pH Electrodes: Studies and Applications in the Upper G.I. Tract

The use of electrodes for measuring pH in the gastrointestinal tract is on the increase. Improvement in electrode manufacturing techniques has coincided with the miniaturisation of recording equipment and the widespread availability of personal computers. Commercial software packages of ever increasing sophistication enable the clinician to analyse data recorded from these electrodes with an ease which would scarcely have been thought possible only a decade ago. The temptation to use these systems to generate data without a full understanding of their limitations is a real one. I have set out to clarify some of these fundamental problems. The thesis opens with an overview of the currently available methods for measuring G.I. pH. The advantages and disadvantages of aspiration, in situ titration, dialysis and ion-exchange resins are discussed. The theoretical basis of pH electrodes, both ion-selective and metal-metal oxide, along with methods of recording and display are then presented. The two electrodes most commonly used in clinical practice are glass and antimony. A comparative study is presented in which the in vitro operating characteristics (response, sensitivity and drift) of glass are shown to be superior to those of antimony. This study also investigates the influence of the siting of the reference electrode on recorded pH values and concludes that combined electrodes should be used in preference to electrodes with either a skin or buccal reference electrode. Measurement of pH using aspiration and electrodes are then compared by passing two electrodes and a feeding tube to the same level in the stomach of a volunteer. Both techniques are used simultaneously to follow pH changes during fasting and after ingestion of liquids and solids. This study shows close correlation between the two methods as long as aspiration is possible. In the fasting state there may not be sufficient gastric content for aspiration and after a solid meal the tube can obstruct. For these reasons, the glass electrode is superior to aspiration. The difference between hydrogen ion concentration and activity and the significance of bile staining in the acid stomach are also discussed. An account of recent developments in pH sensor technology is then given. The working principles of two devices that, although at present unavailable for clinical use, may become useful in the future are described. These are ion-selective field effect transitors and optrodes. Steep pH gradients are characteristic of the proximal duodenum and many attempts have been made to maintain electrode position there. After reviewing these various methods, it is concluded that to date there is no satisfactory means of achieving this goal. There follows an account of debate surrounding regional variations in intragastric pH. There is no agreement in the literature as to which areas of the stomach are characterised by high or low pH and some authors question the existence of these differences. In order to answer these questions, the author has developed a new technique which enables accurate placement and fixation of pH electrodes at any point in the upper G.I.T. within the range of a gastroscope. The electrodes are localised endoscopically and held in place by stainless steel clips applied through the biopsy channel of a wide-channel gastroscope. preliminary studies on cadaver stomachs to assess the safety of the technique are described, as are the results of attempted fixation of pH electrodes for a period of 24 hours in the antrum of 40 patients. Refinements which converted an initial success rate of one only out of the first ten to the point where failure is now virtually unknown are ennumerated. (Abstract shortened by ProQuest.)