pH Measurement with a fiber-optic tissue-pH monitor and a standard blood-pH meter.

Continuous monitoring of the pH of tissue with a glass electrode was initially introduced by Stamm et al. (1). Its potential value in monitoring the high-risk fetus during labor has been demonstrated in several studies (2-4). However, this technique has not been clinically accepted for routine use in labor, mostly because of problems with the application and sustained proper attachment of the relatively bulky electrode at a right angle to the fetal scalp. A new fiber-optic pH probe has been developed, based on the concept of monitoring the color of a pH-sensitive dye (phenolsulfonphthalein) surrounded by a hydrogen ion-permeable membrane contained in a 22-gauge hypodermic needle, which is implanted in the tissue of interest (5, 6). Access to the membrane surface is provided by a slot in the needle wall that is just proximal to the sharp end of the needle. The other end of the needle is connected to a thin and highly flexible Teflon cord containing two singlestrand optical fibers. Alterations of the dye color are detected by illuminating the dye with light transmitted along one fiber while monitoring the backscattered light with the adjacent fiber. The illumination system is incorporated in the pH monitor, together with a photodetection device and signal-processing electronics. In the fiber-optic pH measurement technique, pH is linearly related to the ratio of the intensities of the green and red light that is returned. It allows measurement of the pH of any tissue in which the needle probe is implanted. Before this probe is used as a clinical tool, however, a good correlation must be established with the pH as measured with a regular clinical pH meter. We have investigated the in vitro correlation between pH as measured with fiber-optics with an ATL-Cranbury tpH Monitor (Advanced Technology Laboratories, Inc., Cranbury Division, Cranbury, NJ 08512) with use of a straight 22-gauge needle probe and as measured with a Model 165 pHlBlood Gas Analyzer (Corning Medical, Modfield, MA 02052). The probe was calibrated a 37 #{176}C in pH 7.04 and pH 7.44 phosphate buffer solutions. In a firstexperiment, these two buffers were mixed in various proportions: 34 samples were thus prepared, spanning the pH range from 7.04 to 7.44. The pH of each of the samples was then measured at 37 #{176}C with the Monitor and with the Analyzer. The relationship between measurements by the two methods was very close for the 34 buffer samples. The slope of the regression line of the fiberoptic pH vs the regular pH measurement was 1.043 (95% confidence interval, ± 0.034). The correlation coefficient (r) was 0.996, and the mean difference (regular p11-fiber-optic pH) was ±0.010 pH unit,with a standard deviation of 0.013 pH unit. In a second experiment, we collected 24 heparinized samples of human blood (adult venous and umbilical cord blood) with a pH range of 7.06 to 7.46, and again measured the pH of each sample anaerobically at 37 #{176}C with the Monitor and the Analyzer. The slope of the regression line for the fiber-optic pH vs the regular pH measurement was 1.057 (95% confidence interval, ± 0.043). The correlation coefficient was 0.996, and the mean difference (regular pH-fiber-optic pH) was -0.005 pH unit, with a standard deviation of 0.013 pH unit. These are within the limits of variation between different readings of the same sample with the same instrument, in clinical blood pH measurement, and thus they have no practical significance. We conclude that in vitro pH measurements of buffer solutions and of blood from adults and from the umbilical cord with the two systems do not differ significantly over the pH range of 7.04 through 7.46. Further studies are needed to check the accuracy of the fiber-optic pH monitor when it is used in vivo, especially if modified into a spiral for continuous monitoring of fetal scalp pH during labor.