Abstract
Electrical bio-impedance can be considered as a physiological measurement of significant value in research and clinical applications. This is because certain structures of living organisms and the human body such as tissues and membranes are characterized by their specific impedance, which can be easily measured with existing technology. In addition, many body functions (physiological processes) involve changes in their ionic and electrolytic content and size, features that are followed by changes in their electrical characteristics indicated by their impedance values. The use of surface electrodes for measuring impedance in humans makes the method more attractive. Thus, during the 2nd half of the last century, impedance techniques have been developed to measure the blood flow in limbs, to form cross-sectional images of human body sections and to define body composition. The imaging techniques suffer from poor resolution due to the anisotropic electrical properties of the tissues. Epigastrography, based on determination of electrical impedance changes, is a simple, inexpensive, radiation free technique, which can be repeated many times without any trauma to the patient and is a valuable research tool. This work involves the generation, analysis and interpretation of epigastric electrical impedance signals. The signals recorded represent the epigastric impedance of a fasting volunteer before, during the oral intake of a liquid meal of typically 450 mL and for periods between 45 minutes to 2 hours post-prandially. The half emptying times (T50s) were calculated for a variety of test meals, and the statistically significant differences between the T50s were found for meals which varied in calorific content in a total of 7 studies with at least 9 subjects per study. Significant differences between T50s were found also when testing the same meal but under different conditions, namely, intravenous infusion of peptides (GLP-1, loxiglumide) or placebo (physiological saline) and the release in the gastric cavity of amino acids, free or bound with an orally taken gelatine capsule. However, the impedance T50s calculated were found to be considerably shorter than half emptying times presented in the literature using other techniques. Simultaneous application of scintigraphy confirmed that the T50s based on impedance were in comparison shorter. Similarly when experiments were carried out, simultaneously with the octanoic acid breath test and application of paracetamol absorption the same shorter, in comparison, values of T50s were obtained. The comparison with scintigraphy strengthened the suspicion that impedance values were being strongly influenced by the presence of gastric acid. Gastric acid studies undertaken for a number of conditions supported the considerable influence that gastric acid played. In vitro measurements of conductivity and pH of mixtures of test meals with gastric juice aspirations from subjects showed the acid influence. Finally, the physiology of the gastric mucosa from the resting state to secretion leads to the hypothesis that epigastric impedance is in the main controlled by the conductivity of the gastric content rather than by the volume of the meal content in the stomach and is supported by the laws of physics underlying the electrical impedance of a bulk object. In conclusion, both experimental and clinical results from the studies and investigations undertaken in this research convinced the author that the epigastric impedance reflects the conductivity and the acidity of the gastric content. Further by carrying out signal processing of the epigastric impedance signals employing fast Fourier transformation information was extracted about the gastric contractions in the range of 2.5 to 4.5 cycles per minute, confirming the usefulness of the technique in the study of gastric motility. In addition studies are necessary, to establish the validity of the results in the clinical environment and to answer conclusively the question "whether epigastric impedance can be developed as an external monitoring technique of gastric acidity and motility".