Biomedical Engineering Reference
In-Depth Information
Section through Sensor Tip
Exterior microporous
polyethylene membrane
Carmeda™ covalently bonded
heparin surface coating
+ Bio-compatible
+ Improves bio-compatibility of the
sensor, inhibits protein and
thrombin formation on the
sensor
+ Outside diameter less than
0.5 mm (nominal average)
+ Indwelling sensor length:
approximately 15-20 mm
Temperature thermocouple
pH, PCO
2
, PO
2
optical cell miniature
sensing elements
No sensing elements at the tip
+ Prevents elements from detecting
erroneous data from tissue
+ Type T (Copper and constantan)
+ Used to report blood gas values at
37°C or patient temperature
+ Each is 0.175 mm diameter
+ Acrylic optical fiber construction
+ pH: Phenol red in polyacrylamide gel
+ PCO
2
: Phenol red in bicarbonate solution
+ PO
2
: Ruthenium dye in silicone matrix
+ Covers 360° spiral along sensor's length
(rather than merely at the tip) to eliminate
“wall effect”
+ Mirror encapsulated within the tip of the
optical fiber to return light back along the
same fiber for detection.
FIGURE 10.39
Principle of an indwelling arterial optical blood gas catheter. A heparin-coated porous polyeth-
ylene membrane encapsulates the optical fibers and a thermistor.
Courtesy of Diametrics, Inc., St. Paul, MN.
(phosphorescence). A wide variety of substances act as fluorescence quenchers. One of the
best-known quenchers is molecular oxygen.
A typical fiber optic sensor for measuring
O
2
using the principle of fluorescence quench-
ing consists of a dye that is excited at 470 nm (blue) and fluoresces at 515 nm (green) with an
emitted intensity that depends on the
p
O
2
. If the excited dye encounters an oxygen molecule,
the excess energy will be transferred to the oxygen molecule, decreasing the fluorescence sig-
nal. The degree of quenching depends on the concentration of oxygen. The optical informa-
tion is derived from the ratio of light intensities measured from the green fluorescence and
the blue excitation light, which serves as an internal reference signal. The ratio of green to
blue intensity is described by the Stern-Volmer equation
p
I
o
=
I
¼
1
þ
Kp
O
2
ð
10
:
26
Þ
where I
o
and I are the fluorescence emission intensities in the absence (i.e.,
p
O
2
¼
0) and
presence of the oxygen quencher, respectively.
is the Stern-Volmer quenching coefficient,
which is dependent on temperature. The method provides a nearly linear readout of
K
O
2
over the range of 0-150 mmHg (0-20 kPa), with a precision of about 1 mmHg (0.13 kPa).
The slope of the plot described by Eq. (10.26) is a measure of the oxygen sensitivity of the
sensor. Note that the sensor will be most sensitive to low levels of oxygen.
p
