Background: South Asians have a low prevalence of atrial fibrillation (AF) in comparison with Caucasians despite a higher burden of hypertension, diabetes mellitus and coronary artery disease. The reason for this disparity is unclear but may relate to variations in autonomic tone.
Methods: A prospective cohort study was performed on 80 South Asian and 80 Caucasian healthy volunteers aged 18–40 years. All subjects underwent 24-hour ambulatory ECG to assess the burden of supraventricular ectopy (SVE) and heart rate variability (HRV), a quantitative marker of autonomic activity. Fifty subjects in each cohort underwent exercise testing to assess exercise capacity and heart rate recovery (HRR), an indicator of parasympathetic activity.
Results: Subjects in each cohort were well-matched in terms of age (males: South Asian 28.0 (13) years versus Caucasian 28.0 (7) years, p=0.757; females: 22.0 (8) years versus 23.5 (8) years, p=0.687) and there was no significant difference in systolic (males p=0.674; females p=0.313) or diastolic blood pressure (males p=0.781; females 0.720). Compared with Caucasians, South Asians had a lower burden of SVE (Table 1). South Asians had a higher minimum and mean heart rate and South Asian males had lower time-domain and frequency-domain measures of HRV (Table 1). Total exercise time (males: South Asian 803.2 ± 167.5 secs versus Caucasian 961.0 ± 180.7 secs, p= 0.002; females: 633.0 ± 115.6 secs versus
772.0 ± 147.1 secs, p<0.001) and maximum metabolic equivalents (METS) achieved (males: South Asian 15.7 ± 3.1 versus Caucasian 18.7 ± 3.3,
p=0.001, females: 12.5 ± 2.1 versus 15.2 ± 2.9, p<0.001) were both significantly lower in South Asians. There was a trend towards lower HRR at 1 minute in South Asian males (South Asian 166.2 ± 12.1 bpm versus Caucasian
160.7 ± 13.4 bpm, p=0.122) but no difference was seen in females.
Conclusions: The combination of a higher mean heart rate in South Asians and lower HRV in South Asian males may reflect differences in autonomic function and would be most consistent with South Asians having sympathetic predominance. In individuals with structurally normal hearts, the onset of AF is typically preceded by an increase in parasympathetic activity. Therefore, if South Asians have a predominance of sympathetic tone, this may confer protection against the development of AF. The burden of SVE, which has previously been shown to independently predict AF, was lower in South Asians although as one might expect in healthy cohorts, the overall burden of SVE was low in both groups. Whether this influences South Asians risk of developing AF is uncertain. Further research is warranted to assess variations in autonomic tone and SVE within South Asians and its association with the development of AF.