Mechanical effects of pharyngeal constrictor activation on pharyngeal airway function

The mechanical effects of pharyngeal constrictor (PC) muscle activation on pharyngeal airway function were determined in 20 decerebrate, tracheotomized cats. In 10 cats, a high-compliance balloon attached to a pressure transducer was partially inflated to just occlude the pharyngeal airway. During progressive hyperoxic hypercapnia, changes in pharyngeal balloon pressure were directly related to phasic expiratory hyopharyngeus (middle PC) activity. In two separate protocols in 10 additional cats, the following measurements were obtained with and without bilateral electrical stimulation (0.2-ms duration, threshold voltage) of the distal cut end of the vagus nerve's pharyngeal branch supplying PC motor output: 1) pressure-volume relationships in an isolated, sealed upper airway at a stimulation frequency of 30 Hz and 2) rostrally directed axial force over a stimulation frequency range of 0-40 Hz. Airway compliance determined from the pressure-volume relationships decreased with PC stimulation at and below resting airway volume. Compared with the unstimulated condition, PC stimulation increased airway pressure at airway volumes at and above resting volume. This constrictor effect progressively diminished as airway volume was brought below resting volume. At relatively low airway volumes below resting volume, PC stimulation decreased airway pressure compared with that without stimulation. PC stimulation generated a rostrally directed axial force that was directly related to stimulation frequency. The results indicate that PC activation stiffens the pharyngeal airway, exerting both radial and axial effects. The radial effects are dependent on airway volume: constriction of the airway at relatively high airway volumes, and dilation of the airway at relatively low airway volumes. The results imply that, under certain conditions, PC muscle activation may promote pharyngeal airway patency.

Respiratory-related pharyngeal constrictor muscle activity in decerebrate cats

Respiratory-related activity of the hyopharyngeus (middle pharyngeal constrictor) and thyropharyngeus (inferior pharyngeal constrictor) muscles was determined in decerebrate, tracheotomized adult cats and compared with the electromyographic activity of the thyroarytenoid, a vocal cord adductor. During quiet breathing, the hyopharyngeus and usually the thyroarytenoid exhibited phasic activity during expiration and tonic activity throughout the respiratory cycle. Respiratory-related thyropharyngeus activity was absent under these conditions. Progressive hyperoxic hypercapnia and progressive isocapnic hypoxia increased phasic expiratory activity in both pharyngeal constrictor (PC) muscles but tended to suppress thyroarytenoid activity. Passively induced hypocapnia and the central apnea that followed the cessation of the mechanical hyperventilation were associated with tonic activation of the hyopharyngeus and thyroarytenoid but no recruitment in thyropharyngeus activity. The expiratory phase of a sigh and progressive pneumothorax were associated with an increase in phasic thyroarytenoid activity but no change in phasic PC activity. The results indicate that a variety of stimuli modulate respiratory-related PC activity, suggesting that the PC muscles may have a role in the regulation of upper airway patency during respiration.

Respiratory-related pharyngeal constrictor muscle activity in normal human adults

Electromyographic activity of the superior, middle, and inferior pharyngeal constrictor (PC) muscles was examined in 10 normal adult humans during wakefulness and sleep. Wire electrodes were inserted close to the midline of the posterior pharyngeal wall at the level of the soft palate (superior PC), tip of the epiglottis (middle PC), and corniculate tubercle (inferior PC). In general, the three PC muscles exhibited similar patterns of activation. The PCs were activated during swallows, repetitive "pa" sounds, changes in head position, and the last portions of slow inspiratory and expiratory vital capacity maneuvers. Respiratory-related PC activity was infrequent during quiet breathing during wakefulness; variable and inconsistent phasic activation in expiration in one or more of the PCs was present in seven of the 10 subjects, particularly after a swallow. Progressive hyperoxic hypercapnia and progressive isocapnic hypoxia were associated with recruitment of phasic PC activity, which was predominantly expiratory; however, variable discharge patterns were observed within a given muscle and a given subject. When phasic PC activity was present, preactivation during late inspiration was frequently observed. PC activity was absent in NREM sleep and exhibited sporadic, nonrespiratory-related bursts of activity during REM sleep. Passively induced hypocapnic hyperventilation in NREM sleep was not associated with PC activation. The results indicate that the PCs have very similar patterns of activation and exhibit phasic expiratory activity during relatively high ventilatory output states in wakefulness.

