Kristina Kairaitis

Tissue vibration induces carotid artery endothelial dysfunction: a mechanism linking snoring and carotid atherosclerosis?

STUDY OBJECTIVES We have previously identified heavy snoring as an independent risk factor for carotid atherosclerosis. In order to explore the hypothesis that snoring-associated vibration of the carotid artery induces endothelial dysfunction (an established atherogenic precursor), we utilized an animal model to examine direct effects of peri-carotid tissue vibration on carotid artery endothelial function and structure. DESIGN In supine anesthetized, ventilated rabbits, the right carotid artery (RCA) was directly exposed to vibrations for 6 h (peak frequency 60 Hz, energy matched to that of induced snoring in rabbits). Similarly instrumented unvibrated rabbits served as controls. Features of OSA such as hypoxemia, large intra-pleural swings and blood pressure volatility were prevented. Carotid endothelial function was then examined: (1) biochemically by measurement of tissue cyclic guanosine monophosphate (cGMP) to acetylcholine (ACh) and sodium nitroprusside (SNP); and (2) functionally by monitoring vessel relaxation with acetylcholine in a myobath. MEASUREMENT AND RESULTS Vessel cGMP after stimulation with ACh was reduced in vibrated RCA compared with unvibrated (control) arteries in a vibration energy dose-dependent manner. Vibrated RCA also showed decreased vasorelaxation to ACh compared with control arteries. Notably, after addition of SNP (nitric oxide donor), cGMP levels did not differ between vibrated and control arteries, thereby isolating vibration-induced dysfunction to the endothelium alone. This dysfunction occurred in the presence of a morphologically intact endothelium without increased apoptosis. CONCLUSIONS Carotid arteries subjected to 6 h of continuous peri-carotid tissue vibration displayed endothelial dysfunction, suggesting a direct plausible mechanism linking heavy snoring to the development of carotid atherosclerosis.

Decreased surface tension of upper airway mucosal lining liquid increases upper airway patency in anaesthetised rabbits.

The obstructive sleep apnoea syndrome (OSA) is a disorder characterised by repetitive closure and re‐opening of the upper airway during sleep. Upper airway luminal patency is influenced by a number of factors including: intraluminal air pressure, upper airway dilator muscle activity, surrounding extraluminal tissue pressure, and also surface forces which can potentially act within the liquid layer lining the upper airway. The aim of the present study was to examine the role of upper airway mucosal lining liquid (UAL) surface tension (γ) in the control of upper airway patency. Upper airway opening (PO) and closing pressures (PC) were measured in 25 adult male, supine, tracheostomised, mechanically ventilated, anaesthetised (sodium pentabarbitone), New Zealand White rabbits before (control) and after instillation of 0.5 ml of either 0.9 % saline (n= 9) or an exogenous surfactant (n= 16; Exosurf Neonatal) into the pharyngeal airway. The γ of UAL (0.2 μl) was quantified using the ‘pull‐off’ force technique in which γ is measured as the force required to separate two curved silica discs bridged by the liquid sample. The γ of UAL decreased after instillation of surfactant from 54.1 ± 1.7 mN m−1 (control; mean ±s.e.m.) to 49.2 ± 2.1 mN m−1 (surfactant; P < 0.04). Compared with control, PO increased significantly (P < 0.04; paired t test, n= 9) from 6.2 ± 0.9 to 9.6 ± 1.2 cmH2O with saline, and decreased significantly (P < 0.05, n= 16) from 6.6 ± 0.4 to 5.5 ± 0.6 cmH2O with surfactant instillation. Findings tended to be similar for PC. Change in both PO and PC showed a strong positive correlation with the change in γ of UAL (both r > 0.70, P < 0.001). In conclusion, the patency of the upper airway in rabbits is partially influenced by the γ of UAL. These findings suggest a role for UAL surface properties in the pathophysiology of OSA.

