Differential effects of respiratory and electrical stimulation-induced dilator muscle contraction on mechanical properties of the pharynx in the pig

Abstract

Respiratory stimulation (RS) during sleep often fails to discontinue flow limitation, whereas electrical stimulation (ES) of the hypoglossus (HG) nerve frequently prevents obstruction. The present work compares the effects of RS and HG-ES on pharyngeal mechanics and the relative contribution of tongue muscles and thoracic forces to pharyngeal patency. We determined the pressure-area relationship of the collapsible segment of the pharynx in anesthetized pigs under the following three conditions: baseline (BL), RS induced by partial obstruction of the tracheostomy tube, and HG-ES. Parameters were obtained also after transection of the neck muscles and the trachea (NMT) and after additional bilateral HG transection (HGT). In addition, we measured the force produced by in situ isolated geniohyoid (GH) during RS and HG-ES. Intense RS was recognized by large negative intrathoracic pressures and triggered high phasic genioglossus and GH EMG activity. GH contraction produced during maximal RS less than a quarter of the force obtained during HG-ES. The major finding of the study was that RS and ES differed in the mechanism by which they stabilized the pharynx: RS lowered the pressure-area slope, i.e., reduced pharyngeal compliance (14.1 ± 2.9 to 9.2 ± 1.9 mm(2)/cmH2O, P < 0.01). HG-ES shifted the slope toward lower pressures, i.e., lowered the calculated extraluminal pressure (17.4 ± 5.8 to 9.2 ± 7.4 cmH2O, P < 0.01). Changes during RS and HG-ES were not affected by NMT, but the effect of RS decreased significantly after HGT. In conclusion, HG-ES and RS affect the pharyngeal site of collapse differently. Tongue muscle contraction contributes to pharyngeal stiffening during RS.

Alteration in upper airway dilator muscle coactivation during sleep: comparison of patients with obstructive sleep apnea and healthy subjects

“…Combined activation of dilator muscles may protect airway patency primarily by stiffening the tongue and other peripharyngeal structures. This conclusion was supported by animal studies that compared the mechanical effects of respiratory stimulation to electrical stimulation of the hypoglossus (2).…”

“…This principle also applies to use of hypoglossus nerve stimulation devices currently in use to treat OSA. Although electrical stimulation produces tongue muscle contraction that well exceeds levels achieved during periods of intense central respiratory drive (2), isolated activation of the GG may not always be sufficient to prevent pharyngeal collapse when other peripharyngeal muscles remain inactive. Accordingly, attempts should be undertaken to stimulate tongue muscles so as to produce patterns of muscle activation that stiffen the tongue and change its shape favorably, rather than only contracting the GG.…”