Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 26

2.9.1 Pulse Oximetry

A pulse oximeter is a medical device that indirectly measures the oxygen level in a
patient's blood and changes in blood volume in the skin, producing a photoplethys-
mograph. As noninvasive measurement instruments they are particularly conve-
nient for wide use.
Typically a pulseoximeter has a pair of small light-emitting diodes (LEDs) facing
a photodiode through a translucent part of the patient's body, usually a fingertip or
an earlobe. One of the LEDs is red, with wavelength of 660 nm, and the other is
infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly
between oxyhemoglobin and its deoxygenated form; therefore, from the ratio of the
absorption of the red and infrared light, the oxy/deoxyhemoglobin ratio can be calcu-
lated. The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isos-
bestic point) for the wavelengths of 590 and 805 nm.
The monitored signal bounces in time with the heart beat because the arterial
blood vessels expand and contract with each heartbeat. By examining only the
varying part of the absorption spectrum, a monitor can ignore other tissues or nail
polish and discern only the absorption caused by arterial blood. Thus, detecting a
pulse is essential to the operation of a pulse oximeter and it will not function if there
is none (Fig. 2.3 ).
A pulse oximeter is useful in any setting where a patient's oxygenation is unstable,
including intensive care, operating, recovery, emergency and hospital ward settings,
pilots in unpressurized aircraft, for assessment of any patient's oxygenation, and
determining the effectiveness of or need for supplemental oxygen. Assessing a
patient's need for oxygen is the most essential element to life; no human life thrives
in the absence of oxygen (cellular or gross). Although a pulse oximeter is used to
monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount
of oxygen being used by a patient. For this purpose, it is necessary to also measure
carbon dioxide (CO 2 ) levels.

R R

Photodetector

Fig. 2.3 Operating principle of a pulseoximeter. Explanation: Red and infrared light emmitors are
positioned above the fingernail. A photodetector is positioned below the finger. Absorption of red
and infrared light differs significantly between oxyhemoglobin and its deoxygenated form; there-
fore, from the ratio of the absorption of the red and infrared light, the oxy/deoxyhemoglobin ratio
can be calculated