Differential activity of the pars recta and pars oblique in fundamental frequency control

This study was designed to determine if differences exist in pars recta and pars oblique muscle activity during speech and singing. Hooked wire electrodes were implanted in the muscle bundles under direct vision during thyroid surgery in two men and three women. It was found that the pars recta and pars oblique do not function in a similar manner across fundamental frequencies (fo's), tasks, or subjects. Large inter- and intrasubject variability was evident in the contribution of the cricothyroid bundles to fundamental frequency (fo) control. It is speculated that the effect of pars recta and pars oblique contraction may be a function of individual anatomic variations.

Activation of intrinsic laryngeal muscles during cough

We studied the pattern of discharge of the posterior cricoarytenoid (PCA), cricothyroid (CT), thyroarytenoid (TA), and arytenoideus transversus (AR) muscles during cough in 12 anesthetized dogs. Diaphragm electromyographic (EMG) activity was also recorded, together with subglottic and esophageal pressures. Trains of repetitive coughs were induced by mechanically stimulating the tracheobronchial airway. Trials with the upper airway isolated from and connected to the lower airway were performed before and following bilateral sectioning of the internal branch of the superior laryngeal nerve (SLN). The immediate effect of tracheal stimulation was an "apneic" period at FRC, during which the PCA, a laryngeal abductor, showed a progressive increase in activity accompanied by small, variable increases in the activity of the CT and the laryngeal adductors, the TA and AR. The subsequent cough efforts were divided into three phases: inspiration, glottic narrowing, and forced expiration. PCA activity was greatest during the inspiratory phase and CT activity was greatest during the expiratory phase. Peak subglottic pressure occurred during glottic narrowing and coincided with the greatest activation of the TA and AR during the cough effort, and suppression of the PCA and CT. The patterns of EMG activation were not affected by the route of breathing or SLN section. The results suggest the presence of a uniquely central process controlling laryngeal muscles during cough, independent of laryngeal sensory feedback.

Comparison of direct and indirect calculations of laryngeal airway resistance in connected speech

Direct measures of subglottal pressure obtained through a tracheal puncture were used to calculate laryngeal airway resistance. Six subjects completed tasks including syllable trains and more natural speech samples produced at three loudness levels. Direct calculations of natural speech resistance values were compared with indirect estimates obtained during syllable train production. The degree of correspondence between direct and indirect calculations varied by subject. Overall, the smallest relative errors among calculations occurred for syllable trains, with higher relative errors for the monologue and sentence. For loudness conditions, the smallest and largest relative errors occurred for soft and loud productions, respectively. The clinical utility of indirect estimation is questioned and suggestions for improving its validity are provided.

Comparison of direct and indirect calculations of laryngeal airway resistance in various voicing conditions

Calculation of laryngeal airway resistance using intraoral pressure divided by airflow has become a widely accepted clinical tool. The estimation is based largely on theoretical assumptions, particularly regarding the relationship between intraoral and subglottal pressure. To determine the estimate's validity, direct measures of airflow and subglottal, intraoral and pharyngeal pressures were obtained for four men and four women. Subjects produced normal, loud, soft and simulated breathy and strained syllable trains. Comparison of direct and indirect calculations of laryngeal airway resistance revealed generally good correspondence, particularly for [pi] syllables, although the estimate's accuracy varied among individual subjects. Some interpretive caution is warranted for the extremes of laryngeal airway resistance, with resistance likely to be underestimated in the strained condition, and overestimated in the breathy condition.

Effect of hypercapnia on laryngeal airway resistance in normal adult humans

Laryngeal airway resistance (Rlar) was measured in eight normal adult humans during progressive hyperoxic hypercapnia. In most subjects, the translaryngeal pressure-flow relationship appeared linear under normocapnic conditions. During hypercapnia, the pressure-flow relationship on inspiration and expiration was curvilinear with increasing translaryngeal pressure associated with progressively smaller increments in flow. Translaryngeal pressure-flow relationships at different CO2 levels were compared over their common flow ranges by performing a least-squares linear regression on data throughout inspiration and expiration. During normocapnia, the mean slope, i.e., mean Rlar, was 0.50 +/- 0.21 (SD) cmH2O.l-1.s. A moderately significant decrease in Rlar was present at 9% end-tidal CO2 (P = 0.08). In a separate series of experiments, subjects breathed oxygen- and helium-based gas mixtures through a face mask attached to a pneumotachograph. Data analysis over the flow range present during normocapnia revealed no difference in Rlar between nose and mouth breathing and similar decreases in Rlar under hypercapnic conditions with the oxygen- and helium-based gas mixtures. The decrease in Rlar from normocapnic to hypercapnic conditions found over common, but relatively low, ranges of flow predicts that even greater increases in Rlar would occur at high flow rates in the absence of increasing glottic aperture.

