Blood gas monitoring sensors

Colorimetric, fiber optic sensors for measuring pH, PCO.sub.2 and/or other chemical parameters of the blood. The sensors are fabricated using a single optical fiber, which is provided with a chamber at its distal end containing a pH sensitive dye. Located distal to the chamber is a white reflective surface located within 0.04" from the end of the optical fiber, which enhances the performance of the sensor.

CROSS REFERENCE TO COMMONLY ASSIGNED, CO-PENDING APPLICATION 
Reference is made to U.S. patent application Ser. No. 07/314561 for a 
"POLYMER DYE FOR FIBER OPTIC SENSOR" by Fogt et al, filed Feb. 23, 1989, 
now U.S. Pat. No. 4,906,249. 
BACKGROUND OF THE INVENTION 
This invention relates generally to blood gas monitoring, and more 
particularly to fiber optic reflectance sensors. 
Early designs of fiber optic pH and CO.sub.2 sensors were done by Peterson, 
as described in the article "FIBER OPTIC PH PROBE FOR PHYSIOLOGICAL USE", 
by Peterson et al, published in Analytical Chemistry, Vol. 52, pp. 
864-869, 1980 and by Vurek, as disclosed in the article "A FIBER OPTIC 
PCO2 SENSOR", by Vurek et al, published in Annals of Biomedical 
Engineering, Vol. 11, pp. 499-510, 1983. The pH sensor, described by 
Peterson, employed two optical fibers, connected to a semipermeable 
membrane pouch filled with phenol red dye bonded to a polyacrylamide 
powder substrate and mixed with reflective glass microspheres. The Vurek 
sensor employs the same dye (phenol red) in a bicarbonate buffer solution 
encased by a gas permeable, ion impermeable membrane. As carbon dioxide 
passes through the membrane, the pH of the buffer solution is altered. 
In both the Peterson and Vurek sensors, the light absorption properties of 
phenol red are utilized in order to produce a color change, which can be 
monitored optically. Phenol red is a weakly ionizing acid which 
disassociates into an acid form having an absorption peak at 430 nm and a 
base form having an absorption peak at 560 nm. The proportions of acid and 
base forms are determined by the pH of the solution. Therefore, the pH of 
the solution containing phenol red can be monitored using light from a 
green LED, preferably having a frequency band centered at about 560 nm. In 
both sensors, the green light is provided to the sensor capsule by means 
of one of the optical fibers, and reflected light is monitored using the 
second optical fiber. 
Recently, single optical fiber sensors employing fluorescence based dye 
systems have been introduced, as described in "OPTICAL FLUORESCENCE AND 
ITS APPLICATION TO AN INTRAVASCULAR BLOOD GAS MONITORING SYSTEM", by 
Gehrich et al, published in IEEE Transactions on Biomedical Engineering, 
Vol. BME-33, No. 2, pp. 117-132, 1986. This article is incorporated herein 
by reference in its entirety. 
SUMMARY OF THE INVENTION 
The present invention provides a single fiber colorimetric optical sensor 
for sensing chemical parameters of the blood. In the two disclosed 
embodiments, it is configured as pH and PCO.sub.2 sensors, and employs dye 
systems based on phenol red. In conjunction with both sensors, the 
absorption of green light having a wavelength band centered at 
approximately 560 nm is measured. In use, the sensor is coupled to a 
monitoring apparatus which includes one or more light sources and a light 
measuring apparatus. The optical fiber carries light from the light source 
to the sensor and reflected light from the sensor back to the apparatus 
for measuring light intensity. 
The sensor is provided with a dye chamber containing a colorimetric dye, 
typically phenol red. The dye is located between the end of the optical 
fiber and a reflective surface, perpendicular to the axis of the optical 
fiber. This configuration allows the construction of a single fiber 
colorimetric sensor, having adequate characteristics for use in an vivo 
blood gas monitoring. Moreover, this structure allows for simple and 
economical sensor fabrication.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a side, cutaway view of a PCO.sub.2 sensor according to the 
present invention. The sensor assembly is mounted at the distal end of an 
optical fiber 10, which is adapted to be coupled to light generating and 
measuring equipment, such as that disclosed in the above cited Gehrich et 
al article. Surrounding the distal end of the optical fiber 10 is a 
cellulosic semipermeable membrane of the type typically used for dialysis 
tubing. Sealing the distal end of the membrane 12 is a plug of titanium 
dioxide impregnated epoxy 16. The addition of titanium dioxide to the 
epoxy provides a white color to allow the proximal surface 18 to function 
as a reflective surface, reflecting light emitted from the distal end of 
optical fiber 10. Alternatively, other reflective surfaces, such as 
miniature mirrors or metallized layers, might be used. However, the 
construction illustrated is both economical and readily produced. 
