Biomedical Engineering Reference
In-Depth Information
beds. Under normal conditions, oxygen saturation in the pulmonary artery is normally
around 75 percent, and oxygen consumption is less than or equal to the amount of oxygen
delivered. However, in critically ill patients, oxygen delivery is often insufficient for the
increased tissue demands because many such patients have compromised compensatory
mechanisms. If tissue oxygen demands increase and the body's compensatory mechanisms
are overwhelmed, the venous oxygen reserve will be tapped, and that change will be
reflected as a decrease in SvO
2
. For this reason, SvO
2
is regarded as a reliable indicator of
tissue oxygenation and, therefore, can be used to indicate the effectiveness of the cardiopul-
monary system during cardiac surgery and in the ICU.
The fiber optic SvO
2
catheter consists of two separate optical fibers; one fiber is used for
transmitting incident light to the flowing blood, and the other directs the backscattered light
to a photodetector. The catheter is introduced into the vena cava and further advanced
through the heart into the pulmonary artery by inflating a small balloon located at the distal
end. The flow-directed catheter also contains a small thermistor for measuring cardiac out-
put by thermodilution.
The principle is based on the relationship between SvO
2
and the ratio of the infrared-to-
red (IR/R) light backscattered from the red blood cell in blood
SvO
2
¼
A
B
ð
IR
=
R
Þ
ð
10
:
27
Þ
where,
are empirically derived calibration coefficients.
Several problems limit the wide clinical application of intravascular fiber optic oximeters.
These include the dependence of the optical readings on motion artifacts due to catheter tip
“whipping” against the blood vessel wall. Additionally, the introduction of the catheter into
the heart requires an invasive procedure and can sometimes cause arrhythmias.
A
and
B
10.6.4 Intravascular Fiber Optic Pressure Sensors
Pressure measurements provide important diagnostic information. For example, pres-
sure measurements inside the heart, cranium, kidneys, and bladder can be used to diagnose
abnormal physiological conditions that are otherwise not feasible to ascertain from imaging
or other diagnostic modalities. In addition, intracranial hypertension resulting from injury
or other causes can be monitored to assess the need for therapy and its efficacy. Likewise,
dynamic changes of pressure measured inside the heart, cranial cavities, uterus, and blad-
der can help to assess the efficiency of these organs during contractions.
Several approaches can be used to measure pressure using minimally invasive sensors.
The most common technique involves the use of a fiber optic catheter. Fiber optic pressure
sensors have been known and widely investigated since the early 1960s. The major chal-
lenge is to develop a small enough sensor with high sensitivity, high fidelity, and adequate
dynamic response that can be inserted either through a hypodermic needle or in the form
of a catheter. Additionally, for routine clinical use, the device must be cost-effective and
disposable.
A variety of ideas have been exploited for varying a light signal in a fiber optic probe
with pressure. Most designs utilize either an interferometer principle or measure changes
in light intensity. Interferometric-based pressure sensors are known to have a high sensitiv-
ity, but they involve complex calibration and require complicated fabrication. On the other
