Optical sensors - Biomedical Engineering Desk Reference - page 543

Optical Fibers

Thermocouple

PH Optrode

pCO 2 Optodre

pO 2 Optodre

Figure 6.2-24 Fiber-optic vascular sensor for measuring blood oxygen and carbon dioxide.

the quantity of oxygen delivered to the tissues and the
quality of the perfusion is of great importance. The pH is
detected by a chromophore, which changes its optical
spectrum as a function of the pH. The absorption-based
indicators or fluorophores are usually used. The carbon
dioxide is detected indirectly, since its diffusion in a car-
bonate solution fixed at the optical fiber tips alters the
pH, which means that the CO 2 content can be de-
termined by measuring the pH. Oxygen is measured
using a separate chemical transducer; oxygen is detected
via fluorescence techniques that exploit the quenching
produced by oxygen on fluorophores.
The first intravascular sensor for simultaneous and
continuous monitoring of the pH, pO 2 , and pCO 2 is
shown in Figure 6.2-24 . It is composed of three optical
fibers encapsulated in a polymer enclosure, including also
a thermocouple used for temperature monitoring. Three
fluorescent indicators are used as chromophores. The pH
and pCO 2 are measured by the same fluorophore. The
optoelectronics is composed of three modules, one
for each sensor. A suitable filtered xenon lamp, when
modulated, provides illumination for the modules. The
source of light is focused through a lens onto a prism
beam splitter and coupled through fiber to the sensor
tip. The deflected light is collected by a reference
detector for source control. The returning fluorescence
is deflected by the prism beam splitter onto the signal
detector.
There have been problems concerning the usage of
intravascular fiberoptic sensors during clinical trials on
volunteers in critical care and on surgical patients and
these remain unresolved.

Respiratory monitoring
In intensive care units there is a need to continuously
monitor breathing condition. It is possible to accomplish
this monitoring from the nurse's station in such a way
that patients can be kept under observation without the
need for the nurses to be physically present. An optical
fiber with a moisture-sensitive cladding can be developed
for that purpose. The cladding is a plastic film doped
with umbelliferous dye, which is a moisture-sensitive
fluorescent material when pumped with UV light. The
sensitive fiber section is placed over the patient's mouth
and excited with He-Cd laser on a halogen lamp. Since
the water vapor content in the human exhalation exceeds
that in the room, the patient's exhalation produces
a fluorescent signal that is detected by an electro-optical
unit at the nurse's station. This monitoring is very useful
in detecting abnormal breathing in patients.

Angiology
Blood vessels that are obstructed by atherosclerotic
plaques can be recanalized by means of a pulse excimer
laser radiation, guided by an optical fiber (laser angio-
plasty). Despite its widespread uses there is the possi-
bility of blood vessel perforation, which occurs in
between 20% and 40% of patients. To minimize the risk
there is the usage of laser-induced fluorescence diagnosis
of the vessel wall. This is part of identifying the target
under irradiation. An all-optical approach to target iden-
tification is suggested by the fact that short optical pulses,
when absorbed by the tissue, generate ultrasonic ther-
moelastic waves. The amplitude and temporal charac-
teristics of the acoustic signal are dependent on the target
composition and can be detected by a pressure fiber-
optics system. In this approach an optical fiber tipped
with a Fabry-P ´ rot cavity is inserted in the lumbar of the
artery, and the sensor is in contact with the tissue. As
shown in Figure 6.2-25 , two signals are guided by the
optical fiber: (1) pulse light of a Nd:YAG laser used
to generate the thermoelastic wave in the tissue and
(2) low-power continuous wave light of a tunable laser

1. blood flow decreases due to peripheral
vasoconstriction lasting for several hours after
surgical operations, which can give rise to a
contamination by flush solutions
2. the wall effect, which affects the oxygen count
3. the formation of a clot around the sensor tip, which
can alter the value of all the analytical values.