Mass loading of the upper airway extraluminal tissue space in rabbits: effects on tissue pressure and pharyngeal airway lumen geometry

Lateral pharyngeal fat pad compression of the upper airway (UA) wall is thought to influence UA size in patients with obstructive sleep apnea. We examined interactions between acute mass/volume loading of the UA extra-luminal tissue space and UA patency. We studied 12 supine, anesthetized, spontaneously breathing, head position-controlled (50 degrees ), New Zealand White rabbits. Submucosal extraluminal tissue pressures (ETP) in the anterolateral (ETPlat) and anterior (ETPant) pharyngeal wall were monitored with surgically inserted pressure transducer-tipped catheters (Millar). Tracheal pressure (Ptr) and airflow (V) were measured via a pneumotachograph and pressure transducer inserted in series into the intact trachea, with hypopharyngeal cross-sectional area (CSA) measured via computed tomography, while graded saline inflation (0-1.5ml) of a compliant tissue expander balloon in the anterolateral subcutaneous tissue was performed. Inspiratory UA resistance (Rua) at 20 ml/s was calculated from a power function fitted to Ptr vs. V data. Graded expansion of the anterolateral balloon increased ETPlat from 2.3 +/- 0.5 cmH(2)O (n = 11, mean +/- SEM) to 5.0 +/- 1.1 cmH(2)O at 1.5-ml inflation (P < 0.05; ANOVA). However, ETPant was unchanged from 0.5 +/- 0.5 cmH(2)O (n = 9; P = 0.17). Concurrently, Rua increased to 119 +/- 4.2% of baseline value (n = 12; P < 0.001) associated with a significant reduction in CSA between 10 and 70% of airway length to a minimum of 82.2 +/- 4.4% of baseline CSA at 40% of airway length (P < 0.05). We conclude that anterolateral loading of the upper airway extraluminal tissue space decreases upper airway patency via an increase in ETPlat, but not ETPant. Lateral pharyngeal fat pad size may influence UA patency via increased tissue volume and pressure causing UA wall compression.

Pharyngeal muscle contraction modifies peri-pharyngeal tissue pressure in rabbits

We examined the influence of pharyngeal dilator muscle activity on upper airway extraluminal tissue pressure (ETP) distribution and upper airway patency. We studied seven anaesthetised, supine, spontaneously breathing NZ white rabbits. ETP was measured via pressure transducer tipped catheters in lateral (ETP(lat)) and anterior (ETP(ant)) pharyngeal wall tissues. Airflow (V) and tracheal pressure (P) were monitored and upper airway resistance (RUA) calculated. Genioglossus (GG) or bilateral sternohyoid (SH) muscles were electrically stimulated. Tongue protrusion (TP) during GG stimulation was measured. With GG stimulation, RUA decreased to 57.8+/-10.9% (mean+/-S.E.M.) of baseline and TP increased to 4.8+/-0.5mm (both p<0.05), but ETP(lat) (2.6+/-1.5 cm H(2)O) and ETP(ant) (1.4+/-0.8 cm H(2)O) were unchanged. SH stimulation reduced RUA to 53.6+/-6.8%, and ETP(lat) fell by 1.0+/-0.4 cm H(2)O (both p<0.05). ETP(ant) was unchanged. GG muscle contraction decreased RUA without altering ETP, whereas SH contraction altered RUA and ETP(lat), but not ETP(ant). Contraction of the upper airway dilator muscles results in improvements in upper airway patency associated with changes in peri-pharyngeal tissue pressure.

A threshold lung volume for optimal mechanical effects on upper airway airflow dynamics: studies in an anesthetized rabbit model.

