S
a
O
2
and
R
R
is programmed by the manu-
facturer into the pulse oximeter.
10.4.2 pH Electrodes
pH describes the balance between acid and base in a solution. Acid solutions have an
excess of hydrogen ions (H
รพ
), whereas basic solutions have an excess of hydroxyl ions (OH
).
In a dilute solution, the product of these ion concentrations is a constant (1.0
10
14
).
