Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 27

Because of their simplicity and speed, pulse oximeters are of critical importance
in emergency medicine and are also very useful for patients with respiratory or
cardiac problems, especially COPD, or for diagnosis of some sleep disorders such
as apnea and hypopnea. Portable, battery-operated pulse oximeters are useful for
pilots operating in a non-pressurized aircraft above 10,000 ft (12,500 ft in the US),
where supplemental oxygen is required.
A pulse oximeter does, however, have a number of drawbacks, limiting their use,
or calling for caution when oximetry results are used and interpreted.
First, it should be noted that oximetry is not a complete measure of respiratory
sufficiency. A patient suffering from hypoventilation (poor gas exchange in the lungs)
given 100% oxygen can have excellent blood oxygen levels while still suffering
from respiratory acidosis due to excessive carbon dioxide.
It is also not a complete measure of circulatory sufficiency. If there is insufficient
blood flow or insufficient hemoglobin in the blood (anemia), tissues can suffer
hypoxia despite high oxygen saturation in the blood that does arrive.
Furthermore, a high level of methemoglobin will tend to cause a pulse oximeter to
read closer to 85% regardless of the true level of oxygen saturation. It also should be
noted that the inability of two-wavelength saturation level measurement devices to
distinguish carboxyhemoglobin due to carbon monoxide inhalation from oxyhemoglo-
bin must be taken into account. To counter this problem, CO-oximeters have recently
been developed. These devices use additional wavelengths to distinguish CO from O 2 .

2.9.2 Continuous Glucose Monitors

A continuous blood glucose monitor (CGM) determines blood glucose levels on a
continuous basis (every few minutes). These devices include a disposable glucose
sensor placed just under the skin and worn for a few days until replacement, a link
from the sensor to a non-implanted transmitter which communicates to a radio
receiver and an electronic receiver worn like a pager (or insulin pump) that displays
blood glucose levels on a practically continuous manner.
The sensor can be implanted in the subcutaneous tissue using a specialized tool
designed to minimize tissue damage. The tip of the sensor is made of a membrane
selectively permeable to glucose. Once the glucose passes through the membrane,
it is oxidized by the enzyme glucose oxidase. Reduced glucose oxidase can then
be oxidized by reacting with molecular oxygen, forming hydrogen peroxide as a
by-product. At the electrode surface, hydrogen peroxide is oxidized into water,
generating a current which can be measured and correlated to the glucose concen-
tration outside the membrane (see above right).
This type of device requires at least four finger sticks per day for calibration.
Continuous glucose monitors measure glucose levels in the interstitial fluid, not
directly the blood. This causes a temporal lag behind actual blood glucose values of
about 5 min.