Increasing lung volume improves upper airway airflow dynamics via passive mechanisms such as reducing upper airway extraluminal tissue pressures (ETP) and increasing longitudinal tension via tracheal displacement. We hypothesized a threshold lung volume for optimal mechanical effects on upper airway airflow dynamics. Seven supine, anesthetized, spontaneously breathing New Zealand White rabbits were studied. Extrathoracic pressure was altered, and lung volume change, airflow, pharyngeal pressure, ETP laterally (ETPlat) and anteriorly (ETPant), tracheal displacement, and sternohyoid muscle activity (EMG%max) monitored. Airflow dynamics were quantified via peak inspiratory airflow, flow limitation upper airway resistance, and conductance. Every 10-ml lung volume increase resulted in caudal tracheal displacement of 2.1 ± 0.4 mm (mean ± SE), decreased ETPlat by 0.7 ± 0.3 cmH(2)O, increased peak inspiratory airflow of 22.8 ± 2.6% baseline (all P < 0.02), and no significant change in ETPant or EMG%max. Flow limitation was present in most rabbits at baseline, and abolished 15.7 ± 10.5 ml above baseline. Every 10-ml lung volume decrease resulted in cranial tracheal displacement of 2.6 ± 0.4 mm, increased ETPant by 0.9 ± 0.2 cmH(2)O, ETPlat was unchanged, increased EMG%max of 11.1 ± 0.3%, and a reduction in peak inspiratory airflow of 10.8 ± 1.0%baseline (all P < 0.01). Lung volume, resistance, and conductance relationships were described by exponential functions. In conclusion, increasing lung volume displaced the trachea caudally, reduced ETP, abolished flow limitation, but had little effect on resistance or conductance, whereas decreasing lung volume resulted in cranial tracheal displacement, increased ETP and increased resistance, and reduced conductance, and flow limitation persisted despite increased muscle activity. We conclude that there is a threshold for lung volume influences on upper airway airflow dynamics.

Is the pharynx a muscular hydrostat

Failure to maintain the patency of the pharyngeal airway during sleep is central to the pathogenesis of obstructive sleep apnoea (OSA). This failure is hypothesised to be due to the combination of a small pharyngeal airway and inadequate state-dependent neuro-mechanical control. Little is known of how the pharyngeal muscles function in an integrated function to alter the size and shape of the pharyngeal airway. We hypothesise that the muscles of the pharynx function as a muscular hydrostat. Muscular hydrostats are organs that are composed almost entirely of muscle, with a complex muscular arrangement within the organ. Examples of muscular hydrostats include the mammalian tongue, octopus tentacles, elephant trunks and the medicinal leech. During muscle contraction the organ will maintain a constant volume as muscle tissue is mostly water and hence incompressible. The mechanical effect of contraction of individual muscles within the muscular hydrostat is dependent on the integrated activity of all other muscles, as muscle orientation is dependent on the organ shape. Functionally the significance of the muscular hydrostat model lies in the concept that alterations in organ shape are achieved via muscle contraction driven redistribution of hydrostatic tissue pressure. The tissues which comprise the pharynx are predominantly muscle, and thus incompressible. The pharynx is composed of 20 muscles that are arranged in a complex fashion. Within the peri-pharyngeal tissues the only bony structure is the hyoid bone and in adult humans this is a free-floating bone. Evidence already exists that the functional outcome of contraction of some of the pharyngeal muscles is dependent on stage of respiration, the intra-luminal pressure, or the position of the hyoid bone when the muscle is activated. There is also evidence that muscle contraction can alter the pressure in the tissues surrounding the pharynx in a non-uniform fashion. However, it has not been demonstrated for the pharynx that pharyngeal luminal shape is determined by muscle contraction determined transmural pressure distribution. The consequences of this hypothesis are that reported pharyngeal anatomical abnormalities in subjects with OSA, such as increased peri-pharyngeal fat deposition or thickening of the lateral pharyngeal walls, could result in alteration in integrated muscular function and thus a failure to maintain upper airway patency. In addition, nocturnal pharyngeal airway obstruction may result from a failure of cross muscle activation. This novel paradigm may lead to greater insights into the pathogenesis of OSA as well as opening new avenues for exploration of novel therapeutic strategies.

