Evidence that Young Normotensive South Asians Show Early Signs of Disturbed Arterial Blood Pressure (ABP) Regulation of in Acute Mental Stress Relative to White Europeans

Abstract

The risk of cardiovascular disease (CVD) including hypertension is higher in South Asians (SAs) than White Europeans (WEs). But, whether disturbances in autonomic control of the cardiovascular system are present in early adulthood in SAs has not been tested. Thus, we performed studies on young 20 WEs and 21 SAs aged (21.9±0.36, 20.7±0.41 respectively, equal numbers of men and women in each group) in whom arterial blood pressure (ABP), electrocardiogram (ECG), heart rate (HR) and respiration were recorded at rest, during mental stress (3 min Colour Stroop test) and during 5 min slow breathing (6 breaths/min). Stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were computed from the pulse contour. Baroreflex sensitivity (BRS) was calculated by the sequence method as change in R-R interval evoked by spontaneous up and down sequences in systolic pressure (SP) at rest and during mental stress and as the relationship between R-R interval and the fall in SP evoked by standing from a squat before and at 0, 3, 6 min following the stress test. HRV was computed in time-domain during mental stress and slow breathing. Comparisons within and between WEs and SAs were done by paired and un-paired t-tests respectively. At rest, WEs and SAs had similar mean ABP (mABP, 86.9±1.42 vs 87.1±1.77mmHg), HR (71.1±2.45 vs 76.2±3.56 beats/min), CO (4.8±0.21 vs 5.2±0.21L/min), SV (76.5±3.86 vs 81.5±2.97ml), TPR (1.14±0.06 vs 1.01±0.04 resistance units), and BRS (up-sequence: 1.2±0.05, down-sequence: 1.2±0.05 vs 1.0±0.09 and 1.2±0.09ms/mmHg). Further, mental stress evoked similar increases in mABP and HR in WEs and SAs but was accompanied by an increase CO in WEs and by an increase in TPR in SAs. Concomitantly, vagal indices of HRV were depressed during mental stress in SAs only (RMSSD: 66.2±7.8 to56.2±11.4 and 67.6±8.7 to 50.0±6.5*ms, *: P<0.05 within WE or SAs). Further during up-sequences in SP, BRS was depressed in both SAs and WEs during mental stress, but during down-sequences, BRS was depressed in WE only (to 0.59±0.10 vs 0.96±0.08*ms/mmHg). Also, following mental stress, BRS during squat to stand, an index of vagosympathetic activity, was depressed in WEs only (0.5±0.04 at rest to 0.3±0.04* vs 0.5±0.06 to 0.5±0.06ms/mmHg). On the other hand, slow breathing, tended to decrease ABP in both WEs and SAs (82.1±1.8 to 79.2±2.1mmHg: P<0.05; 79.4±2.0 to 77.4±2.0mmHg: P=0.09 respectively) and increased vagal indices of HRV in both WEs and SAs (66.3±8.6 to 86.0±13.2*ms; 67.6±8.7 to 86.48.8*ms). These results suggest that during and after mental stress, young normotensive SAs show greater sympathetic vasoconstriction than WEs, and attenuated respiratory modulation of HR, while their ability to evoke baroreflex tachycardia by vagal inhibition and sympathetic activation is preserved relative to young WEs. Such disturbances in would be expected to increase the risk of future CVD and hypertension in SAs, particularly in those with high levels of daily stress. The finding that slow breathing increased vagal activity in young SAs raises the possibility that regular slow breathing may help to normalize autonomic regulation of ABP in SAs and limit future CVD.