Figure 7 - Simultaneous readings of esophageal pH, muscle layer thickness, and manometric pressure in patients with heartburn.

Sustained esophageal contractions (SECs) detected by an increase in esophageal muscle thickness on high-frequency intraluminal ultrasound precede symptoms of heartburn in the absence of pathologic acid reflux detected on pH recording. (Source: Pehlivanov et al.,⁵⁶ with permission from American Physiological Society.)