Sleep disordered breathing has minimal association with retinal microvascular diameters in a non-diabetic sleep clinic cohort

INTRODUCTION

Obstructive sleep apnea (OSA) may increase stroke risk; retinal arteriolar (central retinal arteriolar equivalent, CRAE) diameter narrowing and/or retinal venular (central retinal venule equivalent, CRVE) widening may predict stroke. We examined relationships between sleep disordered breathing (SDB) and CRAE and CRVE and in a diabetes-free sleep clinic cohort.

METHODS

Patients for SDB assessment were recruited (Main Group, n = 264, age: 58.5 ± 8.9 yrs [mean ± SD]; males: 141) for in-laboratory polysomnography (standard metrics, eg apnea hypopnea index, AHI) and retinal photographs (evening and morning). A more severe SDB sub-group (n = 85) entered a 12-month cardiovascular risk factor minimisation (hypertension/hypercholesterolemia control; RFM) and continuous positive airway pressure (CPAP) intervention (RFM/CPAP Sub-Group); successfully completed by n = 66 (AHI = 32.4 [22.1-45.3] events/hour, median[IQR]). Univariate (Spearman's correlation, t-test) and multiple linear regression models examined non-SDB and SDB associations with CRAE and CRVE measures.

RESULTS

Main Group: Evening CRAE predictors were: systolic blood pressure (0.18μm decrease per mmHg, p = 0.001), age (2.47μm decrease per decade, p = 0.012), Caucasian ethnicity (4.45 μm versus non-Caucasian, p = 0.011), height (0.24 μm decrease per cm increase, p = 0.005) and smoking history (3.08 μm increase, p = 0.052). Evening CRVE predictors were: Caucasian ethnicity (11.52 μm decrease versus non-Caucasian, p>0.001), diastolic blood pressure (0.34 μm increase in CRVE per mmHg, p = 0.001), hypertension history (6.5 μm decrease, p = 0.005), and smoking history (4.6 μm increase, p = 0.034). No SDB metric (all p>0.08) predicted CRAE or CRVE measures. RFM/CPAP Sub-Group: A one-unit increase in ln(AHI+1) was associated with a 0.046μm increase in CRAE (n = 85; p = 0.029). Mean evening CRAE and CRVE values did not change across the intervention (n = 66), but evening CRVE decreased ~6.0 μm for individuals with AHI >30 events/hr.

CONCLUSION

No major SDB associations with CRAE or CRVE were identified, although the RFM/CPAP intervention reduced evening CRVE for severe OSA patients. Implications for cerebro-vascular disease risk remain uncertain.

TRIAL REGISTRATION

The protocol was registered with the Australian New Zealand Clinical Trials Registry (Trial Id: ACTRN12620000694910).

Predictors for carotid and femoral artery intima-media thickness in a non-diabetic sleep clinic cohort

Introduction

The impact of sleep disordered breathing (SDB) on arterial intima-media thickness (IMT), a surrogate measure for cardiovascular disease, remains uncertain, in part because of the potential for non-SDB vascular risk factor interactions. In the present study, we determined predictors for common carotid (CCA) and femoral (CFA) artery IMT in an adult, sleep clinic cohort where non-SDB vascular risk factors (particularly diabetes) were eliminated or controlled.

Methods

We recruited 296 participants for polysomnography (standard SDB severity metrics) and CCA/CFA ultrasound examinations, followed by a 12 month vascular risk factor minimisation (RFM) and continuous positive pressure (CPAP) intervention for participants with a range of SDB severity (RFM Sub-Group, n = 157; apnea hyponea index [AHI]: 14.7 (7.2–33.2), median [IQR]). Univariable and multivariable linear regression models determined independent predictors for IMT. Linear mixed effects modelling determined independent predictors for IMT change across the intervention study. P<0.05 was considered significant.

