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Interpretation of an HRM study is aided by several quantifiable metrics and patterns that can be assessed by analysis software after appropriate placement of manometric landmarks by an interpreter. These include the IRP, DCI, and distal latency (DL) (Figure 8.2):

 The IRP is the measure of deglutitive EGJ relaxation and is the primary metric to define an EGJ outflow obstruction. The IRP is calculated as the mean of the 4 s (contiguous or noncontiguous) of the lowest pressure at the EGJ during a 10 s window following the swallow and referenced to gastric pressure [33, 34]. The axial borders of the IRP measurement window should span the length of the EGJ, including swallow‐associated esophageal shortening, if observed. Because the IRP is referenced to gastric pressure, proper placement of the gastric reference is important, typically placed 2 cm below the distal border of the EGJ. However, placement should be adjusted if there appears to be a pressure artifact.

 The DCI is a measure of peristaltic (contractile) vigor and calculated as the pressure amplitude x duration x axial length (mmHg•s•cm) of the distal esophageal contraction proximal to the EGJ and referenced to 20 mmHg so as to exclude pressure artifacts [3, 35]. The DCI is the primary determinant of hypercontractile swallows (high DCI) as well as weak and failed peristalsis (low DCI) [3, 4, 36, 37].

 The DL reflects the duration of deglutitive inhibition of the distal esophageal contraction and is used to define “spastic” or premature contractions [4, 38, 39]. The DL is the time interval (seconds) from the onset of the swallow (as noted by the relaxation of the UES) to the contractile deceleration point (CDP) [39, 40]. The CDP is the most distal inflection point in the velocity of the peristaltic wavefront (localized on the 20 mmHg isobaric contour within 3 cm of the EGJ) [41]. If compartmentalized pressurization (Figure 8.3B) is present, the CDP should be localized on an isobaric pressure contour set to be greater than the pressure within the zone of compartmentalized pressurization.Figure 8.3 Stepwise classification of individual swallows on HRM/EPT. Application of EPT metrics provides a scheme with which to classify individual swallow types. Specific swallow types are identified in dark purple boxes.Contractile front velocity: The velocity (cm/s) of the contractile front can also be measured; rapid contractions are sometimes considered simultaneous or “spastic” contractions. However, owing to concerns about the specificity and variability of this measure, the Chicago Classification uses only the DL to define “spastic” contractions [4, 21, 39].

 Peristaltic breaks: Using the isobaric contour tool set at 20 mmHg, the peristaltic wave is assessed for peristaltic breaks, or gaps in the isobaric contour. Breaks are considered significant if they are > 5 cm in axial length because these are associated with impaired bolus clearance as determined by HRIM.

 Pressurization pattern: Pressurization of > 30 mmHg spanning from the UES to the EGJ is termed panesophageal pressurization and is the defining feature of type II achalasia (see below). Compartmentalized pressurization, i.e. pressurization between a progressing esophageal contraction and the EGJ (Figure 8.2B), may be indicative of esophageal outflow obstruction.