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9 Esophageal Testing Using Multichannel Intraluminal Impedance
Mohamed Khalaf1,2 and Amit Agrawal1,3
1 Medical University of South Carolina, Charleston, SC, USA
2 University of Alexandria, Alexandria, Egypt
3 Northern Light Eastern Maine Medical Center, Bangor, ME, USA
Introduction
Multichannel intraluminal impedance (MII) is a relatively new technique that enables detection of retrograde and antegrade bolus movement along the esophagus. This chapter reviews the basic principles of intraesophageal impedance measurement and its application to esophageal function testing (EFT) and gastroesophageal reflux (GER) monitoring. The clinical and research applications of impedance combined with manometry for EFT and impedance combined with pH‐metry for GER monitoring are discussed in greater detail in Chapter 11.
Basic principles
The technique for assessing intraluminal bolus movement using impedance measurements was initially described by Silny et al. in the early 1990s [1]. Intraesophageal impedance is determined by measuring electrical conductivity across a pair of closely spaced electrodes within the esophageal lumen. Impedance in an esophageal measuring segment depends on the cross‐sectional area of the organ and the conductivity of the material through which the electrical current must travel. Conductivity, in turn, is determined by the ionic content of the material surrounding the impedance measuring segment. Since the esophageal mucosa, air, and any given bolus material (i.e. swallowed food, saliva, or refluxed gastric contents) each has different ionic content and thus different electrical conductivity, they all produce a different change in impedance.
Combined videofluoroscopy–impedance measurements have validated changes observed with bolus entry, presence, and clearing in the impedance‐measuring segment [1, 2] (Figure 9.1). In the absence of bolus, impedance is determined by the electrical conductivity of the esophageal lining. Upon arrival of bolus between the electrodes, impedance may increase abruptly as a result of the presence of a pocket of air in front of the head of the bolus. Intraluminal impedance then rapidly decreases as the high ionic content of the bolus provides good electrical conductivity. While the bolus is present in the impedance measuring segment, intraluminal impedance remains low. Esophageal contractions clearing the intraluminal content may increase the impedance, with a slight “overshoot” caused by a decrease in esophageal cross‐section during contraction before returning to baseline.
By placing a series of conducting electrodes in a catheter that spans the length of the esophagus (Figure 9.2), changes in impedance at multiple segments (hence the term multichannel intraluminal impedance, or MII) can be recorded in response to movement of intraesophageal material in either the antegrade or retrograde direction. Antegrade bolus movement (i.e. a swallow) is detected by impedance changes of bolus presence progressing from the proximal to the distal esophagus (Figure 9.3a). Retrograde bolus movement (i.e. reflux) is detected by changes in impedance progressing distal to proximal, followed by proximal to distal clearance of bolus (Figure 9.3b).
By incorporating impedance electrodes into classic manometry or pH catheters, MII now complements traditional esophageal motility and pH testing. Combined MII and manometry (MII–EM) provides simultaneous information on intraluminal pressure changes and bolus transit, whereas combined MII and pH (MII–pH) allows detection of reflux episodes regardless of their pH values, i.e. acid and nonacid (also called weakly acidic) reflux.
Figure 9.1 Impedance changes observed during bolus transit over a single pair of measurement rings separated by 2 cm. A rapid rise in impedance is noted when air traveling in front of the bolus head reaches the impedance measuring segment followed by a drop in impedance once higher conductive bolus material passes the measuring site. Bolus entry is considered at the 50% drop in impedance from baseline relative to nadir and bolus exit at the 50% recovery point from nadir to baseline. Lumen narrowing produced by the contraction transiently increases the impedance above baseline.
Figure 9.2 Multiple impedance measuring segments within the esophagus allow determination of direction of bolus movement within the esophagus; i.e. multichannel intraluminal impedance (MII).
Figure 9.3 Movement of intraesophageal material detected by multichannel intraluminal impedance. (A) Antegrade bolus movement (i.e. a swallow) is characterized by sequential drops in impedance beginning at the proximal esophagus and progressing toward the distal esophagus. (B) Retrograde bolus movement (i.e. gastroesophageal reflux) is characterized by sequential drops in impedance beginning at the distal esophagus and moving upward toward the proximal esophagus.
High‐resolution impedance manometry
In the early years, combined MII‐EM and high‐resolution manometry (HRM) were developed in competition with one another until technical and software development enabled device companies to offer combined high‐resolution impedance manometry (HRiM) systems.
The combined HRiM gives a unique tool that can assess pressure changes from the hypopharynx to the proximal stomach and decide the effectiveness of esophageal peristalsis to move boluses through the esophagus. The increased number of impedance channels in HRiM allows intraluminal impedance to be displayed as a color contour plot. The display of the impedance data can be done by overlaying data from individual impedance‐measuring segments on the pressure topography colors (Figure 9.4). Impedance is displayed using a single‐color gradient (i.e. gray or purple) to overlay on pressure data or using split screens. In contrast to the line tracing of the conventional intraluminal impedance, complete bolus transit is defined in the color contour plot by the absence of residual bolus color after a swallow, allowing a more reproducible and easier interpretation [32]. A study of 10 healthy volunteers