September 2003

From American Physiological Society

New study shows why hypertension affects black males disproportionately

Bethesda, MD Hypertension (HT) remains a public health challenge because it is so prevalent and leads to increases in cardiovascular disease. It is a disease that disproportionately affects African-American males. Not only does HT occur more frequently among this group, it also presents itself earlier in their life, and causes increased complications of cardiovascular diseases compared with white Americans.

Because changes in the arterial wall compliance and autonomic function often precede the onset of HT, a team of researchers has sought to identify whether differences in these areas exist among young, health African-American males who show no evidence of hypertension and compared them with their non-African-American counterparts. A joint research team from Columbia and Howard Universities has proven that indeed such differences exist. Moreover, as changes in these regions may be antecedent markers of HT, a safe, relatively inexpensive screening and detection program should be considered to target individuals at risk. Such early screening may help reduce the staggering human and financial impact the disease has upon black men in America.

A New Study

The authors of a new study, "Low Arterial Compliance in Young African-American Males," are Adrienne S. Zion, Richard P. Sloan, Matthew N. Bartels, John A. Downey, Ronald E. De Meersman, and Robert E. Fullilove, of Columbia University, New York, NY, and Vernon Bond, Richard G. Adams and Deborah Williams, of Howard University, Washington, DC. Their findings appear in the August 2003 edition of the American Journal of Physiology -- Heart and Circulatory Physiology, one of 14 scientific journals published monthly by the American Physiological Society (APS) (http://www.the-aps.org).

Methodology

Thirty-two African-American (AA) volunteers were compared with a similar group of 29 non-African-American (NAA) males recruited from the staff and student body of Columbia and Howard Universities. All participants were American born, listed English as their primary language, and had no familial history of HT. In accordance with the Institutional Review Boards, subjects provided written informed consent before being tested.

The AA group consisted of 32 males and the NAA group consisted of 29 males. Via self-report, individuals with two parents or two grandparents of African descent were assigned to the AA group, and those who did not have any parent or grandparent of African descent were assigned to the NAA group. The racial distribution of the non-African-American group was 55% Caucasian (16/29), 31% Asian (9/29), and 14% Hispanic of non-African descent (4/29).

Subjects arrived at the laboratory between 7:00 and 10:00 AM Tuesday through Friday after an overnight fast. Caffeinated products were not consumed before the testing protocol. After anthropometric measurements were taken, subjects were instrumented with electrocardiograms and beat-by-beat radial blood pressure (BP). Respiratory recording data were captured and stored on a computer. After a 15-min seated equilibration period in which BP, heart rate, and respiratory rate fluctuated <5%, 5 min of resting data were acquired in accordance with published recommendations and sampled. The area under the diastolic pressure waveform of the radial artery was utilized to estimate compliance.

Resultant values were averaged and yielded an estimate of arterial compliance. Power spectral density analysis of heart rate variability (HRV) and blood pressure variability (BPV) were used to derive measures of autonomic modulation. A priori power spectra of R-R intervals within the 0.15- to 0.4-Hz range were defined as the high-frequency (HF) component of HRV (denoted as HFRR), representing primarily parasympathetic modulation.

All spectral data were log transformed to remove skewness and minimize the large standard deviations customarily present in these data. Digitized R-R intervals and in-phase systolic peaks determined spontaneous BRS by a modification of the sequence technique shown to have a high correlation to invasive methods. After these assessments were completed, maximum oxygen consumption ( O2 max) capacity tests were performed and classic criteria were used to determine whether a maximal effort had been achieved.

Independent t-tests were performed for all variables pertaining to the hypotheses. As a secondary analysis, a discriminant analysis was computed to determine the relative strength of the arterial compliance, autonomic variables, and BRS in distinguishing the members of one racial group from another and in correctly predicting the racial group to which each sample participant belonged.

Results

The investigators found that:

  • the AA group had lower arterial compliance, HFRR (parasympathetic modulation), and BRS in addition to a higher LF/HF power ratio (sympathovagal balance);

  • mean ages, heights, weights, and body mass indexes were similar in both groups, and there were no differences between groups for O2 max values, resting heart rates, systolic BP (SBP), and respiratory rates; and

  • the LFRR and LFSBP components (sympathetic modulation) failed to reach levels of significance.

To further assess the accuracy of the analysis, the researchers also computed discriminant scores for each sample participant and compared each participant's predicted racial group with his actual race. Approximately 77 percent of the cases were correctly classified (P < 0.05) within the discriminant analysis model; baroreflex sensitivity was the single best predictor of race and was closely followed by arterial compliance.

Conclusions

These results demonstrate that young, normotensive African-American males have differences in arterial compliance and autonomic modulation compared with a similar group of non-African-American males. The loss of arterial compliance has been proposed as a possible mechanism in the initiation, progression, and etiology of HT, and as a prognostic marker for cardiovascular disease. The findings in this cross-sectional investigation may be suggestive that the establishment of clinical disease may occur much earlier than believed.

On the basis of public health statistics, it is conceivable that these subjects may develop HT at a future point in their lives. Accordingly, when preclinical markers of disease risk are verified in young asymptomatic African-Americans, aggressive behavioral adaptations should be made early to minimize the progression and onset of cardiovascular disease.

The methods used to evaluate arterial compliance and autonomic function in the study were safe, noninvasive and caused no discomfort to the subjects. The results of this study and the methods used for evaluation could thus lead to a screening and detection program targeting individuals at risk.

Source: August 2003 edition of the American Journal of Physiology -- Heart and Circulatory Physiology.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.











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