Miniaturized pH and pCO₂ intravascular catheter using optical monitoring and a dual concentric-flow microdialysis approach

Hollow fiber membranes are found to allow the development of extremely small diameter (0.22 mm) probe and catheter-type pH and pCO₂ sensors. These types of sensors can be comparable to the size of fiber optic sensors for these analytes. This work employs a concentric-flow geometry to direct a micro-flow of sweep solutions through hollow fiber membranes made of cuprammonium rayon and silicone. The sweep solutions establish a diffusive equilibrium with the external environment and then flow into optical absorbance cells. The optical system, which is remote from the sensor membranes, consists of two separate transparent capillaries, 555-nm surface-mount LEDs and photodetectors to monitor absorbance changes. In buffer, a ±0.01 pH unit resolution is achieved over the range of 7.02-7.87 with a 5-min 90% response time, and a 1.5-mm Hg pCO₂ resolution is achieved from 0 to 80 mm Hg with a 15-min 90% response time.