Capsule pH monitoring: is wireless more?

S ince its introduction in 1974, ambulatory oesophageal pH monitoring has secured a valuable but complementary role in the diagnosis of gastro-oesophageal reflux disease. Monitoring of oesophageal pH allows not only the detection of excessive levels of acid exposure but also, and more importantly, assessment of the relationship between acid reflux and symptoms. It is especially useful for clarification of the diagnosis in patients with typical or atypical reflux symptoms who do not response to empirical therapy with a proton pump inhibitor, and for assessment of the level of acid suppression in patients with refractory symptoms or oesophagitis despite appropriate antisecretory therapy. 3 The traditional method of pH monitoring uses a pH electrode mounted on a catheter that is passed transnasally into the oesophagus and positioned 5 cm above the manometrically defined upper border of the lower oesophageal sphincter. The catheter is connected to a portable data logger thereby allowing ambulatory recordings to be made. Ideally, the circumstances under which the pH recordings are made should reflect, as best as possible, the usual living conditions and physical activities normally undertaken by the patient. However, because the catheter is conspicuous and uncomfortable, most patients restrict their activities and dietary intake, 5 which could potentially lead to false negative results. In addition, because the electrode is not fixed to the oesophageal wall but rather to the nose, it can become displaced, or transiently ‘‘migrate’’ into the stomach in patients with large mobile hiatus hernias, thereby altering the amount of reflux that is recorded and potentially leading to erroneous recordings. Recently, a new pH monitoring system has been developed that overcomes these limitations of the catheter based system. The Bravo pH monitoring system uses a radiotelemetric capsule that is attached to the oesophageal wall and transmits pH data to a small receiver that is attached to the patient’s belt. The device is mounted on a delivery system that is passed transnasally or transorally into the oesophagus. The oesophageal mucosa is sucked into a well on the back of the capsule which is then fixed with a spring loaded pin that is passed tangentially through the mucosa. Once placed, the capsule is well tolerated by patients, with most reporting only a minor foreign body sensation. However, a small proportion (,5%) of patients can develop severe chest pain or odynophagia that requires removal of the capsule. 9 The capsule offers two principal potential advantages over conventional pH monitoring. Firstly, diet, physical activity, and quality of life are significantly less affected by the capsule compared with conventional pH monitoring. 10 Thus the monitoring period should much better reflect the patient’s usual circumstances of daily living. The major potential advantage of capsule pH monitoring however is the longer period of monitoring. Oesophageal acid exposure exhibits significant day-to-day variability 12 and 30%–50% of patients may have a different diagnosis if repeat 24 hour monitoring studies are performed, particularly if acid exposure values are in the region of the upper limit of normal. For undifferentiated patients with reflux disease, in comparison with 24 hour recordings, analysis of 48 hour recordings increases the sensitivity and discriminative value of acid exposure when considering either the total 48 hour period or the worst of the two 24 hour periods. 13 This benefit however does not appear to be present in patients with endoscopy negative reflux disease. A longer monitoring period also increases both the likelihood that symptoms will occur during the study and the number of symptom episodes available for association with reflux events, and has been reported to enhance the likelihood of detecting a positive symptom association. While comparisons of 24 with 48 hour recordings have been made using the capsule monitoring system, direct comparison between the performance of capsule and conventional pH monitoring methods has been lacking. In this issue of Gut, there are two reports of direct and simultaneous comparisons of the two techniques. Bruley des Varannes and colleagues studied 40 patients with symptoms suggestive of reflux disease (see page 1682). Concurrent conventional and capsule monitoring were performed for the first 24 hours after which the pH catheter was removed and capsule recordings were continued for a further 24 hours. Comparison of conventional with capsule recordings over the first 24 hours showed that the capsule recorded substantially (30%) less reflux whether measured by acid exposure or number of reflux episodes. Given that the capsule is reported to cause less interference with both diet and physical activity, this is a somewhat surprising result. However, the difference could be explained by failure of the capsule to record a large number of reflux episodes of short duration. As the study did not include a control group, an upper limit of normal had to be calculated by regression equation using published values for conventional pH recordings. After this adjustment, the concordance of the diagnosis of reflux disease based only on acid exposure was 88%. Symptom association, assessed using the symptom association probability, was similar with the two techniques, possibly because the recording time was similar. Unfortunately, no comparison was made between 24 hour conventional recordings and 48 hour capsule recordings. In the second study, Pandolfino and colleagues analysed in more detail the performance characteristics of the two recording techniques (see page 1687). They found that the capsule recorded almost three times the number of reflux episodes than did the catheter system, with the excess consisting predominantly of episodes of relatively short duration. While some of the difference between the two techniques could be explained by the lower sample rate of the capsule system (0.16 v 0.25 Hz), the major cause that accounted for 40% of the discrepancy was due to a calibration error in the catheter system that consistently measured a pH value 0.77 units below the ex vivo calibration. The capsule system on the other hand had minimal offset. How should the new technology be viewed? Is catheter based pH monitoring now obsolete? 1672 COMMENTARIES