an important advance in GER testing because it enables accurate detection of GER at all pH levels: impedance detects retrograde bolus movement (i.e. reflux), while pH measurement establishes the acidity of the reflux episode (acid if pH < 4.0; nonacid otherwise). The technique has been validated fluoroscopically and manometrically to detect bolus movement in the esophagus, both in the oral and aboral direction [24], thus allowing measurement of and distinction between swallows and reflux. As discussed earlier, a catheter with multiple pairs of impedance electrodes can record impedance changes in response to movement of intraesophageal material in either antegrade or retrograde direction. As shown in Figure 9.3, this means that a swallow can be clearly distinguished from reflux, i.e. retrograde movement of gastric contents into the esophagus. Because MII records retrograde flow of gastric contents into the esophagus in a pH‐independent fashion, combining the technique with pH‐metry enables detection of acid as well as nonacid reflux. Some authors classify reflux with a pH above 4.0 as either weakly acidic (pH ≥ 4 but <7) or weakly alkaline (pH ≥ 7). In this chapter, nonacid reflux refers to any reflux with a pH of 4 or greater.
Table 9.2 Normative data for reflux using combined impedance‐pH monitoring.
| Study | n | Number of reflux episodes: upper limit of normal* | ||
|---|---|---|---|---|
| Total | Acid | Nonacid | ||
| United States (Shay et al. [27]) | 60 | 73 | 59 | 27 |
| France–Belgium (Zerbib et al. [28]) | 72 | 75 | 50 | 48 |
* Upper limit of normal for the number of reflux episodes was based on the 95th percentile for the healthy volunteers in each study.
Because of its ability to measure GER at all pH levels, MII–pH monitoring has emerged as a useful diagnostic and research tool, particularly in patients with ongoing symptoms despite acid suppression [12]. Additionally, MII–pH provides very meticulous characterization of the reflux episode, including determination of the composition (gas, liquid, mixed), proximal extent (height reached), velocity, and clearance time [9]. Based on the very detailed information that MII–pH monitoring provides, a recently convened panel of experts concluded that it is the most sensitive tool for measuring GER [25]. Assessment of reflux with MII–pH has been found to be reproducible [26], and normal values for ambulatory 24‐h MII–pH monitoring obtained by independent multicenter studies are similar (Table 9.2) [27, 28].
MII–pH catheter characteristics and placement
MII–pH monitoring can be performed with several different catheters (usually no more than 2 mm in diameter) that incorporate a varying number of impedance and pH electrodes in different configurations. The catheter is placed transnasally and is connected to a data logger that the patient carries for 24 h. Ordinarily, the catheter has a pH electrode for placement 5 cm above the LES (similar to conventional pH testing), with the possibility of additional pH sensors in the stomach or proximal esophagus. Catheters may incorporate six or more impedance measuring segments (each composed of two metal electrodes usually spaced 2 cm apart) to detect impedance changes along variable lengths of the esophagus. While there are no published studies comparing the accuracy of different catheter configurations, a catheter with six impedance measuring segments and one pH electrode (Figure 9.10) enables accurate detection of reflux episodes with an assessment of proximal extent of reflux, and it permits distinction between swallowing and reflux episodes; a smaller number of impedance measuring segments may compromise accuracy [29].
Figure 9.10 Schematic representation of the 2 mm diameter multichannel intraluminal impedance (MII)–pH catheter with impedance electrodes (4 mm in length) set in pairs at 2 cm intervals, allowing for six impedance measuring segments, as well as one pH electrode. Once properly positioned, this catheter allows recording of pH at 5 cm above the lower esophageal sphincter (LES) and impedance in six measuring segments, with their centers 3, 5, 7, 9, 15, and 17 cm above the LES.
Source: Vela [9] with permissions of Wolters Kluwer.
MII–pH interpretation
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,