Dear Editor,

Pandolfino et al. and Bruley des Varannes et al. present data from simultaneous pH monitoring by wireless Bravo™ and catheter mounted antimony electrode systems.[1,2] Both studies demonstrate that the wireless system records significantly fewer reflux events; the effect on overall acid exposure was more limited. Further analysis revealed that events ‘missed’ by the Bravo™ system were shorter and less acidic than those detected by both systems. The authors suggest that the different recording characteristics of the two systems explain the apparent 'higher sensitivity' of the catheter pH system, including the lesser sampling rate of the Bravo™ system,[1,2] and systematic inaccuracy in catheter electrode calibration.[1] (Calibration in vivo was not checked by Bruley des Varannes et al; this group also used Medtronic equipment and the same systematic error could also have explained some of the discrepancy between the systems).

Beyond these technical concerns, the significance of short ‘reflux events’ is questionable because short drops to pH<4 can be caused by factors unrelated to gastro-oesophageal reflux. First, ingestion of mildly acidic fluids (despite instruction). Second, movement of the catheter relative to the mucosa;[1] i.e. ‘drying’ or ‘loss of contact’ of the catheter electrode (the internal reference of Bravo™ reduces this source of error). Third, movement of the catheter relative to the gastro- oesophageal junction (GOJ) during swallowing. The last point requires explanation: on swallowing the oesophagus shortens by several centimetres due to longitudinal muscle contraction,[3,4] an event that stabilizes the oesophageal wall and increases the effectiveness of peristaltic contraction and bolus transport.[5] Even greater shortening can occur during oesophageal spasm (6) and transient lower oesophageal sphincter relaxation (with or without acid reflux). As the oesophagus shortens the catheter electrode moves distally towards the GOJ and may pass into the proximal stomach, a region in which highly acidic conditions may be present.[7,8] Thus the catheter electrode may dip into the ‘acid pocket’[8] at the GOJ before relaxation of the oesophagus returns the catheter into its original position. This cannot occur with Bravo™ because it is fixed to the oesophageal wall. As a result a short drop in pH is recorded by the catheter electrode but not the Bravo™ system.

As stated by the authors, pH studies alone cannot explain the discrepancies between the catheter and Bravo™ systems. Studies that combine pH monitoring (chemical reflux and clearance) and multichannel intraluminal impedance (volume reflux and clearance) with manometry are required to define the physiology of these events and help determine their relevance (if any) in eliciting symptoms. Studies using polymodal measurements have shown that oesophageal volume clearance is considerably faster than chemical clearance.[9] Chemical clearance usually progresses by stepwise increases in pH as swallowing activity brings bicarbonate containing saliva into contact with acid reflux. However short pH drops (often <20s) can occur with peristaltic contractions and resolve without further swallowing activity. Short pH drops can also be seen during transient LOS relaxation, again resolving without swallowing activity. These observations strongly suggest that many short ‘reflux events’ recorded by catheter systems may be artefacts, related to oesophageal shortening rather than gastro-oesophageal reflux.

The Bravoâ„¢ system is a well tolerated alternative to catheter-based pH measurement, experience with the technique is increasing and normal values are being established. If the accuracy (specificity) of catheter based detection of reflux events is shown to be limited, the Bravoâ„¢ system may establish itself as the new standard for pH measurement in the investigation and diagnosis of gastro-oesophageal reflux disease.

Mark Fox MD markfox{at}doctors.org.uk

Figure 1. Concurrent high resolution manometry (HRM) and pH recording during a transient lower oesophageal sphincter relaxation (TLOSR) in a normal volunteer. The positions of the Bravo™ capsule, catheter electrode and (LOS) are indicated on the schematic diagram and the HRM plot. The TLOSR is associated with a brief oesophageal contraction with ~5cm shortening. The position of the Bravo™ capsule remains constant relative to the LOS and the Bravo™ pH recording remains at ~pH6; however the catheter electrode approaches the LOS and records a short pH drop to <pH4 as it enters the ‘acid pocket’. Relaxation of the oesophagus restores the position of the LOS and the catheter pH recording normalizes without swallowing activity. This ‘reflux event’ is an artifact, related to oesophageal shortening rather than gastro-oesophageal reflux.

References

1. Pandolfino J, Zhang Q, Schreiner M, Ghosh S, Roth M, Kahrilas PJ. Acid reflux event detection using the Bravoâ„¢ wireless vs the Slimlineâ„¢ catheter pH systems: why are the numbers so different? Gut 2005.

2. Bruley des Varannes S, Mion F, Ducrotte P, Zerbib F, Denis P, Ponchon T, Thibault R, Galmiche JP. Simultaneous recordings of oesophageal acid exposure with conventional pH monitoring and a wireless system (Bravo(R)). Gut 2005.

3. Pouderoux P, Lin S, Kahrilas PJ. Timing, propagation, coordination, and effect of esophageal shortening during peristalsis. Gastroenterology 1997;112:1147-54.

4. Kahrilas PJ, Wu S, Lin S, Pouderoux P. Attenuation of esophageal shortening during peristalsis with hiatus hernia. Gastroenterology 1995;109:1818-25.

5. Pal A, Brasseur JG. The mechanical advantage of local longitudinal shortening on peristaltic transport. J Biomech Eng 2002;124:94-100.

6. Fox M, Hebbard G, Janiak P, Brasseur JG, Ghosh S, Thumshirn M, Fried M, Schwizer W. High-resolution manometry predicts the success of oesophageal bolus transport and identifies clinically important abnormalities not detected by conventional manometry. Neurogastroenterol Motil 2004;16:533- 42.

7. Fletcher J, Wirz A, Henry E, McColl KE. Studies of acid exposure immediately above the gastro-oesophageal squamocolumnar junction: evidence of short segment reflux. Gut 2004;53:168-73.

8. Fletcher J, Wirz A, Young J, Vallance R, McColl KE. Unbuffered highly acidic gastric juice exists at the gastroesophageal junction after a meal. Gastroenterology 2001;121:775-83.

9. Koek GH, Vos R, Flamen P, Sifrim D, Lammert F, Vanbilloen B, Janssens J, Tack J. Oesophageal clearance of acid and bile: a combined radionuclide, pH, and Bilitec study. Gut 2004;53:21-6.

10. Sifrim D, Castell D, Dent J, Kahrilas PJ. Gastro-oesophageal reflux monitoring: review and consensus report on detection and definitions of acid, non-acid, and gas reflux. Gut 2004;53:1024-31.