Intravascular pH-ISFET, a method of the future

Since blood pH reflects the metabolic state as well as the functioning of the respiratory and circulatory systems, it appears to be one of the vital parameters that should be continuously monitored. The point to point measurement of pH by sampling has limitations, since results are available after some time only. A continuous pH signal is necessary in rapidly changing situations such as often occur during intensive treatment. The application of ion-selective field-effect transistor (ISFET) technology to intravascular pH measurement seemed attractive since these devices offer a short response time, are suitable to miniaturization and fabrication in large quantities, and have a low output impedance. The system was tested in dog experiments. Two to four pH-ISFET catheters were tested simultaneously in each dog. Arterial samples for blood-gas analysis with a Radiometer ABL2 system were taken during periods in which plasma-pH was stable. Clinical trials were performed in patients who were admitted to the intensive care unit after coronary artery bypass grafting. Testing the pH-ISFET system in dogs (N = 12) and comparing it with ABL2 measured samples (n = 132 with a pH range of 6.72-7.86), resulted in a delta pH (ISFET-ABL2) of 0.016 +/- 0.024 pH with a maximum deviation of 0.06. The clinical trials proved that the system is well-suited for monitoring intravascular pH in critically ill patients.