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A novel dual-lumen catheter electrode design suitable for the simultaneous measurement of PCO2 (partial pressure of carbon dioxide) and pH in flowing blood is described. The probe is fabricated from a single segment of dual-lumen silicone rubber or polyurethane tubing that is impregnated with the proton ionophore tridodecylamine. The impregnation step imparts H+ permselectivity to both inner and outer walls of the tubing. By filling each lumen with a suitable buffer/electrolyte solution and Ag/AgCl reference electrode wire, simultaneous potentiometric detection of both PCO2 and pH is achieved. Careful optimization of incorporated proton carrier (tridodecylamine), plasticizer (o-nitrophenyl octyl ether), and lipophilic counteranion sites (tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) within the tubing walls yields catheter electrodes with resistance values of 10-20 M omega and relatively high stability in flowing blood. Results from continuous measurements of PCO2 and pH during long-term 30-65-h blood loop experiments demonstrate that, after an initial conditioning period, the catheter exhibits low drift rates (PCO2, 4.7 +/- 1.7 mV/30 h; pH, 1.4 +/- 0.5 mV/30 h) and yields continuously measured values in good agreement with those obtained on discrete samples with a commercial blood gas analyzer (PCO2, r2 = 0.997; pH, r2 = 0.915). In vivo evaluation of the catheter sensors, performed by implanting silicone rubber dual-lumen probes in the arteries of anesthetized dogs, indicates that the proposed catheter design can closely follow PCO2/pH changes induced in the animals during 6-13 h of continuous monitoring.

The development and analytical performance of a novel potentiometric combination ion/pCO2 sensor design for in vitro and in vivo measurements are reported. The design is based on incorporating an appropriate ionophore within the outer silicone gas permeable membranes of both conventional macro and new catheter-type pCO2 sensors. Simultaneous measurement of the potentials across the ion-selective/gas permeable membrane and the inner glass or polymer pH sensitive membrane provides the basis for continuous monitoring of both ionic and pCO2 levels with the same device. A macro-sized K+/pCO2 embodiment of the sensor is constructed from a commercial Severinghaus CO2 sensor and is used to demonstrate the principles and capabilities of the proposed design. A flexible, miniaturized (outer diameter = 1.2 mm) combination K+/pCO2 catheter sensor is also described. The catheter-type sensor is fabricated by inserting a tubular polymer membrane pH electrode into an outer silicone rubber tube doped with valinomycin. Continuous measurements of K+ and pCO2 during 6-h blood pump studies using both the macro and catheter-type combination sensors correlate well with those of conventional bench-top analyzers. In addition, continuous (4 h) intravascular measurements with the combination catheter sensor in dogs show good agreement with those of commercial blood analyzers (R = 0.984 and 0.962 for pCO2 and K+, respectively.

Background: Minimally invasive interstitial fluid (ISF) sampling and glucose measurement technologies were integrated into a hand-held device for diabetic glucose monitoring investigations. Methods: Conventional electrochemical test strip technology (Bayer Glucometer Elite®) was adapted to measure glucose in small (0.5–2.0 μL) samples of ISF. Test strip glucose measurements were performed on a commercial potentiostat and were compared to various reference glucose methodologies (YSI 2300 analyzer, microhexokinase procedure, Bayer Glucometer Elite). Characterizations of the integrated ISF sampling-glucose test strip design included accuracy and precision in various sample media (saline, ISF surrogates, diabetic ISF samples), sample volume dependence, test strip sterilization studies (electron beam, γ irradiation), and diabetic ISF sampling and glucose measurements. Results: Glucose measurements were free from significant media effects. Sample volume variations (0.6–3.2 μL) revealed only modest dependence of glucose measurement bias on sample volume (−1.5% per microliter). Sterilization treatments had only a minor impact on glucose response and test strip aging and no significant impact on interferent responses of the glucose test strips. Diabetic subject testing under minimum fasting conditions of at least 2 h with integrated ISF sampling and glucose measurement gave low ISF glucose measurement imprecision (CV, 4%) and mean glucose results that were indistinguishable from reference (microhexokinase) ISF glucose measurements and from capillary blood glucose measurements (Glucometer Elite). Conclusions: Conventional single-use, electrochemical glucose test strip and ISF collection technologies can be readily integrated to provide real-time ISF sampling and glucose measurements for diabetic monitoring applications.

A new continuous-flow automated enzymatic method suitable for the direct determination of creatinine in physiological samples is described. The proposed system utilizes an on-line gas predialysis unit in conjunction with a flow-through enzyme reactor coil and a potentiometric ammonia detector. The enzyme reactor contains immobilized creatinine iminohydrolase (EC 3.5.4.21) which converts creatinine to ammonia and N-methylhydantoin. Ammonia liberated from this reaction is detected downstream with the membrane electrode-based detector. The novel gas predialysis unit effectively removes >99.8% of endogenous ammonia (up to 1 mM) present in the sample. Thus, final peak potentials recorded by the electrode detector are directly proportional to the logarithm of creatinine concentrations present. The method is shown to be precise (< 3%), selective, and capable of accurately determining creatinine in serum and urine samples containing abnormally high endogenous ammonia levels. Determinations of creatinine in serum samples (n = 30) using this new method correlate well with an existing Technicon AutoAnalyzer calorimetric method (r = 0.996). *Mark E. Meyerhoff obtained his B.A. (Chemistry) at the Herbert H. Lehman College (CUNY system), and his Ph D. m 1979 at the State University of New York at Buffalo under the direction of Prof. G. A. Rechnitz. After postdoctoral work at the University of Delaware, Newark, DE he moved to Michigan in 1979 and became an Associate Professor of Chemistry at the University of Michigan in 1985. His research interests are in the areas of ionselective electrodes, gas sensors, bioanalytical chemistry, enzyme-labelled competitive binding assays, and novel toxicity bioassays.