In absence of hiatal hernia, pressure signals resulting from the lower esophageal sphincter (LES) and crural diaphragm (CD) are not distinguishable. As a consequence the term “EGJ relaxation” is used. Moreover, during swallowing, the pressure measured at the level of the EGJ is the composite of LES pressure, CD contraction, and intrabolus pressure as the swallowed bolus traverses the EGJ. Finally, the LES elevates during swallowing due to esophageal longitudinal muscle contraction. Hence, accurate identification of the axial limits of the EGJ is essential to avoid the artifact of pseudo-relaxation created when the sphincter elevates above the zone of measurement.

The EPT metric which best discriminated normal EGJ relaxation from impaired EGJ relaxation is the integrated relaxation pressure (IRP) [5]. The IRP is derived from an e-sleeve measurement applied at the level of the EGJ within a 10-s window beginning with deglutitive UES relaxation. The IRP is the mean pressure for the 4 s (continuous or noncontinuous) during which the e-sleeve value is least within this window. It is influenced not only by LES relaxation but also by CD contraction and intrabolus pressure. By convention, the IRP is referenced to intragastric pressure. Using the Given adult version circumferential HRM sensing device (Given Imaging, Los Angeles, CA), the upper limit of normal for the IRP is a mean of 15 mmHg for ten swallows in the supine posture. Appropriate normal values for other devices and postures need to be established.

Although the Chicago Classification was not intended to describe EGJ pressure morphology or barrier function, this has been characterized to some degree with EPT. The EGJ can be classified into three subtypes based on the axial relationship between the LES and the CD [6]. With EGJ type I, the LES and CD are superimposed. With EGJ type II there is a minimal separation between LES and CD (1–2 cm). Finally, EGJ type III is characterized by a separation >2 cm at inspiration between the LES and CD. Type III is the HRM signature of hiatal hernia. In a logistic regression analysis, the magnitude of inspiratory augmentation of EGJ pressure was found to be the only manometric variable independently associated with GERD [6]. Patients with esophagitis or increased esophageal acid exposure on ambulatory pH monitoring had significantly less inspiratory EGJ augmentation when compared to asymptomatic controls or patients with negative pH monitoring studies.

The architecture of the post-deglutitive esophageal contraction is characterized by 2–3 pressure troughs (proximal, distal, and sometimes one in the middle) [7] and a contractile deceleration point (CDP) [8] (Fig. 3.1). The proximal trough is also called the “transition zone.” The CDP is an inflection point in the contractile wavefront velocity corresponding to the transition from the peristaltic propagation to the late phase of esophageal emptying, which proceeds much slower than the peristaltic propagation. Thus the contractile front velocity (CFV) is measured from the proximal pressure trough to the CDP as the slope of the tangent to the 30-mmHg isobaric contour of the contractile wavefront. However, a more important indicator of the integrity of the inhibitory pathway in the distal esophagus is the contraction latency at the CDP. Hence, the distal latency (DL), measured from UES relaxation to the CDP, is used in the Chicago Classification to define premature esophageal contractions, indicative of spasm [9] (Fig. 3.2).

The metric developed to describe the vigor of the distal esophageal contraction in the Chicago Classification is the distal contractile integral (DCI) [10]. It is measured as the “volume” of the esophageal contraction spanning from the proximal pressure trough to the EGJ. The DCI is the product of the integral of the amplitude (greater than 20 mmHg), the duration, and the length of the contractile segment between the proximal trough and the EGJ (Fig. 3.1). The upper limit of normal (95th percentile) is 5,000 mmHg-s-cm and a value greater than 8,000 mmHg-s-cm was never encountered in a normal population. Lower limits for the DCI were also recently defined: a DCI <150 mmHg-s-cm identifies failed peristalsis and a DCI <450 mmHg-s-cm defines weak peristalsis [11].

Each swallow is characterized for the integrity of the contraction, contraction pattern, and intrabolus pressure pattern (Table 3.1). Contraction integrity should first be assessed based on the integrity of the 20-mmHg isobaric contour. Interruptions, or breaks, in the 20-mmHg isobaric contour between the UES and the EGJ are characterized by their axial length; breaks 2 cm or longer are considered significant based on correlation with impaired bolus transit [12]. Large breaks are >5 cm in length and small breaks are 2–5 cm. Contractions with small or large breaks are defined as weak contractions.

Intact contractions and weak contractions with small breaks are further characterized by DL, DCI, and CFV. These metrics are used to define premature contractions (DL <4.5 s; Fig. 3.2), hypercontractility (DCI >8,000 mmHg-s-cm; Fig. 3.3), and rapid contraction (CFV >9 cm/s), summarized in Table 3.1. Finally, each swallow is characterized according to the associated pattern of intrabolus pressure, assessed with the 30-mmHg isobaric contour. Intrabolus pressure is qualified as panesophageal pressurization if it spans from UES to EGJ, as compartmentalized pressurization when it is restricted to the segment between the esophageal contraction and one sphincter, or normal when there is no bolus pressurization to greater than 30 mmHg. Panesophageal pressurization can be distinguished from a simultaneous contraction by analyzing the corresponding spatial pressure variation plot. In instance of panesophageal pressurization, the spatial pressure variation plot between sphincters is flat, without regional variation. On the other hand, in instances of simultaneous esophageal contractions, pressure variations are visualized along the esophageal body (Fig. 3.4).