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ORIGINAL ARTICLES |
From the Department of Psychology (J.A.S., C.R.F.), Ohio University, Athens, OH; and the Department of Psychiatry (J.C.S.), University of Pittsburgh School of Medicine, Pittsburgh, PA.
Address correspondence and reprint requests to Julie A. Suhr, PhD, Department of Psychology, 200 Porter Hall, Ohio University, Athens, OH 45701. E-mail suhr{at}ohio.edu
| ABSTRACT |
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METHODS: Participants were selected from a subsample of 5662 NHANES III respondents who completed an initial home interview, a medical examination, and a series of computerized cognitive tests of visuomotor reaction time, complex psychomotor speed (ie, symbol digit substitution), and verbal learning/attention (ie, serial digit learning).
RESULTS: Hierarchical regressions tested the contribution of resting blood pressure to the prediction of performance on each of the cognitive tests. Results indicated that, after controlling for demographic variables (age, sex, race/ethnicity, education) and resting blood pressure, the interaction of systolic blood pressure by age was a significant predictor of performance on the test of verbal learning/attention. Follow-up analyses revealed that higher systolic blood pressure was associated with poorer performance in those younger than 40 years.
CONCLUSIONS: Expanding on findings from previous epidemiological studies, the present study reports a small but significant relationship between resting blood pressure and cognitive performance that is particularly evident in younger healthy adults.
Key Words: blood pressure, cognition, hypertension.
Abbreviations: NHANES III = Third National Health and Nutrition Examination Survey;; NES = Neurobehavioral Evaluation System;; CNS = central nervous system;; SBP = systolic blood pressure;; DBP = diastolic blood pressure.
| INTRODUCTION |
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It has been hypothesized that the mild neuropsychological impairments associated with hypertension are related to hypertrophic changes of central nervous system (CNS) blood vessels resulting from recurrent autoregulation of blood pressure within the brain (27). These vascular changes may result in small but significant decreases in blood flow to the CNS, and, perhaps more importantly, may render blood vessels less efficient at producing dilatory responses required to enhance blood flow to focal brain regions in response to specific cognitive tasks. In line with this explanation, Jennings et al. (27,28) reported that hypertensive individuals (with a mean age of 60 years) exhibit a general reduction of cerebral blood flow relative to those with normal blood pressure and show smaller increases in cerebral blood flow to active areas of the brain during completion of cognitive tasks. Interestingly, evidence from cognitive testing suggests that this effect may be more pronounced in younger adults. For example, in two epidemiological studies that included individuals with a much wider range of ages (ie, from the 20s to the 80s), an interaction between blood pressure and age was detected, with a stronger relationship between blood pressure and cognitive performance observed among younger adults (29,30). Similarly, at least one case-control study has demonstrated that differences in cognitive performance between individuals with and without hypertension are more pronounced in younger cohorts (10).
The present study sought to replicate and extend previous findings by assessing the relationship between blood pressure and cognitive function in 2727 men and women between the ages of 20 and 59 years who participated in the Third National Health and Nutrition Examination Survey (NHANES III). The specific goals of this study were to reexamine the previously reported linear relationship between blood pressure and cognitive performance in a larger and more diverse sample of young to middle-aged adults who were carefully screened for alternative medical explanations for cognitive impairment and to determine whether the relationship between blood pressure and cognition, if observed, is more pronounced in younger versus older adults.
| METHODS |
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The sample for this study was taken from all respondents between the ages of 20 and 59 years who completed the Household Adult Questionnaire, the medical evaluation, and the NES tests (N = 5662). From this sample, 283 respondents were excluded because they had incomplete NES data. To control for potential alternative causes of cognitive impairment, we also excluded, in the following order, respondents who reported a history of stroke (N = 29), who reported taking 3 or more prescription medications (N = 412), who reported taking any prescription medications other than antihypertensive medications (N = 1530), who reported being exhausted on the questionnaire administered immediately before the cognitive tests (N = 69), who reported consuming 2 or more alcoholic drinks in the 3 hours directly preceding administration of the cognitive tests (N = 9), who reported putting forth no effort on the cognitive tests (N = 85), and who did not take the cognitive tests in English (N = 495). It should be noted that another 23 respondents who were statistical outliers on the measures of cognitive performance were excluded from analyses (see below), leaving a final sample of 2727 individuals. Also of note, we planned to exclude individuals with a self-reported diagnosis of diabetes, but other exclusionary criteria above eliminated all such individuals from the sample.
Measures
Resting Blood Pressure
A total of 6 blood pressure readings were obtained from each respondent. The procedures for the blood pressure assessments as well as the specific instrumentation are described in detail elsewhere (31). Briefly, using a Baumanometer Gravity Rx sphygmomanometer with calibrated V-Lok cuff, 3 seated blood pressure readings were taken by a trained interviewer during the household interview after administration of the Household Adult Questionnaire. Respondents were asked to refrain from smoking or drinking coffee or alcohol during the interview, and they were instructed to remain relaxed during the blood pressure measurements. During a subsequent medical examination, a physician recorded 3 more seated blood pressure readings using a Baumanometer wall model standard sphygmomanometer with a calibrated V-lok cuff. Because blood specimens were also obtained during the medical examination, respondents were instructed to fast for at least 6 hours before their examination.