Results

Age, systolic blood pressure and waist:hip ratio were identified as non-SDB predictive factors for CCA IMT and age, weight and total cholesterol:HDL ratio for CFA IMT. No SDB severity metric emerged as an independent predictor for either CCA or CFA IMT, except in the RFM Sub-Group, where a 2-fold increase in AHI predicted a 2.4% increase in CFA IMT. Across the intervention study, CCA IMT decreased in those who lost weight, but there was no CPAP use interaction. CFA IMT, however, decreased by 12.9% (95%CI 6.8, 18.7%, p = 0.001) in those participants who both lost weight and used CPAP > = 4hours/night.

Conclusion

We conclude that SDB severity has little impact on CCA IMT values when non-SDB vascular risk factors are minimised or not present. This is the first study, however, to suggest a potential linkage between SDB severity and CFA IMT values.

Trial registration

Australian New Zealand Clinical Trials Registry, ACTRN12611000250932 and ACTRN12620000694910.

Modelling mucosal surface roughness in the human velopharynx: a computational fluid dynamics study of healthy and obstructive sleep apnea airways

We previously published a unique methodology for quantifying human velopharyngeal mucosal surface topography and found increased mucosal surface roughness in obstructive sleep apnea (OSA) patients. In fluid mechanics, surface roughness is associated with increased frictional pressure losses and resistance. This study used computational fluid dynamics (CFD) to analyse the mechanistic effect of different levels of mucosal surface roughness on velopharyngeal airflow.

METHODS

Reconstructed velopharyngeal models from OSA and Control subjects were modified, giving each model three levels of roughness, quantified by the curvature based surface roughness index (CBSRI_0.6; range 24.8-68.6mm^-1). CFD using the k-ω shear stress transport (SST) turbulence model was performed (unidirectional, inspiratory, steady state, 15l/min volumetric flow rate), and the effects of roughness on flow velocity, intraluminal pressure, wall shear stress and velopharyngeal resistance (R_v) were examined.

RESULTS

Across all models, increasing roughness increased maximum flow velocity, wall shear stress and flow disruption, while decreasing intraluminal pressures. Linear mixed effects modelling demonstrated a log-linear relationship between CBSRI_0.6 and R_v, with a common slope (log(R_v)/CBSRI_0.6) of 0.0079 (95%CI 0.0015-0.0143; p=0.019) for all subjects, equating to a 1.9-fold increase in R_v when roughness increased from Control to OSA levels. At any fixed CBSRI_0.6, the estimated difference in log(R_v) between OSA and Control models was 0.9382 (95%CI 0.0032-1.8732; p=0.049), equating to an 8.7-fold increase in R_v.

CONCLUSION

This study supports the hypothesis that increasing mucosal surface roughness increases velopharyngeal airway resistance, particularly for anatomically narrower OSA airways, and may thus contribute to increased vulnerability to upper airway collapse in OSA patients.

Feedback modulation of surrounding pressure determines the onset of negative effort dependence in a collapsible tube bench model of the pharyngeal airway

Negative effort dependence (NED), decreased airflow despite increased driving pressure, has been proposed as a specific obstructive sleep apnea (OSA) phenotypic characteristic. We examined conditions under which NED occurs in a collapsible tube, pharyngeal airway bench model with the chamber enclosed, focusing on relationships with surrounding pressure levels and longitudinal strain. Using a vacuum source, graded airflows (V̇; 0-5 l/s) were generated through a thin-walled latex tube enclosed within a rigid, cylindrical chamber, sealed with initial chamber pressures (Pci) of 0-5 cmH₂O (separate runs), or opened to the atmosphere. Upstream minus downstream pressure (Pu - Pd), maximum airflow (V̇_max), and chamber pressure (Pc) were measured at 0-50% longitudinal strain. NED occurred across the range of Pci and strains studied but was most pronounced for the chamber open condition. With a sealed chamber, V̇ increased and Pc decreased with increasing Pu - Pd until the onset of NED at V̇_max and a Pc value that was designated as critical (Pcc). Pcc was lowest (-17 cmH₂0) and V̇_max was highest (~5 l/s) with chamber sealed: Pci = 0 cmH₂O and 12.5 to 25% strain. We conclude that for our collapsible tube model, the achievable V̇_max before the onset of NED depends on both the initial conditions (Pci and strain) and the dynamics of feedback between driving pressure and chamber pressure (chamber sealed vs. open). NED-based phenotypic analyses for OSA may need to focus on potential feedback control mechanisms (eg lung volume change, muscle activity) that may link peripharyngeal tissue pressure levels to driving pressures for airflow.NEW & NOTEWORTHY A collapsible tube, pharyngeal airway bench model was used to study the role of surrounding pressure and longitudinal wall strain at the onset of negative effort dependence (NED). NED occurred to varying degrees across all conditions tested, but maximum airflow was achieved with 1) low initial surrounding pressure, 2) a feedback mechanism between surrounding pressure and driving pressure; and 3) a moderate amount of strain applied. Potential impacts on OSA phenotypic analyses are discussed.

