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Impedance sensors can detect luminal air or liquid by measuring the associated changes in electrical resistance within the esophageal lumen. Incorporation of multiple impedance electrodes onto HRM catheters allows for the simultaneous imaging of bolus transit and clearance as they relate to EPT. Early versions of HRIM assessed bolus transit and bolus clearance in a dichotomized manner: complete or incomplete. Bolus entry at the site of impedance measurement was indicated by a decrease in impedance by 50% from baseline and bolus exit from the site if/when the impedance signal returned to baseline [9, 10, 59]. Bolus transit could then be considered complete if the bolus entered at the most proximal of intraesophageal impedance site and exited at the most distal of the intraesophageal impedance site. In contrast, bolus transit was considered incomplete if the bolus did not exit the distal esophageal impedance site. When utilizing this paradigm for dichotomizing bolus transit, abnormal bolus transit was detected among patients with manometrically defined motor disorders such as IEM and achalasia [10, 59]. However, the clinical significance of the added impedance findings remained undefined.

Table 8.3 Summary of adjunctive and provocative maneuvers used in clinical manometry studies.

Maneuver	Description	Yield	Clinical utility
Upright swallows	5 ml swallows in an upright, seated position	EGJ obstruction	Clarify EGJ outflow obstruction
Multiple rapid swallows	5 sequential 2 ml swallows 2 3 s apart	Contractile reserve	GERD Before antireflux surgery
Rapid drink challenge	Free drinking 200 ml as fast as possible	EGJ obstruction	Dysphagia
Solid swallows	Cracker Bread (1 cm3) Marshmallow	EGJ obstruction Peristaltic disorders	Dysphagia
Viscous swallows	Applesauce	EGJ obstruction Peristaltic disorders	Dysphagia
Test meal	Standardized (pasty; 200 g soft rice) Nonstandardized (symptom‐inducing meal)	EGJ obstruction Peristaltic disorders Rumination and belching disorders	Dysphagia GERD (regurgitation or belch predominant)

An updated methodology for HRIM interpretation was developed to objectively measure components of bolus flow timing, bolus retention, pressurization, and luminal distension using a pressure‐flow analysis paradigm termed automated impedance manometry (AIM) [11,60–62]. These techniques have been applied to bolus flow during both oropharyngeal and esophageal bolus transit [13, 63, 64]. Multiple pressure‐flow parameters are generated using AIM, which have demonstrated differences between healthy controls and patient cohorts, including post‐fundoplication dysphagia and nonobstructive dysphagia [11, 61, 62]. Additionally, improvement in symptom perception associated with test swallows was demonstrated when compared to standard HRM parameters, particularly with viscous and solid rather than with liquid boluses [62, 65, 66].

The bolus flow time and esophageal impedance integral are additional HRIM parameters developed to utilize the impedance data optimally in relation to manometric data. Bolus flow time provides a measurement of trans‐EGJ bolus flow by determining bolus presence within the EGJ using impedance and then a flow‐permissive pressure gradient (i.e. higher pressures in the esophageal body than at the EGJ or stomach) to determine the duration of bolus flow associated with a test swallow [12]. The esophageal impedance integral ratio quantifies esophageal clearance by measuring the residual bolus following a 5 ml liquid or viscous test swallow [67]. The bolus flow time and esophageal impedance integral ratio have been shown to correlate with symptom scores and clinical outcomes in patients with achalasia and major motor disorders as well as with symptom scores in patients without major motor disorders [68–70]. Yet another HRIM measure, the impedance bolus height, quantifies esophageal retention after a 200 ml rapid liquid drink in an upright posture by measuring the height of the residual fluid column after five minutes, analogous to a timed‐barium esophagram [71].

Another clinical application of HRIM that may be particularly useful is in the evaluation of suspected GERD that is not responding as expected to therapy, especially when the predominant symptom is regurgitation or belching. In addition to detecting major motor disorders and compromised EGJ barrier function in such patients, HRIM facilitates the detection of behavioral disorders, particularly rumination syndrome and supragastric belching [72]. Both rumination events and supragastric belches have an objective appearance on HRIM (Figure 8.7) that can be detected during extended HRIM monitoring after ingestion of a patient‐identified provocative meal, i.e. a test meal with post‐prandial monitoring [73–76]. While both are behavioral disorders that can often be clinically diagnosed from a patient’s history and/or observation of eating, HRIM gives objective evidence of these diagnoses to both clinicians and patients and provides a method for biofeedback during initiation of the behavioral therapy, typically diaphragmatic breathing [77].

Figure 8.7 Rumination and supragastric belching. Examples: a rumination event (A) and supragastric belching (B) observed during a post‐prandial HRIM test. The impedance signals are depicted by both line tracings and the interpolated purple coloration. (A) The rumination event is evident by: (1) an increase in gastric pressure (> 30 mmHg); (2) retrograde flow of gastric contents (dashed arrow) with the presence of liquid identified by the decrease in impedance signal; (3) an increase in intra‐esophageal pressure (difficult to visualize in the example due to the impedance signal); and (4) relaxation of the upper esophageal sphincter (UES). (B) A supragastric belch can be identified by: (1) negative intra‐thoracic pressure; (2) UES relaxation; (3) antegrade air flow (impedance increase); and (4) retrograde air flow – the injected air does not cross the EGJ (hence, supragastric belch). In some cases, such as in this example, the supragastric belching behavior may be repetitive (dashed box).

Source: Used with permission from the Esophageal Center at Northwestern University. Data from Kessing, Bredenoord, and Smout [74]; Kessing, Bredenoord, and Velosa [75].