A pressure-regulated model of normal and pathologic phonation

Vocal Efficiency Measurements in Subjects with Vocal Polyps and Nodules: A Preliminary Report

Vocal efficiency is a quantitative measure of the ability of the larynx to convert subglottal power to acoustic power. On the basis of the scant previous literature and clinical intuition, we tested the hypothesis that vocal efficiency, as an indicator of the functional status of the larynx, is abnormally reduced in persons with vocal nodules and polyps. Because the most difficult aspect of obtaining measures of vocal efficiency has been the determination of subglottal pressure, we applied a noninvasive airflow interruption technique for this purpose. Subjects with normal voices (n = 22), vocal polyps (n = 14), and vocal nodules (n = 16) phonated at different intensities into a mask connected by way of piping to a flow meter, a pressure transducer, and an acoustic microphone. Inflation of a balloon-type valve located within the piping provided interruption of phonation. The intraoral pressure plateau occurring during flow interruption was used to estimate subglottal pressure. Subglottal power and acoustic power were determined, and their quotient provided a measure of vocal efficiency. The vocal efficiency in the normal subjects averaged 1.15 × 10−5 at 70 dB, 3.17 × 10−5 at 75 dB, 7.52 × 10−5 at 80 dB, and 1.41 × 10−4 at 85 dB. The vocal efficiency in the patients with vocal polyps averaged 3.62 × 10−6 at 70 dB, 8.34 × 10−6 at 75 dB, 2.10 × 10−5 at 80 dB, and 4.26 × 10−5 at 85 dB. The vocal efficiency in the patients with vocal nodules averaged 4.32 × 10−6 at 70 dB, 1.57 × 10−5 at 75 dB, 4.26 × 10−5 at 80 dB, and 8.34 × 10−5 at 85 dB. As compared to the normal subjects, the patients with laryngeal polyps or vocal nodules had significantly reduced vocal efficiency. These results provide quantitative verification of the clinical impression of inefficient phonation in patients with mass lesions of the vocal folds.

The Effects of Hyper- and Hypocapnia on Phonatory Laryngeal Airway Resistance in Women

PURPOSE

The larynx has a dual role in the regulation of gas flow into and out of the lungs while also establishing resistance required for vocal fold vibration. This study assessed reciprocal relations between phonatory functions-specifically, phonatory laryngeal airway resistance (Rlaw)-and respiratory homeostasis during states of ventilatory gas perturbations.

METHOD

Twenty-four healthy women performed phonatory tasks while exposed to induced hypercapnia (high CO2), hypocapnia (low CO2), and normal breathing (eupnea). Effects of gas perturbations on Rlaw were investigated as were the reciprocal effects of Rlaw modulations on respiratory homeostasis.

RESULTS

Rlaw remained stable despite manipulations of inspired gas concentrations. In contrast, end-tidal CO2 levels increased significantly during all phonatory tasks. Thus, for the conditions tested, Rlaw did not adjust to accommodate ventilatory needs as predicted. Rather, stable Rlaw was spontaneously accomplished at the cost of those needs.

CONCLUSIONS

Findings provide support for a theory of regulation wherein Rlaw may be a control parameter in phonation. Results also provide insight into the influence of phonation on respiration. The work sets the foundation for future studies on laryngeal function during phonation in individuals with lower airway disease and other patient populations.

Effects of implant stiffness, shape, and medialization depth on the acoustic outcomes of medialization laryngoplasty

OBJECTIVE

Medialization laryngoplasty is commonly used to treat glottic insufficiency. In this study, we investigated the effects of implant stiffness (Young modulus), medialization depth, and implant medial surface shape on acoustic outcomes.

STUDY DESIGN

Basic science study using ex vivo laryngeal phonation model.

METHODS

In an ex vivo human larynx phonation model, bilateral medialization laryngoplasties were performed with implants of varying stiffness, medial surface shape (rectangular, divergent, and convergent), and varying depths of medialization. The subglottal pressure, the flow rate, and the outside sound were measured as the implant parameters were varied.

RESULTS

Medialization through the use of implants generally improved the harmonic-to-noise ratio (HNR) and the number of harmonics excited in the outside sound spectra. The degree of acoustic improvement depended on the implant insertion depth, stiffness, and to a lesser degree implant shape. Varying implant insertion depth led to large variations in phonation for stiff implants, but had much smaller effects for soft implants.