Peripharyngeal tissue deformation and stress distributions in response to caudal tracheal displacement: pivotal influence of the hyoid bone?

Caudal tracheal displacement (TD) leads to improvements in upper airway (UA) function and decreased collapsibility. To better understand the mechanisms underlying these changes, we examined effects of TD on peripharyngeal tissue stress distributions [i.e., extraluminal tissue pressure (ETP)], deformation of its topographical surface (UA lumen geometry), and hyoid bone position. We studied 13 supine, anesthetized, tracheostomized, spontaneously breathing, adult male New Zealand white rabbits. Graded TD was applied to the cranial tracheal segment from 0 to ∼ 10 mm. ETP was measured at six locations distributed around/along the length of the UA, covering three regions: tongue, hyoid, and epiglottis. Axial images of the UA (nasal choanae to glottis) were acquired with computed tomography and used to measure lumen geometry (UA length; regional cross-sectional area) and hyoid bone displacement. TD resulted in nonuniform decreases in ETP (generally greatest at tongue region), ranging from -0.07 (-0.11 to -0.03) [linear mixed-effects model slope (95% confidence interval)] to -0.27 (-0.31 to -0.23) cmH2O/mm TD, across all sites. UA length increased by 1.6 (1.5-1.8)%/mm, accompanied by nonuniform increases in cross-sectional area (greatest at hyoid region) ranging from 2.8 (1.7-3.9) to 4.9 (3.8-6.0)%/mm. The hyoid bone was displaced caudally by 0.22 (0.18-0.25) mm/mm TD. In summary, TD imposes a load on the UA that results in heterogeneous changes in peripharyngeal tissue stress distributions and resultant lumen geometry. The hyoid bone may play a pivotal role in redistributing applied caudal tracheal loads, thus modifying tissue deformation distributions and determining resultant UA geometry outcomes.

Pharyngeal wall fold influences on the collapsibility of the pharynx

Obstructive sleep apnoea (OSA) is a disease that is characterised by recurrent pharyngeal obstruction during sleep. The pharynx is a hollow muscular tube lined with epithelium that performs the competing functions of breathing, where it is required to be open and swallowing where it is required to close. The mechanical process by which these large changes in luminal dimensions occur have not been considered, however in other biological tubes such as the oesophagus and the bronchial airways narrowing and closure occurs via folding of the mucosal surface. The transmural pressure (P) required to collapse a tube is related to the number of folds (n) formed during collapse by the equation P=n(2)-1, so that the more folds formed during narrowing and closure, the greater the transmural pressure required to collapse the tube. In biomechanical models, the bronchial airway is modelled as a 2-layer tube with an inner epithelial lining and an outer layer of muscle. These models predict that fold numbers will be reduced with thickening and stiffening of the outer layer, accompanied by an increase in collapsibility. We hypothesise that, similar to other biological tubes the pharynx narrows and closes via folding of the surface of the tube, and that the pharynx can also be modelled as a 2-layer tube. We further hypothesise that when compared to healthy subjects, subjects with OSA will have less pharyngeal wall folds during narrowing and closure, and that this reduction in fold numbers will contribute to an increase in pharyngeal collapsibility. In the absence of muscle activity, subjects with OSA have increased pharyngeal collapsibility when compared with healthy subjects, supporting an anatomical contribution to pharyngeal collapse. Histopathological studies of the pharyngeal epithelium in subjects with OSA demonstrate that, compared with age matched subjects, there is thickening of the epithelial surface with oedema of the submucosal layer, with a loss of tethering of the epithelium to the submucosal layer. These changes would tend to decrease fold numbers. There are no measurements of pharyngeal folds, however previous imaging data supports that narrowing of the pharynx occurs via folding. This hypothesis defines a novel anatomical risk for OSA. It also suggests a new therapeutic paradigm for the treatment of OSA, aimed at increasing the folds formed during narrowing and closure.

