Subglottic Pressure and Immersion Depth: Key Determinants of Voice Outcomes in Water Resistance Tube Exercise

The present study aimed to evaluate the effects of different immersion depth levels and subglottic pressure (P_sub) levels on the oscillatory features of oral pressure (P_oral; frequency, amplitude, and regularity), the degree of glottal adduction, and the subjective perception of phonatory ease when performing a water resistance tube (WRT) exercise.

Forty participants were recruited: 20 having normal voice and 20 diagnosed with functional dysphonia confirmed by laryngoscopic examination. Assessment protocol included (a) the self-assessment of ease of phonation and (b) the objective assessment of air pressure-related variables and electroglottographic (EGG) contact quotient (CQ). Participants in both groups were instructed to select and produce a sustained-vowel-like phonation into a tube immersed in a recipient filled with water at five different depths: 2, 4, 6, 8, and 10 cm (Experiment 1). Then, they were required to phonate into the tube with three different P_sub targets (10, 15, and 20 cm H₂O) while keeping a constant immersion level (Experiment 2). Participants were asked to self-assess the degree of ease of phonation. Objective measurements of air pressure-related variables and CQ were acquired during WRT exercises.

Changes in depths and P_sub levels showed significant effects on CQ and massage-like sensation in both groups. P_oral oscillation variables (amplitude, frequency, jitter, and shimmer of oscillation) were significantly influenced by changes in the depth of immersion and P_sub values. No significant differences were found for most variables when comparing both groups.

Findings confirm that modifying immersion depth and P_sub during WRT exercises induces measurable changes in vocal physiology. Across both experiments, higher loads (greater immersion or increased P_sub) led to a progressive increase in CQ, accompanied by higher P_oral instability and a significant reduction in perceived ease of phonation. Also, both P_sub and immersion depth were shown to influence bubble characteristics, P_oral oscillations, and potentially the massage-like sensation associated with WRT. Thus, careful control of these variables appears essential to optimize therapeutic outcomes in patients with voice disorders.