Clinical Utility of the Ratio of Sound Pressure Level to Subglottal Pressure in Patients Surgically Treated for Phonotraumatic Vocal Fold Lesions

Abstract

Purpose: This study aimed to determine whether a simplified, and potentially more stable, acoustic-aerodynamic voice outcome ratio (ratio of sound pressure level [SPL] to subglottal pressure) is comparable to a traditional vocal efficiency measure (ratio of acoustic power to the product of average subglottal pressure and average phonatory airflow) in terms of the ability to detect change in vocal function following surgical removal of bilateral phonotraumatic lesions.

Method: Pre- and postoperative acoustic and aerodynamic measures were analyzed retrospectively from 75 female patients who underwent surgical removal of bilateral phonotraumatic lesions. A 2 × 2 repeated-measures analysis of variance was conducted for each of three acoustic-aerodynamic voice outcome ratios-traditional vocal efficiency, an SPL-based ratio with both airflow and subglottal pressure, and a simplified SPL-based ratio with subglottal pressure only-to investigate the main effects of treatment stage (pre- and postsurgery), loudness condition (comfortable and loud), and their interaction. Post hoc paired samples t tests were conducted for statistically significant interactions. The within-subject variability of the measures was assessed using the coefficient of variation.

Results: Although exhibiting an expected main effect of loudness (higher values in the loud condition), the traditional vocal efficiency ratio did not exhibit a main effect of treatment. For both SPL-based ratios, there were significant main effects of treatment stage (higher values postoperatively) and loudness condition (lower values in the loud condition). Within-subject, postoperative changes in the two SPL-based ratios moderately correlated with one another. The simplified ratio of SPL to subglottal pressure (without airflow) exhibited the least within-subject variability relative to the other two acoustic-aerodynamic ratios.

Conclusions: These findings indicate that SPL-based acoustic-aerodynamic voice outcome ratios increase significantly following the surgical removal of phonotraumatic vocal fold lesions. The simplified ratio of SPL to subglottal pressure exhibits the least variability and can be easily obtained without requiring the measurement of airflow.

Figure 1.

Plots of mean values for the (A) ratio of acoustic sound pressure level to aerodynamic power (SPL/AP) and (B) ratio of acoustic sound pressure level to subglottal pressure (SPL/Ps) for the patient group in comfortable (Comf) and Loud loudness conditions before and after surgical intervention for phonotraumatic vocal fold lesions. Error bars represent 1 SD above and below the mean within each condition.

Figure 2.

Plot of the change (postsurgical value minus presurgical value) in SPL/AP and SPL/Ps in the (A) comfortable (Comf) and (B) Loud loudness conditions. Linear regression lines indicate the slope, intercept, and strength of the correlation between the two measures.