Morphological and electrophysiological differences between the Caucasian and South Asian Atrium

Introduction: South Asians (SAs) have a low prevalence of atrial fibrillation (AF) compared with Caucasians despite a higher prevalence of hypertension, diabetes mellitus and coronary artery disease. The aim of this thesis was to determine whether this was related to an under-detection of the arrhythmia and if not, whether differences in left atrial (LA) size, electrophysiological properties or autonomic function in SAs might help to explain this disparity. Methods: Retrospective and prospective cohort studies were performed on SA and Caucasian participants using data from implantable cardiac devices, cardiac magnetic resonance imaging scans, invasive electrophysiology studies and a range of non-invasive cardiac investigations. Results: The cumulative incidence of subclinical AF was significantly lower in SAs compared with Caucasians (log rank p=0.002) with an annual event rate of 6.9% versus 13.9%. In comparison with Caucasians, SAs were of a smaller height with lower lean body mass and higher waist:hip ratio; had lower minimum (27.7±11.1 ml vs 34.9±12.3 ml, p=0.002) and maximum LA volumes (64.7±21.1 ml vs 80.9±22.5 ml, p<0.001) even after matching for body surface area; lower P wave dispersion (males 28.0(12)ms vs 25.0(12)ms, p=0.039; females 24.0(12)ms vs 22.0(12)ms, p=0.004) and P wave terminal force in lead V1 (males 0.031(0.04)mm•s vs 0.021(0.03) mm•s, p=0.023; females 0.036(0.04)mm•s vs 0.034(0.04)mm•s, p=0.030), electrophysiological variations related to the inhomogeneity of LA conduction and LA size respectively; increased heart rate (82.5(18)bpm vs 78.0(18)bpm, p=0.024), lower atrioventricular (280(50)ms vs 300(60)ms, p=0.001) and ventriculoatrial (300(60)ms vs 320(93)ms, p=0.013) effective refractory periods and lower heart rate variability (in SA males), suggestive of sympathetic predominance. Conclusions: SAs have reduced LA size and evidence of increased sympathetic tone and reduced inhomogeneity in LA conduction. The morphological, electrophysiological and autonomic differences identified in SAs may help to explain why this ethnic group has a lower prevalence of AF.