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When MII is combined with pH, a distinction between acid and nonacid reflux can be made because MII detects the presence of the refluxate, whereas pH simply determines the acid or nonacid nature of the refluxate. Furthermore, MII–pH enables the measurement of additional reflux episodes during an ongoing acid reflux episode, so‐called re‐reflux. Examples of GER of three types (acid, nonacid, and re‐reflux) recorded with a six‐impedance/one‐pH catheter are shown in Figure 9.11.

Figure 9.11 Impedance changes in ohms during three episodes of reflux. The six impedance measuring segments (Z1–Z6) and pH changes are shown on the y‐axis. The dotted line marks a pH of 4.0. (A) Point a indicates the proximal extent of the reflux event. It is preceded by a sequential drop in impedance starting at the most distal measuring segment that proceeds toward the proximal esophagus. Arrival of the refluxate into the distal esophagus causes a fall in pH to below 4.0 (point b), an acid reflux episode. (B) The proximal extent is indicated by point a. This is not accompanied by a fall in pH to below 4.0 and is thus considered an episode of nonacid reflux. (C) Reflux detected by multichannel intraluminal impedance (MII) (point a) causes a fall in pH to below 4.0 (point b). A second MII‐detected reflux episode (re‐reflux, point c) occurs before the pH returns to 4.0.

Assisted by the observation that air and gastric contents each produce a different change in impedance, reflux episodes can be characterized as containing gas, liquid, or both. Air conducts electricity poorly and therefore has high impedance, whereas liquid gastric contents have low impedance. As illustrated in Figure 9.12, gas increases intraluminal impedance, and liquid gastric contents decrease impedance.

Identification of reflux episodes requires visual analysis of changes in the multiple impedance measuring segments, making interpretation of MII–pH tracings more time‐consuming compared to that of conventional pH, which is fully automated. There is available software that enables automated detection of reflux episodes in the MII–pH tracing. However, the only published study evaluating automated analysis found that it tends to overestimate the number of reflux episodes [30]. Therefore, the recommended approach is to use the automated analysis software to mark the MII–pH tracing, and this is then reviewed and edited manually. Relying solely on the automated analysis software is not currently advised, but it is possible that software improvements will result in full automation in the future. It is important to mention that a low impedance baseline, which may be seen in patients with esophagitis, Barrett’s esophagus, and fluid stasis (i.e. achalasia) can make interpretation of the MII–pH tracings quite difficult [49].

Figure 9.12 Impedance changes in ohms during reflux of gas, liquid, and mixed contents, obtained with a catheter incorporating six impedance measuring segments (Z1–Z6), which are shown on the y‐axis. Impedance values for the second measuring segment (Z2) are shown for the three reflux episodes. (A) Reflux of gas is characterized by sharp increases in impedance beginning in the most distal recording segment and rapidly progressing upward toward the proximal esophagus. In Z2, impedance rises from a baseline of 1360 ohms to a peak of 10,000 ohms. (B) Reflux of liquid is characterized by sequential impedance falls, starting in the distal esophagus and moving upward toward the mouth. In Z2, impedance falls from a baseline of 3600 ohms to a trough of 260 ohms. (C) Reflux of gas mixed with liquid. In Z2, impedance rises from a baseline of 2860 ohms to a peak of 10,000 ohms; this is followed by an impedance fall to a trough of 350 ohms.

In summary, MII–pH enables refined characterization of the reflux episode. Because it measures both volume presence (through MII) as well as changes in acidity (through pH), it enables detection of reflux of all types—acid, nonacid, and re‐reflux—while providing details about the volume and acid clearance, as well as the composition and height reached by the refluxate.