Mean blood pressure readings obtained in the home and at the medical examination were significantly correlated for both systolic blood pressure (SBP, r = 0.74, p < .001) and diastolic blood pressure (DBP) (r = 0.61, p < .001), and differed by less than 2 mm Hg between settings (SBPhome, mean = 119.2, SD = 14.4; SBPexam, mean = 117.0, SD = 14.7; DBPhome, mean = 74.9, SD = 11.9; DBPexam, mean = 73.8, SD = 11.8). As can be seen in Table 1, simple correlations between blood pressure and cognitive variables were similar for both the home and medical examination blood pressure readings. As a result, to provide a single, reliable estimate of resting blood pressure levels for all participants, the mean of all six blood pressure readings (home + medical examination) was used in the subsequent analyses.
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Because the measures of performance on the cognitive tests were not normally distributed, they were transformed to approximate a normal distribution. Specifically, the performance measures on the visual reaction time test (ie, mean latency to respond to a visual stimulus across 50 trials) and the symbol digit substitution test (ie, mean latency to complete a trial across five trials) were inverse transformed such that lower scores on the transformed variables were indicative of worse performance. The performance measure on the serial digit learning test (ie, total number of errors across trials required to learn the criterion) was square root transformed such that higher scores were indicative of worse performance. As noted above, seven statistical outliers on the symbol digit substitution test and 16 outliers on the visual reaction time test were excluded from all analyses.
Data Analysis
As an initial exploration of the NES data and to compare performance in our smaller sample to that obtained in larger NHANES III reports (34), we examined the relation of demographic characteristics (age, education, sex, self-reported racial/ethnic background) to the NES variables using simple correlation and t tests or one-way analysis of variance, where appropriate. To test specific hypotheses regarding 1) the relation of blood pressure to cognitive performance and 2) interaction between blood pressure and age in explaining cognitive performance, we first conducted three hierarchical regression analyses (one for each NES performance measure) with age, race/ethnicity, education, and sex entered on the initial step as control variables. Resting SBP was then offered as a potential predictor on the second step of the regression. The age by blood pressure interaction was offered on step three. Finally, SBP by sex and SBP by self-reported race/ethnicity interactions were entered on step 4. Significant blood pressure by age interactions were examined using hierarchical regressions for separate age groups (ie, 2029, 3039, 4049, and 5059 years). In addition, significant hierarchical regressions were followed up with parallel hierarchical logistic regression analyses, using clinical impairment on cognitive tests as the dependent variable. Clinical impairment was defined as a score that might be interpreted as clinically meaningful in a testing setting (performance falling 1.5 SD or greater from the mean for the task based on data from the entire sample).
| RESULTS |
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Demographic Variables and Neurobehavioral Test Performance
Consistent with prior research (34), significant correlations were observed between age, education, and performance on the three NES tests. Higher age and lower educational level were related to worse performance on all three tests (age rs = 0.04 to 0.43, education rs = 0.17 to 0.44). Males performed better than females on all three tasks, and self-reported non-Hispanic whites performed better than individuals in the three other self-reported racial-ethnic groups. Thus, age, education, sex, and self-reported race-ethnicity (recoded as non-Hispanic/white vs. all other groups, given the above findings) were entered on the first step of each of the regression analyses.
Blood Pressure and Neurobehavioral Test Performance
As Table 1 demonstrates, elevations in SBP or DBP were significantly related to worse performance on both the symbol digit substitution task (r = 0.16 to 0.23, p < .001) and the serial digit learning task (r = 0.11 to 0.13, p < .001). The following regression analyses use SBP as a predictor; however, it should be noted that a similar pattern of relationships was observed when the analyses were repeated using DBP instead of SBP.
Visual Reaction Time
As can be seen in Table 2, the demographic control variables of age, race/ethnicity, education, and sex combined to account for 9% of the variance in performance on the visual reaction time test. Neither SBP nor the blood pressure by age interaction significantly added to the variance explained in test performance. The interactions between sex and SBP as well as race/ethnicity and SBP also did not add significantly to variance explained in performance on this task.
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Serial Digit Learning
For the serial digit learning test, the set of demographic predictors accounted for 20% the variance in performance (Table 2). Although SBP did not explain significant additional variance in performance on this task, the SBP by age interaction was significant. Neither the SBP by sex interaction nor the SBP by race/ethnicity interaction added to variance explained in performance on the serial digit learning task.
