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Impedance has shown its ability to detect the movement of gas within the esophageal lumen as a high‐impedance signal, and intraluminal impedance has allowed for better characterization of belching. Two distinct patterns of belching have been described by using intraluminal impedance: aerophagia resulting in air moving into the stomach and then being expelled (gastric belching); and a second in which air rapidly enters the esophagus and is immediately expelled without reaching the stomach (supragastric belching) [39]. Interestingly, the frequency of gastric belching is similar in patients with excessive belching and healthy volunteers. This suggests that in the majority of patients, markedly excessive belching is likely related to supragastric belching [39]. Identification of supragastric belching with intraluminal impedance has also allowed for the development of effective therapies, most notably baclofen and diaphragmatic breathing [40–42]. Intraluminal impedance monitoring can also be used to diagnose rumination syndrome by finding a large increase in gastric (“R wave”) [43], leading to reversal of the esophagogastric pressure gradient. This is associated with impedance‐detected movement of gastric contents proximally through the esophagus (Figure 9.9). Several studies have subsequently reproduced these findings by administering standard meals during HRIM studies in patients with rumination syndrome [44–47]. This high gastric pressure distinguishes ruminators from patients with gastroesophageal reflux [48]. Further, a recent study suggested that the use of HRIM could not only confirm the diagnosis of rumination syndrome but also be used successfully to give biofeedback with diaphragmatic breathing when the patient is given a solid meal to provoke symptoms [47].

Figure 9.7 Classification of swallows by multichannel intraluminal impedance (MII) criteria. (A) Complete bolus transit if bolus entry is seen at the most proximal site (20 cm above lower esophageal sphincter, LES) and bolus exit points are recorded at all three distal impedance‐measuring sites (i.e. 15, 10, and 5 cm above the LES). (B) Incomplete bolus transit if bolus exit is not identified at any one of the three distal impedance‐measuring sites.

Normal EFT values for impedance parameters were proposed based on the 95th percentile in healthy volunteers (Table 9.1). Impedance parameters define a study as normal if at least 80% of liquid and at least 70% of viscous swallows show complete MII‐detected bolus transit [5]. When using liquid testing substances, a study is considered normal manometrically if it does not contain more than 40% ineffective and 10% simultaneous/premature swallows. It has been suggested that using a viscous swallow rather than a liquid swallow will help to increase the diagnostic yield of intraluminal impedance in the assessment of esophageal bolus clearance. Chen et al. [34] studied 18 patients with dysphagia and 14 healthy controls, and the results showed that viscous bolus transit was more likely to be incomplete among those with a motility abnormality (38% vs. 70%) despite similar measurements with liquids. In another study of 129 patients with non‐obstructive dysphagia and 111 controls, the impedance‐measured viscous bolus clearance among patients with dysphagia was significantly different compared with controls (P = 0.02), whereas no difference was seen using liquid bolus clearance (P = 0.12) [35].

Table 9.1 Normative data of impedance (MII) parameters of esophageal function testing (EFT) for 10 liquid and 10 viscous swallows in 43 healthy volunteers. 95th percentile values can be considered the upper limit of normal for the given parameters.

Source: Tutuian et al. [5] with permissions of Elsevier.

	Liquid	Viscous
	(n = 429)	(n = 425)
	Median	Percentile	Median	Percentile
		25th	75th	5–95th		25th	75th	5–95th
Bolus head advance time (s)
20–15 cm	0.2	0.1	0.3	0.0–0.7	1.0	0.6	1.5	0.2–2.5
20–10 cm	0.6	0.4	0.9	0.1–1.7	3.3	2.4	4.0	0.9–5.1
20–5 cm	1.3	0.8	2.2	0.5–5.0	4.9	4.3	5.6	2.8–7.4
Bolus presence time (s)
at 20 cm	1.7	1.1	2.7	0.6–5.9	1.9	1.2	2.9	0.8–5.0
at 15 cm	4.1	3.0	5.1	1.4–8.8	3.5	2.8	4.1	1.9–5.9
at 10 cm	5.3	4.5	6.3	3.5–9.9	3.4	2.6	4.3	1.9–7.6
at 5 cm	5.8	4.6	6.7	2.3–9.3	3.1	2.3	4.1	1.5–6.3
Segment transit time (s)
20–15 cm	4.4	3.3	5.4	1.6–9.0	4.6	4.0	5.3	2.8–7.3
15–10 cm	5.7	5.0	6.7	3.9–10.5	5.3	4.5	6.3	3.8–10.1
10–5 cm	6.6	5.8	7.6	4.5–10.6	4.9	3.9	6.0	3.0–8.3
Total bolus transit time (s)	7.2	6.6	8.2	5.2–11.9	7.9	7.0	9.0	5.9–12.4

Figure 9.8 Percentage of patients with complete bolus transit in 350 patients with various manometric diagnoses. While all patients with achalasia and scleroderma have incomplete bolus transit, more than half of patients with distal esophageal spasm (DES) and ineffective esophageal motility (IEM) have complete bolus transit (51% and 55%, respectively). Almost all patients with nutcracker esophagus, normal esophageal manometry, and isolated lower esophageal sphincter (LES) abnormalities have complete bolus transit for liquid.

Source: Modified from Tutuian and Castell [6].

In summary, the addition of MII to manometry (MII‐EM) incorporates two complementary techniques that, together, provide a more detailed evaluation of both aspects of esophageal function: esophageal contractile activity and bolus transit. The clinical applications of this approach are discussed in Chapter 5.