The distal end of optical fiber 10, the proximal end of plug 16, and 
membrane 12 together define the outer boundaries of the dye chamber 20. 
Dye chamber 20 is filled with an aqueous mixture of sodium bicarbonate and 
sodium chloride along with a small amount of phenol red. Particular 
proportions which have been successfully used are: 0.030M NaHCO.sub.3 
+0.12M NaCl+0.5 g/liter phenol red. 
Surrounding membrane 12 is a layer of silicone rubber 14, which functions 
as a gas permeable membrane. In alternative embodiments, membrane 12 may 
be omitted, and silicone rubber 14 used as the only membrane. Because 
silicone rubber will allow water vapor to pass through, the sensor can be 
built dry, and hydrated at a later time. 
CO.sub.2 dissolved in the fluid to be measured passes through silicone 
rubber 14 and membrane 12, altering the pH of the dye mixture 22 in 
chamber 20. Light emitted from the distal end of fiber 10 passes through 
dye mixture 22, and is reflected off of reflective surface 18. Depending 
upon the pH of the dye mixture, the amount of light absorbed in the 
measuring chamber 20 will vary. Measurement of the reflected light thus 
provides a measurement of PCO.sub.2 of the fluid in which the sensor is 
located. Reflective surface 18 is preferably generally perpendicular to 
the axis of optical fiber 10, and should be spaced no more than about 
0.004" from the end of optical fiber 10, in order to function with 
adequate efficiency. 
FIG. 2 is a side, cutaway view of a sensor according to the present 
invention configured as a pH sensor. The sensor assembly is mounted at the 
distal end of an optical fiber 30, similar to that discussed above in 
conjunction with the sensor illustrated in FIG. 1. Surrounding the optical 
fiber 30 is a semipermeable membrane 32, which may be a cellulosic 
membrane of the type typically used as dialysis tubing. Binding the 
membrane 32 to the optical fiber 30 is adhesive 34, which may be a UV 
curing epoxy. An epoxy plug 36 is located at the distal end of the sensor. 
Like plug 16 discussed above, it is white in color, and its proximal 
surface 38 serves as a reflector for light emitted by optical fiber 30. 
Dye chamber 40 is filled with a pH indicating dye, phenol red, bound to a 
polymeric composition including substantially equal relative amounts of 
anionic and cationic monomer constituents, along with minor amounts of 
neutral monomer constituents. These constituents are copolymerized 
together in the presence of the dye. The anionic monomer constituent may 
be the sodium salt of 2-acrylamido-2-methyl propane sulfonic acid (Na 
AMPS), the cationic monomer constituent may be 
methylacrylamidopropyl-trimethylamoninium chloride (MAPTAC), and the 
neutral monomer constituent may be acrylamide. One appropriate ratio of 
cationic: anionic: neutral monomers is 2:2:1 by weight. This dye 
composition is discussed in more detail in U.S. patent application Ser. 
No. 07/314561 for a "POLYMER DYE FOR FIBER OPTIC SENSOR" filed on the date 
of this application, by Fogt et al, and now U.S. Pat. No. 4,906,249, 
incorporated herein by reference in its entirety. 
As discussed in conjunction with the sensor illustrated in FIG. 1, the 
distance between the end of optical fiber 30 and the proximal surface 38 
of plug 36 should be no more than about 0.004", in order to assure 
adequate reflection of light from fiber 30. As discussed above, other 
reflective surfaces, such as mirrors or metallized surfaces could be 
substituted for the white epoxy plug 36. 
FIG. 3 shows a cutaway view of the sensors illustrated in FIGS. 1 and 2 
assembled in the form of a multisensor lead. The pH sensors of FIGS. 1 and 
2, mounted to optical fibers 10 and 30, respectively, are bundled with two 
additional optical fibers 50 and 52, which function as a reflectance 
oximeter. The fibers are held in alignment with one another by means of an 
epoxy adhesive 54. The end of the fiber bundle is coated with a 
hydrophylic, gas permeable polymer 56, which allows for free passage of 
dissolved ions and gases from body fluid, to the pH and PCO.sub.2 sensors. 
Polymeric coating 56 may be a hydrophylic polyurethane sold under the 
trade designation BPS 533, by Thoratec, Inc., California. In its preferred 
embodiment, polymer coating 56 is provided with a covalently bound heparin 
coating, applied to the polymeric coating as set forth in U.S. Pat. No. 
4,613,665 issued to Olle Larm for a "PROCESS FOR COVALENT COUPLING FOR THE 
PRODUCTION OF CONJUGATES, AND POLYSACCHARIDE CONTAINING PRODUCTS THEREBY 
OBTAINED". This patent is incorporated by reference in its entirety. The 
bundled optical fibers are covered by a tubular sheath 58, which may be 
formed of heat shrink plastic, and shrunk down over the fibers. Sheath 58 
extends to the proximal end of the sensor lead assembly.