title: The cardiovascular consequences of obstructive sleep apnoea
Creators: name: Robert. Coughlin, Steven
Publisher: name: University of Liverpool Repository
Year: 2023

description: Obstructive sleep apnoea (OSA) is associated with increased cardiovascular morbidity and mortality, possibly because it clusters with a number of cardiovascular risk factors that comprise and underpin the metabolic syndrome. These include, obesity, high blood pressure, insulin resistance, impaired glucose tolerance and dyslipidaemia, and an increased sympathetic activity, altered cardiac autonomic tone and reduced baroreceptor sensitivity. However, it is currently unclear whether OSA clusters with these risk factors independently of obesity as studies investigating these relationships generally used BMI as a surrogate marker of adiposity. Likewise, a lack of intervention studies for the majority of these risk factors makes it difficult to assess whether OSA directly influences their development. The first aim of this thesis was to determine whether OSA was associated with the cardiovascular risk factors that comprise the metabolic syndrome independently of obesity. This was accomplished by measuring these cardiovascular risk factors in two groups of subjects with and without OSA whom were closely matched for a range of obesity related variables. The second aim of this thesis was to determine whether these risk factors resolved with the application of nasal continuous positive airways pressure (CPAP) treatment. This was investigated by randomising subjects to either CPAP or a sub-therapeutic alternative for a six week period and then crossed-over to the alternative treatment for a further six weeks. The cardiovascular risk factors were compared following CPAP and sub-therapeutic therapy. The methodologies used to measure these cardiovascular risk factors included; blood pressure measurements, a HOMA analysis of fasting glucose and insulin values to estimate insulin resistance, a full lipid profile, a urine catecholamine analysis, and spectral analyses of baroreceptor sensitivity and heart rate variability, the latter being an indirect measure of cardiac autonomic tone. The results of these studies demonstrated that whilst OSA was independently associated with a reduced HDL cholesterol, an increased incidence of the metabolic syndrome and values of cardiac autonomic tone that have previously been demonstrated to confer an increased cardiovascular risk following myocardial infarction, only systolic, diastolic and mean arterial blood pressures were significantly reduced by CPAP. Whilst the associations of OSA with these known cardiovascular risk factors may help explain the increased cardiovascular morbidity and mortality associated with this condition, the results of the intervention study suggest that OSA clusters with the majority of these risk factors because of a common cause rather than any direct effect. Whether this common cause is a shared genetic mechanism or because subjects with OSA exhibit a sedentary lifestyle, a known risk factor for the metabolic syndrome which develops as a consequence of their daytime sleepiness, remains unclear. Further intervention using a structured physical activity programme in subjects with OSA is needed to clarify this.