Laryngeal response during forced vital capacity maneuvers in normal adult humans

Previous investigators have reported that transient forced expiration is accompanied by abduction of the vocal cords. To further investigate the laryngeal response during voluntary forced vital capacity maneuvers, intramuscular electromyographic recordings were obtained in 25 normal adult humans from three intrinsic laryngeal muscles: the posterior cricoarytenoid (PCA), a vocal cord abductor, and the thyroarytenoid (TA) and arytenoideus (AR), both vocal cord adductors. All three muscles exhibited sustained activation throughout most of forced expiration from total lung capacity. Forced inspiration from residual volume was associated with a further increase in PCA activity and a marked decrease in adductor muscle activity. To determine the net effect of these electromyographic changes on vocal cord position, simultaneous fiberoptic recordings of vocal cord movement were obtained in five of the subjects. The angle formed by the vocal cords at the anterior commissure was used to assess glottic aperture size. Glottic angle progressively decreased from peak expiratory flow to the end of forced expiration. The angle was 56 +/- 13 degrees (SD) at peak expiratory flow, 34 +/- 4 degrees after forced expiration of 90% of the vital capacity, and 7 +/- 7 degrees at end-expiration. The results indicate that forced expiration is associated with marked activation of not only the PCA but also laryngeal adductor muscles. During forced expiration, the glottis does not decrease below its size during quiet breathing until exhalation of about 75% of forced vital capacity.

Cricothyroid muscle activity during sleep in normal adult humans

Previous investigators reported that cricothyroid (CT) muscle usually exhibits phasic inspiratory activity in normal adult humans during wakefulness. The purpose of this study was to determine respiratory-related CT activity in normal adult humans during sleep. Nighttime polysomnograms were performed in 16 subjects. Hooked-wire electrodes were percutaneously implanted in CT with 21-gauge needle-catheter unit that allowed artifact-free monopolar recordings during electrode placement. During wakefulness, CT was usually phasically active on inspiration, with tonic activity throughout the respiratory cycle. Phasic inspiratory activity was present throughout sleep in all subjects, even those without respiratory-related CT activity during wakefulness. Compared with non-rapid-eye-movement (NREM) sleep, phasic CT activity uniformly increased in rapid-eye-movement (REM) sleep. No differences were apparent in height of phasic CT activity between phasic and tonic REM sleep. Application of nasal continuous positive pressure in stage 3/4 NREM sleep was associated with a decrease in phasic CT activity. Passively induced hypocapnia with positive-pressure ventilation via a nose mask in stage 3/4 NREM sleep was associated with a disappearance of phasic CT activity. Cessation of positive-pressure ventilation under hypocapnic conditions frequently resulted in apnea. Phasic CT activity remained absent during apnea but reappeared coincident with or soon after resumption of spontaneous respiration. In summary, CT's phasic inspiratory activity and respiratory-related response to various stimuli during sleep were very similar to those of posterior cricoarytenoid muscle, the principal vocal cord abductor.

Effect of hypercapnia and hypoxia on arytenoideus muscle activity in normal adult humans

The electrical activity of the arytenoideus muscle, a vocal cord adductor, was measured in 14 normal adult humans during progressive isocapnic hypoxia and progressive hyperoxic hypercapnia. Electromyograms of the arytenoideus were obtained with intramuscular hooked-wire electrodes implanted by means of a fiber-optic nasopharyngoscope. Correct placement of the electrodes was confirmed by discharge patterns during voluntary maneuvers. In three of the subjects, respiratory-related arytenoideus activity was not present during quiet breathing or chemical stimulation. During quiet breathing in the 11 other subjects, the arytenoideus exhibited phasic activity during expiration and usually tonic activity throughout the respiratory cycle. Phasic and tonic arytenoideus activity decreased under hypoxic and hypercapnic conditions. At higher levels of chemical stimulation in many subjects, short abrupt bursts of activity were frequently present at the transitions between inspiration and expiration. To determine the mechanical effect of the latter electromyographic findings, arytenoideus activity and fiber-optic images of the glottic aperture were simultaneously recorded in nine additional normal adult human subjects during progressive hyperoxic hypercapnia. The short abrupt bursts of arytenoideus activity were usually associated with a decrease in glottic aperture, although no change and an increase in glottic aperture were observed in individual subjects. The results suggest that the arytenoideus muscle may have an important role in the control of ventilation in normal human subjects.