Blood Pressure by Age Interaction
To further examine the SBP by age interaction observed on the serial digit learning test, we conducted four additional hierarchical regressions, separately by age groups. In each regression analysis, education, sex, and race/ethnicity were entered on the first step, and SBP was entered on the second step. As can be seen in Table 3, SBP accounted for a significant amount of variance in test performance above and beyond the demographic predictors in the two younger age groups (ie, 2029 years and 3039 years). In contrast, for the two older age groups (ie, 4049 years and 5059 years), blood pressure was not a significant predictor of test performance after controlling for the demographic variables.
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| DISCUSSION |
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Although the relationship between blood pressure and cognitive performance observed in this study was small in magnitude, it may nonetheless have important implications. One significant implication of the current findings is that blood pressure-related cognitive deficits may be observable in a wide range of the population, including young, healthy adults. Whereas previous findings were based largely on older populations with multiple medical conditions, the current results confirm a relationship between elevated blood pressure and diminished cognitive performance even among young adults who were screened for the presence of many potential confounding contributors to decrements in cognitive functioning (eg, medication use, neurological impairment, cardiovascular disease, diabetes, alcohol use, and fatigue and effort during testing).
An additional important finding is the blood pressure by age interaction. In line with prior studies (29,30), our analyses indicate that the most pronounced relationship between elevated blood pressure and diminished cognitive performance was observed among the younger individuals in our sample (ie, those under 40 years of age). Although the mechanism of this effect remains to be elucidated, one possibility is that this reflects increased severity of CNS autoregulatory responses to higher blood pressure in a subset of adults particularly vulnerable to development of hypertension and its future health consequences. Alternatively, our exclusion criteria may have introduced a selection bias such that older participants who show similar relationships between blood pressure and cognitive performance may have been excluded because they were more likely to have developed hypertension-related medical conditions (eg, stroke).
Yet another important implication of our findings is the nature of the blood pressurecognitive performance relationship. Although most studies of older adults also report an inverse, linear relationship between SBP and cognitive performance (1525), others have argued for a curvilinear relationship based on existing evidence that low blood pressure may be related to cognitive decline in adults over 65 (26,3740). This may be particularly true for individuals with cardiovascular disorders such as stroke, myocardial infarction, carotid sinus syndrome, orthostatic hypotension, or vasovagal syndrome, due to the increased likelihood of microvascular pathology (38) or reduced CNS blood flow (41) associated with these conditions. Alternatively, it has also been argued that the apparent relationship between hypotension and cognitive impairment may arise from sample bias inherent in longitudinal studies of older populations (42). That is, participants with lower blood pressure are more likely to survive into a prolonged follow-up evaluation period and, therefore, will also have a greater opportunity to exhibit age-related cognitive impairment. Regardless of the accuracy of these explanations, in our relatively young and healthy sample, a linear relationship provided the best fit to the available data.
Even though our findings provide an interesting extension of the existing literature, there are several limitations that must be noted. First, because only three brief cognitive tests were administered as part of the NHANES III, it is impossible to provide a comprehensive assessment of cognitive abilities or to provide repeated assessment within a particular domain of function. It is important to note that the vast majority of prior epidemiological studies have assessed cognitive performance using measures such as the Mini-Mental State Examination, although at least one used measures of memory and attention (21). Second, the NES tests were not selected specifically for the purpose of assessing potential cognitive consequences of blood pressure and, consequently, are not measures that are frequently used in neuropsychological studies of the effects of hypertension. For example, in the Waldstein et al. (43) comprehensive review of the case-control study literature, few investigations used measures of basic reaction time and only one used a serial digit learning task. Digit symbol and symbol digit substitution tasks are often used in studies of the cognitive effects of hypertension, although consistent group differences have not been observed across studies. Indeed, some of the most consistent findings in the hypertension literature are changes in memory and attention (43), skills that are tapped, at least in part, by the serial digit learning task. Interestingly, prior research with the NES serial digit learning task suggests that it is sensitive to neurological dysfunction (44), and factor analysis suggests that it is more strongly related to performance on traditional neuropsychological instruments than the other two NES variables utilized in the present study, which were more consistently associated with a factor that was interpreted as "computer performance," rather than tapping specific cognitive skills per se (45).
Finally, while multiple blood pressure measurements were obtained on two different occasions, these readings may not be an accurate reflection of typical blood pressure in all participants. Blood pressure reactivity to medical assessment and the minimal control over possible environmental influences on blood pressure levels (eg, recent substance use, eating, drinking, and physical activity) are just two potential sources of variability. While our analyses of the home and medical examination data suggested relative stability of measurement and lack of "white coat" reactivity at the group level, it would be important to replicate findings using more stable measures of blood pressure, such as 24-hour ambulatory monitoring of blood pressure levels.
In conclusion, the findings from the present study provide evidence that increased resting SBP is related to poorer performance on an attentional/learning task in a large sample of young, healthy adults of diverse heritage. These findings provide an important extension to the existing literature on older adults and suggest that comprehensive testing with younger populations may provide important insights into the range and severity of cognitive deficits that can predate the development of hypertension.
Received for publication July 1, 2003.
| REFERENCES |
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