Recently, a system designed to allow ambulatory monitoring of DGER has been developed (Bilitec®; Medtronic, Shoreview, MN). The system uses bilirubin as a marker of duodenal reflux and can be used concomitantly with 24hour pH monitoring.34 Continuous spectropho-tometric measurement of luminal bilirubin concentration is made with a portable optoelectronic data logger and a fiberoptic probe passed transnasally and positioned in the foregut. The Teflon probe head has a 2-mm open groove across which two wavelengths of light are emitted and material is sampled. The light-emitting diodes have wavelengths of 470 and 565nm, allowing the measurement of bilirubin and a reference signal. The photodiode system converts the light in an electrical signal through filtration and amplification, and a difference of the absorbance between the two wavelengths is calculated. This value is directly proportional to the bilirubin concentration. The system samples luminal concentration of bilirubin every 8 seconds. The continuously recorded data are downloaded into a computer and analyzed. A threshold of 0.14 absorbance units has been demonstrated by various authors as having a good correlation with the measurements of increased bile acid concentrations in gastric aspirates, and are considered diagnostic of DGER. Values <0.14 can be caused by the effect of suspended gastric particles or mucus. Other authors have used a threshold of 0.2 based on their studies in healthy volunteers. An increased esophageal exposure to bilirubin is identified when the percentage exposure time to bilirubin exceeds the 95 percentile level of the values obtained in normal subjects.28,33-35,47,48
It has been suggested that the amount of DGER determined by the Bilitec® probe can be underestimated when it is associated with acid reflux. Despite the high precision of this probe, the bilirubin measurement does not necessarily correlate with the amount of bile salts, pancreatic enzymes, and lysolecithin present in the esophageal refluxate.47,49 It should also be noted that there is no standardization of the recommended diet while using the Bilitec®. Theoretically, a solid diet could interfere with the bilirubin measurement and the food could plug the tip of the probe. Therefore, most authors recommend a liquid diet. Some beverages such as coffee, tea, and cola should also be excluded because they have an absorbance close to that of bilirubin.28,33,34,35 However, a liquid diet is less physiologic and could potentially interfere with the results of the 24-hour pH measurements obtained simultaneously with the continuous bilirubin determination. Even with these limitations, the spectrophotometric detection of bilirubin is the best method currently available for the diagnosis and measurement of DGER.
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