Saliva production and surface tension: influences on patency of the passive upper airway

Pharyngeal patency is influenced by the surface tension (γ) of the upper airway lining liquid (UAL), of which saliva is a major component. We investigated the influences of saliva production on γ of the UAL, and upper airway re‐opening and closing pressures. In 10 supine, male, anaesthetized, tracheostomised, mechanically ventilated New Zealand White rabbits, we measured re‐opening and closing of the passive isolated upper airway at baseline and following graded (cumulative) doses of methacholine or atropine. Upper airway liquid volume index (UALVI) was assessed using a standardized suction procedure (secretion weight obtained per second) expressed as the natural logarithm (LnUALVI). The γ of UAL samples were measured using the ‘pull‐off’ force technique. Across all animals, baseline values were: LnUALVI −6.2 (−8.6 to −5.4) median (interquartile range), γ of UAL 58.9 (56.6–59.9) mN m−1, re‐opening 8.6 (6.9–11.1) cmH2O, and closing pressures 3.2 (1.8–5.7) cmH2O. LnUALVI increased by ∼0.17 per μg kg−1 methacholine and decreased by ∼0.14 per 100 μg kg−1 atropine (both P < 0.03, linear mixed effects modelling). Surface tension was unchanged by methacholine but increased by ∼0.6 mN m−1 per 100 μg kg−1 atropine (P < 0.004). When data were analysed across all animals, both re‐opening and closing pressures increased as surface tension increased (by ∼0.4 cmH2O mN−1 and by ∼0.7 cmH2O mN−1, respectively; both P < 0.05). We conclude that saliva production influences upper airway mechanical properties partly via alterations in γ of UAL. We speculate that in obstructive sleep apnoea, altered autonomic activity may reduce saliva production and increase surface tension of the upper airway lining liquid, thus increasing the likelihood of upper airway obstruction.

Pharyngeal mucosal wall folds in subjects with obstructive sleep apnea

Mechanical processes underlying pharyngeal closure have not been examined. We hypothesized that the pharyngeal mucosal surface would fold during closure, and lowering the upper airway lining liquid surface tension would unfold areas of mucosal apposition, i.e., folds. We compared baseline pharyngeal fold numbers and response to reduction in upper airway liquid surface tension in healthy and obstructive sleep apnea (OSA) subjects. Awake, gated magnetic resonance pharyngeal airway images of 10 healthy and 11 OSA subjects were acquired before and after exogenous surfactant administration (beractant). Upper airway liquid surface tension was measured at the beginning and end of image acquisition and averaged. Velopharyngeal and oropharyngeal images were segmented and analyzed separately for average cross-sectional area, circumference, and fold number. Compared with healthy subjects, at baseline, velopharynx for OSA subjects had a smaller cross-sectional area (98.3 ± 32.5 mm(2) healthy, 52.3 ± 23.6 mm(2) OSA) and circumference (46.5 ± 8.1 mm healthy, 30.8 ± 6.1 mm OSA; both P < 0.05, unpaired t-test), and fewer folds (4.9 ± 1.6 healthy, 3.1 ± 1.8 OSA, P < 0.03). There were no differences in oropharynx for cross-sectional area, circumference, or folds. Reduction in upper airway liquid surface tension from 61.3 ± 1.2 to 55.3 ± 1.5 mN/m (P < 0.0001) did not change cross-sectional area or circumference for velopharynx or oropharynx in either group; however, in OSA subjects, oropharyngeal folds fell from 6.8 ± 3.1 to 4.7 ± 1.2 (n = 8, P < 0.05), and velopharyngeal folds from 3.3 ± 1.9 to 2.3 ± 1.2 (P = 0.08), and were unchanged in healthy subjects. Subjects with OSA have fewer velopharyngeal wall folds, which decrease further when surface tension falls. We speculate that reduced pharyngeal wall folds contribute to an increase in pharyngeal collapsibility.