• lower oesophageal sphincter
• acid exposure
• oesophageal squamocolumnar junction
• reflux oesophagitis

For a simple organ like the oesophagus, which functions primarily as a conduit, it is hard to imagine a more unpleasant neighbour than the stomach, a reservoir of concentrated acid constrained by a leaky valve. Studies using pH monitoring techniques developed in the 1970s have shown that the distal oesophagus of normal ambulatory individuals can be exposed to refluxed gastric acid (with pH <4) for up to 5% of a 24 hour monitoring period.¹ Conventionally, oesophageal pH monitoring has been performed with the pH sensor positioned at a level 5 cm above the lower oesophageal sphincter (LOS). In a study reported in this issue of Gut, Fletcher and colleagues² in Glasgow have found that conventional pH monitoring techniques have substantially underestimated the duration of acid exposure for the most distal segment of the oesophagus [see page 168].

To study acid exposure in the most distal oesophagus, the investigators devised a novel endoscopic technique for positioning a modified pH catheter, with two pH electrodes located 5 cm apart. A short prolene loop was attached to the catheter 5 mm below the distal pH electrode and the catheter was passed through the nose into the oesophagus. An endoscopic examination was performed, the junction of squamous and columnar epithelia (the Z line) in the distal oesophagus was identified, and an endoscopic clip was used to fix the prolene loop to the Z line. In this way, the distal pH electrode was anchored in the distal oesophagus at a level 5 mm above the Z line whereas the proximal electrode …