Figure 6.

 Definitions of reflux are based on the pH of the refluxate. Acid reflux (A) is defined as reflux that reduces oesophageal pH to below 4 or reflux that occurs when oesophageal pH is already below 4. Superimposed acid reflux is a subcategory of acid reflux. Weakly acidic reflux (B) is defined as a pH fall of at least 1 unit where the pH does not fall below 4, and a pH of 7 is the cut off between “weakly acidic” and “non-acid reflux”. Weakly alkaline reflux (C) is defined as a reflux episode during which nadir oesophageal pH does not drop below 7.