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Capsule pH monitoring: is wireless more?
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1. R H Holloway
1. Correspondence to:
Dr R Holloway
Department of Gastroenterology, Royal Adelaide Hospital, North Terrace, Adelaide, SA 5000, Australia; rhollowamail.rah.sa.gov.au

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Wireless pH monitoring is a significant advance for intraluminal pH recording. It will provide more meaningful data for evaluation of patients and, hopefully, more discriminative diagnosis

Since its introduction in 1974,1 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.2,3

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,4,5 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 recorded6 and potentially leading to erroneous recordings.7

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.8 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.8,9

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.8,10 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 variability11,12 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.8,13 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.13

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 colleagues14 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,15 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 colleagues16 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?

There is no doubt that capsule pH monitoring offers important advantages over conventional methods. The capsule is better tolerated and allows patients to undertake their recordings with relatively little interruption to their daily activities. Thus the data should better reflect the real patterns of reflux. Given the reported restriction on diet and physical activity by conventional pH monitoring, one might expect values for acid exposure to be higher with capsule recordings. Indeed, that has been the case with two previous studies.8,17 While in the studies of Bruley des Varannes and colleagues14 and Pandolfino and colleagues16 the capsule recorded significantly lower levels of acid reflux, this is not a true test of the capsule as the recordings were performed with a pH catheter in place and only for 24 hours.

Direct comparison of the two methods however has revealed significant differences in the number of reflux episodes recorded, which highlights the need for specific normal values to be derived for the new methodology. Both studies noted a substantial under recording of reflux events by the capsule compared with the catheter based system. While the Pandolfino study16 recorded a larger number of reflux episodes even though it was performed in healthy volunteers, the authors included all drops in pH below 4 greater than one data point (four seconds for the catheter and six seconds for the capsule) whereas Bruley des Varannes and colleagues14 used a minimum duration of six seconds and had a higher threshold for cessation of the reflux event (pH 5 v pH 4.25) which would tend to eliminate a greater number of shorter duration events. In both studies, a proportion of the smaller number of reflux events scored by the capsule can be explained by the lower sample rate of the capsule, which samples pH data only once per six seconds compared with once per four seconds by the catheter, and which thereby reduces the number of short duration reflux events detected by the capsule. More worrying however is the discovery of a systematic calibration error in the catheter system which accounted for the greatest proportion of the discrepancy. It is perhaps surprising that this error was not detected earlier. Whether this is an error specific to the Slimline catheters or applicable to all catheter based systems is not clear and needs to be clarified. From a diagnostic point of view, the contribution of these short duration episodes to overall acid exposure is relatively small and provided that appropriate normal values are derived using the system in question, should not create problems.

The impact of capsule recordings on the assessment of symptom association is unclear at this stage. Depending on the contribution of short duration events to symptom generation, failure to detect such events by the capsule could either improve or impair symptom assessment. Certainly, symptom association needs to be carefully re-evaluated with the capsule system. Current data would suggest that the longer duration of recording appears to enhance the diagnostic efficacy of the test both in terms of acid exposure and symptom association. However, this improvement appears to be largely in patients with erosive disease and it is not clear whether the additional recording time improves the diagnosis in endoscopy negative reflux disease.8 Moreover, nowadays, diagnostic pH monitoring is increasingly restricted to patients who have not responded to a trial of acid suppression with a proton pump inhibitor and there are no data on the performance of capsule pH monitoring in this more select group of patients.

The additional recording power of the capsule comes at an additional cost. In Australia, the single use capsule costs approximately $400 compared with$200 for a multiple use pH electrode. In circumstances where the user pays, this substantial cost differential may not be an issue. However, in laboratories, such as my own, that are not funded on a per patient basis, any additional benefit of the capsule has to be balanced against increased cost. Thus it is not so much more bang for your buck but more bang for more bucks.

In summary, despite some limitations, the capsule is a significant advance for intraluminal pH recording. It will provide more meaningful data for evaluation of patients and, hopefully, more discriminative diagnosis. Details regarding normal values for capsule recordings remain to be finalised, and more data are needed in endoscopy negative patients and in those non-responsive to proton pump inhibitors. Wireless pH monitoring clearly offers more but how much more remains to be determined.

Wireless pH monitoring is a significant advance for intraluminal pH recording. It will provide more meaningful data for evaluation of patients and, hopefully, more discriminative diagnosis

## Footnotes

• Conflict of interest: None declared.