Obstructive Sleep Apnea
What is Obstructive sleep Apnea?
With a prevalence of four percent world wide, sleep apnoea, characterised by prolonged pauses in breathing during sleep that continues throughout the night, has become a growing concern. The impetus to fully understand the problem is precipitated by the more serious known sequelae such as hypertension, stroke and cardiac arrest. Not to be discounted though is the social ramifications of excessive daytime somnolence and snoring which are the hallmark of the more severe manifestation termed obstructive sleep apnoea (OSA). The problem is widely accepted to be mechanical in nature with breathing punctuations occurring as a result of the upper airway collapse. The major issues identified in OSA are associated with:
- airway deformation,
- muscle tone,
- mechanoreceptors and
- central nervous malfunction.
Airway dimensions address the influence of geometry on airway deformation while muscle tone embraces the material properties peculiar to pharyngeal collapse. A further complication that precipitates upper-airway closure is associated with local mechanoreceptor dysfunction which results in inappropriate contraction of the associated muscles. The most complex pathogenesis is central nervous malfunction whose sequelae fall outside the traditional framework of computational mechanics. The latter two indicate a degraded response of the brain to signals from the respiratory tract and is classified as central sleep apnoea (CSA) while the preceding sequelae are commonly associated with mechanical abnormalities and fall within the demarcations of OSA. Of equal importance is the prevalence of mixed sleep apnoea where the eventual obstruction is best realised with empirical descriptions.
- The Centre for Research in Computational and Applied Mechanics (CERECAM) at UCT performs the computational fluid mechanics and finite element simulations of the air flow and pharyngeal deformation.
- Dr Yougan Saman from the Department of Otorhinolaryngology at UKZN has performed several sleep studies on OSA patients and provides the clinical perspective to this project.
- Prof Ernesta Meintjies of the Medical Imaging Research Unit at UCT is actively involved in MRI research that is employed in extracting the geometry of the upper airway.
Research Approaches in OSA
A multidisciplinary approach is generally required for satisfactory quantification of the problem and entails detailed numerical modelling of pharyngeal deformation due to pressure stimulus from airflow during the breathing process. Displacement of the airway tissue is achieved through muscle models and their interplay while the time dependent boundary conditions (the pressure stimulus) are imposed through appropriate fluid flow modelling. The framework for muscle contraction is the finite element method (FEM) which is coupled to computational fluid dynamics (CFD) models of the air during breathing. Geometric characterisation of the pharyngeal airway is usually achieved through imaging modalities such as magnetic resonance imaging (MRI) and computed tomography (CT). The dual value of such approaches, in particular MRI, is the numerical validation from dynamic implementations such as MRI with spatial modulation magnetisation (SPAMM). Such in-situ measurements are well suited for model parameterisation and semi-empirical modelling with the latter becoming increasingly popular in light of the limited understanding of muscle dynamics.
The difficulty in accurately quantifying degraded muscle tone and/or mechanoreceptors in the upper airway of OSA sufferers can only be meaningfully resolved with detailed, in-situ measurement of the deformation and airflow. Semi-empirical relationships for contractile mechanisms and airflow are then easily incorporated into numerical modelling frameworks, yielding meaningful results that can be employed with greater confidence in clinical scenarios . Our research focuses on experimentally mapping out the deformation of the pharyngeal airway during an OSA episode with a view to
- validating the numerical models of the upper airway;
- quantitative descriptions of the underlying mechanisms governing muscle contraction and airflow in the pharyngeal airway.
- M.J. Brennick, S. Pickup, L. Dougherty, J.R. Cater and S.T. Kuna. Pharyngeal airway wall mechanics using tagged magnetic resonance imaging during medial hypoglossal nerve stimulation in rats. Journal of Physiology, 561.2: 597-610, 2004.
- Y. Huang, D.P. White and A. Malhotra. Use of computational modelling to predict responses to upper airway surgery in obstructive sleep apnea. Laryngoscope, 117: 648-653, April 2007.