CONCLUSIONS

Implants with stiffness comparable to vocal folds provided more consistent improvement in acoustic outcomes across different implant conditions. Further investigations are required to better understand the underlying mechanisms.

Estimation of alveolar pressure during speech using direct measures of tracheal pressure

The pressure in the alveoli of the lungs, created by the elastic recoil of the lungs and respiratory muscle activity, is referred to as alveolar pressure (Pa). The extent to which tracheal pressure (Pt) approximates Pa depends on the resistance to airflow offered by structures above and below the point at which tracheal pressure is measured. An understanding of the relationship among Pa, Pt, and upper and lower airway resistance, and how these values fluctuate during speech, could aid in interpretation and modeling of speech aerodynamics. The purpose of this study was to (a) obtain values for lower airway resistance (Rlow), (b) use these Rlow values to estimate Pa during speech, and (c) quantify the degree to which Pt approximates Pa during production of voiced and voiceless sounds, in comparison to inhalation. In addition, the results were discussed in terms of the degree to which the respiratory system functions as a pressure source. Tracheal pressure (obtained with tracheal puncture) and airflow were measured during sentence production in 6 subjects. Using a technique introduced in this paper, Rlow was determined from measures of tracheal pressure and flow obtained during a sudden change in upper airway resistance because of release of a voiceless plosive. Mean Rlow values ranged from 0.14 to 0.32 kPa/(l/s). Each subject's mean Rlow was used to derive a time-varying measure of Pa during speech from continuous measures of tracheal pressure and airflow. Pt was approximately 95% of Pa during phonation (i.e., when the vocal folds were adducted), 75% of Pa during release of the voiceless stop consonant /p/, and 55% of Pa during inhalation (i.e., when the vocal folds were abducted). Therefore, the degree to which the respiratory system functioned as an ideal pressure source varied during speech. The ability to estimate Pa provides a measure of the pressure produced by the respiratory system that is not influenced by laryngeal activity.

Effects of driving pressure and recurrent laryngeal nerve stimulation on glottic vibration in a constant pressure model

Glottic phonatory parameters have been studied in constant flow models; however, the lung-thorax system is better viewed as a constant pressure source. Adjusting the driving pressure and recurrent laryngeal nerve stimulation as independent variables, rather than as dependent variables, may provide a more physiologic understanding of laryngeal function and glottic parameters, including subglottic pressure, airflow, fundamental frequency, and glottic area. In three dogs subglottic pressure and airflow were measured in two separate conditions: with constant recurrent laryngeal nerve stimulation and varying driving pressure, and with constant driving pressure and varying recurrent laryngeal nerve stimulation. Videostroboscopic measures on four dogs assessed glottic areas with constant recurrent laryngeal nerve stimulation at different driving pressures. With constant recurrent laryngeal nerve stimulation, increasing driving pressure had no effect on glottic areas, whereas subglottic pressure, fundamental frequency, and airflow increased significantly. However, changes in subglottic pressure were minimal in comparison with changes in driving pressure. At constant driving pressure, increasing recurrent laryngeal nerve stimulation increased subglottic pressure and fundamental frequency and decreased airflow. These findings suggest that during phonation subglottic pressure is primarily dependent on recurrent laryngeal nerve stimulation and laryngeal muscular contraction, but not on lung driving pressure.

Characteristics of an in vivo canine model of phonation with a constant air pressure source

Many previous studies of laryngeal biomechanics using in vivo models have employed a constant air How source. Several authors have recently suggested that the lung-thorax system functions as a constant pressure source during phonation. This study describes an in vivo canine system designed to maintain a constant peak subglottic pressure (Psub) using a pressure-controlling mechanism. Increasing levels of recurrent laryngeal nerve (RLN) stimulation resulted in a significant rise in resistance followed by a plateau. For a given Psub, flow decreased significantly and precipitously with increasing stimulation and then quickly plateaued. Vocal intensity increased with increasing RLN stimulation until a peak was reached. After this peak, intensity dropped until a plateau was reached, corresponding to the flow minimum. At a given Psub, increasing levels of RLN stimulation resulted in a normal distribution of vocal efficiencies.