Velopharyngeal mucosal surface topography in healthy subjects and subjects with obstructive sleep apnea

Macroscopic pharyngeal anatomical abnormalities are thought to contribute to the pathogenesis of upper airway (UA) obstruction in obstructive sleep apnea (OSA). Microscopic changes in the UA mucosal lining of OSA subjects are reported; however, the impact of these changes on UA mucosal surface topography is unknown. This study aimed to 1) develop methodology to measure UA mucosal surface topography, and 2) compare findings from healthy and OSA subjects. Ten healthy and eleven OSA subjects were studied. Awake, gated (end expiration), head and neck position controlled magnetic resonance images (MRIs) of the velopharynx (VP) were obtained. VP mucosal surfaces were segmented from axial images, and three-dimensional VP mucosal surface models were constructed. Curvature analysis of the models was used to study the VP mucosal surface topography. Principal, mean, and Gaussian curvatures were used to define surface shape composition and surface roughness of the VP mucosal surface models. Significant differences were found in the surface shape composition, with more saddle/spherical and less flat/cylindrical shapes in OSA than healthy VP mucosal surface models (P < 0.01). OSA VP mucosal surface models were also found to have more mucosal surface roughness (P < 0.0001) than healthy VP mucosal surface models. Our novel methodology was utilized to model the VP mucosal surface of OSA and healthy subjects. OSA subjects were found to have different VP mucosal surface topography, composed of increased irregular shapes and increased roughness. We speculate increased irregularity in VP mucosal surface may increase pharyngeal collapsibility as a consequence of friction-related pressure loss.NEW & NOTEWORTHY A new methodology was used to model the upper airway mucosal surface topography from magnetic resonance images of patients with obstructive sleep apnea and healthy adults. Curvature analysis was used to analyze the topography of the models, and a new metric was derived to describe the mucosal surface roughness. Increased roughness was found in the obstructive sleep apnea vs. healthy group, but further research is required to determine the functional effects of the measured difference on upper airway airflow mechanics.

Intrauterine and transmammary transmission ofMycobacterium aviumsubspparatuberculosisin sheep

OBJECTIVE

To investigate intrauterine infection of foetuses with Mycobacterium avium subsp paratuberculosis and the presence of infection in mammary secretions of sheep.

DESIGN

A study of 142 late-pregnant ewes and their foetuses from two heavily infected flocks.

PROCEDURE

Infection of ewes was determined at necropsy by histopathology and culture of tissues and mammary secretions. Antemortem tests (clinical assessment, faecal culture and serology) were also applied. Foetuses from 59 infected ewes and 47 apparently uninfected ewes were examined by culture and histopathology.

RESULTS

Five of five ewes with clinical ovine Johne's disease had infected foetuses. Only one of 54 subclinically affected ewes, and none of 47 uninfected ewes had an infected foetus. M a paratuberculosis was cultured from mammary secretions or mammary glands of only two of 76 ewes, both of which were clinical cases and had infected foetuses.

CONCLUSION

Although intrauterine or transmammary transmission of Mycobacterium avium subsp paratuberculosis may occur frequently in clinically affected sheep, these are less common in subclinically infected ewes. Therefore these modes of transmission are unlikely to compromise existing control programs for ovine Johne's disease on most farms, especially if programs include the immediate culling of clinically affected sheep.