Abstract-We hypothesized that orthostatic tolerance is higher in young, healthy black compared with white women. To determine orthostatic tolerance, 22 women (11 black and 11 white) underwent graded lower body negative pressure to presyncope. We measured blood pressure, heart rate, and R-R interval (ECG) continuously at baseline and through all of the levels of lower body negative pressure. Blood samples were taken at baseline along with presyncope for the measurement of plasma catecholamine concentrations, serum aldosterone concentration, and plasma renin activity. Cumulative stress index, the sum of the product of time and lower body negative pressure, was the indicator of orthostatic tolerance. ; PϽ0.05, for black and white women, respectively). Although heart rate increased and R-R interval decreased to a greater extent during lower body negative pressure in black women compared with white women (ANOVA: PϽ0.05), baroreflex function (ie, slope R-R interval versus systolic blood pressure) was unaffected by race. These data indicate that orthostatic tolerance is greater in black compared with white women, which appears to be a function of greater sympathetic nervous system responses to orthostatic challenges. (Hypertension. 2010; 56:75-81.) Key Words: blood pressure Ⅲ racial differences Ⅲ arterial stiffness Ⅲ sympathetic nervous system O rthostatic tolerance is a measure of the ability to maintain consciousness during changes in posture. Orthostatic stress induced by changes in posture, or by lower body negative pressure (LBNP), causes blood volume shifts to the lower extremities resulting in a fall in central blood volume. This fall in central blood volume stimulates both cardiopulmonary and arterial baroreceptors, leading to compensatory increases in heart rate (HR) and peripheral vasoconstriction. Despite the complex physiological systems evolved to maintain blood pressure during changes in posture, orthostatic intolerance is a relatively common blood pressure dysfunction in healthy young people and is more common in women than in men. 1,2 Racial differences in blood pressure regulation have been well documented with regard to hypertension and on the whole have indicated that hypertension is more prevalent in the black versus white population. 3 Moreover, hypertension manifests at a younger age in black compared with white people, and there are racial differences in the mechanisms that regulate blood pressure. 4,5 With regard to orthostatic tolerance, in response to a 3.75-minute standing test, mean arterial pressure (MAP) increased in black subjects but fell in white and Asian subjects, indicating differences in response to postural challenges across the 3 races. 6 Finally, although black subjects display smaller increases in muscle sympathetic nerve activity (MSNA) during baroreceptor unloading compared with white subjects, 4 forearm vasoconstriction is greater in black subjects, which would suggest enhanced sympathetic vascular transduction. 4,5 Although orthostatic intolerance disproportionally affects w...

We hypothesized that orthostatic tolerance (OT) is higher in young, healthy black (BW) compared to white women (WW). To determine OT, sixteen women (8 BW, 8 WW) underwent graded lower body negative pressure (LBNP) to presyncope. We measured blood pressure, heart rate, and R‐R interval (ECG) continuously at baseline and through all levels of LBNP. We took blood samples at baseline and presyncope for the measurement of plasma catecholamine and serum aldosterone concentrations and plasma renin activity. Cumulative stress index (CSI), the sum of the product of time and LBNP, was the indicator of OT. OT in the BW was ~ 50% greater than in the WW [CSI=−932.1 (402.1) vs. −496.9 (220.2) P < 0.05]. While P[NE] increased in both groups at presyncope, the increase was greater in BW [Δ P[NE] 171.0 (112.3)] versus WW [73.9 (70.9), P < 0.05], as were the increases in PRA [Δ PRA 2.6 (1.0) versus 0.6 (0.9) ng ANG II·ml−1 ·hr−1, P < 0.05, for BW and WW, respectively). Although heart rate increased and R‐R interval decreased to a greater extent during LBNP in BW compared to WW [ANOVA, P < 0.05)], baroreflex function (i.e. slope R‐R interval vs. LBNP) was unaffected by race. These data indicate that OT is greater in black compared to white women, which appears to be a function of greater sympathetic nervous system responses orthostatic stress. HL071159

Diagnostic test, level II.

The Compensatory Reserve Index (CRI) has recently been established to accurately measure the body's integrated capacity to compensate for physiological conditions of reduced central blood volume, and predict hemodynamic decompensation. We previously demonstrated that African American (AA) women have a higher tolerance to orthostatic stress. Therefore, we tested the hypothesis that the CRI would identify racial differences in orthostatic tolerance prior to the onset of traditional signs and symptoms. We measured beat‐by‐beat blood pressure (BP) and heart rate (HR) in 23 AA (22 ± 1 years; 24 ± 1 kg/m2) and 31 Caucasian women (20 ± 1 years; 23 ± 1 kg/m2) during progressive lower body negative pressure (LBNP) to presyncope. Blood pressure waveforms were analyzed using a machine‐learning algorithm to derive the CRI at each LBNP stage; stroke volume (SV) was derived from the Finometer (Model flow). Resting mean arterial BP (AA, 78 ± 3 vs. Caucasian, 74 ± 2 mmHg) and HR (AA, 68 ± 2 vs. Caucasian, 65 ± 2 bpm) were similar between groups. The CRI progressively decreased during LBNP in both groups, however the CRI in Caucasian women was 4% lower than AA women at −15 mmHg LBNP and decreased to 21% lower at −50 mmHg LBNP (P<0.05). The rate of decline in SV during LBNP was greater in Caucasian women (ANOVA P<0.05), and differences were noted as early as −20 mmHg. However changes in BP and HR during LBNP were not different between groups. At presyncope, mean arterial BP (AA, 66 ± 3 vs. white, 65 ± 4 mmHg) and HR (AA, 102 ± 4 vs. white, 97 ± 3 bpm) were similar between groups, as was the change in SV (AA, Δ −33 ± 2 vs. white, Δ −35 ± 3 mL). Finally, the time to presyncope was 1.6 minutes longer in AA women. These data support the notion that greater tolerance to the orthostatic stress induced by LBNP in AA women can be explained by their greater reserve to compensate for progressive central hypovolemia compared to Caucasian women. An important clinical translation is that the CRI provides a more sensitive and specific measure for early assessment of differences in orthostatic tolerance in comparison to traditional vital signs and symptoms or changes in hemodynamics.Support or Funding InformationResearch supported by funding from the Combat Casualty Care Research Program of the US Army Medical Research and Materiel Command and NIH HL071159.