South Asians have adverse cerebrovascular haemodynamics, despite equivalent blood pressure, compared with Europeans. This is due to their greater hyperglycaemia

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Abstract
Background South Asians have a 1.5-fold increased stroke mortality compared with Europeans, despite similar blood pressures (BP). We hypothesized that it is the greater hyperglycaemia in South Asians that increases stroke risk, by adversely affecting cerebrovascular haemodynamics. Methods A population-based sample of 149 Europeans and 151 South Asians underwent metabolic profiling and concurrent measurement of finger BP using a Finapres and middle cerebral artery (MCA) blood flow velocity using transcranial Doppler ultrasound. Cerebrovascular autoregulation, cerebrovascular resistance [resistive index (RI) and pulsatility index (PI)] were calculated. Means of cerebrovascular haemodynamic measures were compared by ethnicity, with the introduction of explanatory variables to a regression model to determine which variable could best account for ethnic differences. Results Cerebrovascular resistance (RI) was 12.9 × 103 (0.9–24.8, P = 0.04) greater in South Asians than Europeans. Systolic, diastolic and mean MCA velocities were also higher in South Asians (mean velocity 41.4 ± 8.0 cm/s vs 38.0 ± 8.0 cm/s, respectively, P = 0.001). Low frequency gain, a measure of autoregulation, was worse in South Asians compared with Europeans (0.50 ± 0.01 cm/s mm/Hg vs 0.45 ± 0.01 cm/s mm/Hg, P = 0.01). RI positively correlated with HbA1c (r = 0.184; P < 0.01). Adjustment for BP could not explain the higher RI in South Asians, but adjustment for HbA1c abolished the ethnic difference in RI (5.8 × 103 (−6.5 to 18.1, P = 0.4). Conclusions Cerebrovascular resistance and autoregulation are worse in South Asians than in Europeans, despite equivalent resting BP. The greater hyperglycaemia in South Asians accounts for their adverse cerebrovascular resistance. This could explain excess stroke in South Asians but requires testing in longitudinal studies.