In vitro evaluation of a fast response, modified pH-Glass electrode designed for continuous measurement of the pCO2 in the gastric lumen
© Current Science Ltd 1999
Published: 16 March 2000
The intramucosal pH (pHi) is a sensitive and early parameter of various shock states. Its prognostic and therapeutic value has been demonstrated. Tonometry relies on the measurement of intragastric pCO2 via a nasogastric probe and the arterial bicarbonate. There are several shortcomings of the tonometric method (e.g. handling and measurement errors, and the need for a long equilibration time (30-90 min)). Therefore, we evaluated a modified pH-glass electrode for fast and continuous pCO2-monitoring in the stomach.
Our in vitro measurements were performed using a special designed pH-metry glass electrode with an outer diameter of 4.5 mm (GK2801C;Radiometer) covered with a thin gas permeable teflon membrane. Thus, CO2 may easily diffuse through the membrane and induces changes of the pH of an interspersed electrolyte solution. At first, the membrane and the membrane covered electrode were tested for chemical and mechanical stability in aggressive and acidic fluids. Secondly, the precision of two one-point calibrated electrodes to measure the pCO2 in the range of 20 to 250 mmHg was tested. For each of five given pCO2-levels a set of five measurements was done. Thirdly, the response time of two electrodes to reach 90% of the maximum (t90) was tested by exposing the electrodes rapidly to two different solutions with a pCO2 of 28.9 and 85.9 mmHg, respectively.
The teflon membrane has proved to be stable against 0.1 N hydrochloric acid, gastric and biliary secretions adjusted to pH 1, and mechanical irritations. In acidic fluids a linear relationship between the measured pCO2 and the defined pCO2 for both electrodes was observed. The slope of the regression line was y=24.43x+7.64 (r = 0.99, n = 25) and 27.57x+4.64 (r = 0.99, n = 25) respectively. The deviation from the line of identity was only caused by the one point calibration. The reponse time t90 of the electrode was 19.5 ± 1.38 s and 25.5 ± 2.42 s (± SD), respectively.
This teflon membrane covered modified pH-glass electrode offers a fast, real time, and continuous measurement of the pCO2 in the acidic environment of the gastric lumen.