Changes Over Time in Masseter Muscle Activity, Symptoms of Discomfort, Stress Level and Salivary Flow Rate Following Intra-Oesophageal Acid Infusion.

Background: Intra-oesophageal acid stimulation increases masseter muscle activity. However, the infusion speeds used in previous studies are significantly slow, with minimal acid volume (30 mL over 30 min). Additionally, it is unclear how masseter muscle activity, symptoms of discomfort, stress levels and saliva quantity change over time.

Objectives: The time course of masseter muscle activity, heartburn and discomfort symptoms, salivary cortisol concentration to assess stress, autonomic nervous system (ANS) activity and saliva quantity under the influence of intra-oesophageal acid infusion were evaluated at a faster injection rate and larger volume than in previous studies.

Methods: Ten healthy adults underwent polygraphic monitoring, consisting of electromyography of the masseter muscle and electrocardiography during intra-oesophageal acid infusion (10 mL/min, 10 min). Symptoms of heartburn and discomfort were assessed using the visual analogue scale (VAS), and saliva quantity was measured. Friedman's test was used for multiple comparisons.

Results: Masseter muscle activity, VAS scores for heartburn and discomfort, as well as saliva quantity during the 10-min acid infusion, increased significantly compared with that before acid infusion (p < 0.001, 0.001, 0.019 and 0.047, respectively) and decreased 10 and 20 min after acid infusion (p = 0.004, 0.004 and 0.007, respectively). No significant changes were observed in the salivary cortisol concentration or ANS activity.

Conclusion: Intra-oesophageal acid infusion stimulated symptoms of heartburn and discomfort and increased masseter muscle activity and saliva production, which may not be related to psychological stress.