Blood gas monitoring using micro-flow colorimetry

Optical monitoring of blood gas concentrations is achieved using sub microliter quantities of a colorimetric indicator continuously perfused through microdialysis hollow fiber membranes in blood contact. The hollow fiber membranes, selected based on their permeabilities in blood, are silicone for both oxygen and carbon dioxide sensing, and cuproammonium rayon for pH sensing. A sweep fluid passed through the lumen of these fibers undergoes a diffusive equilibrium with the blood and then is continuously mixed with an indicator. A 450 nm gallium nitride LED is used to excite an oxygen-sensitive fluorophore, ruthenium tris-(2,2′-bipyridyl) II dichloride, which has a 620 nm emission peak that is analyzed with an orange coated photodetector. Gallium phosphide 555 nm LEDs are used to excite the 550 nm absorbance peak of phenol red for pH measurement and phenol red in a 35 mm bicarbonate buffer for CO ₂ measurement. Accurate measurement of small absorbance and fluorescence changes using small bore capillary tubes allows good resolution of biochemical concentrations. Continuous replenishment of the indicator by flow lends itself to a stable method of biochemical analysis that has potential for long term performance. In vitro buffer studies demonstrate a resolution of +/-0.5 mmHg for pCO ₂, a +/-1.5 mmHg for pO ₂, and a +/-0.003 pH.