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(Stroke. 2006;37:33.)
© 2006 American Heart Association, Inc.
Original Contributions |
From the Laboratory of Epidemiology, Demography and Biometry (M.H.F., R.P., L.J.L.), National Institute on Aging, National Institutes of Health, Bethesda, MD; General Preventive Medicine Residency Program (M.H.F.), Johns Hopkins Bloomberg School of Public Health, Baltimore, MD; Institut für Medizinmanagement und Gesundheitswissenschaften (M.H.F.), Universität Bayreuth, Germany; Pacific Health Research Institute (K.M., H.P., G.W.R., L.R.W.), Honolulu, HI; Honolulu-Asia Aging Study (K.M., H.P., G.W.R., L.R.W.), Kuakini Medical Center, Honolulu, HI; Department of Geriatric Medicine (K.M., H.P., G.W.R., L.R.W.), University of Hawaii, John A. Burns School of Medicine, Honolulu, HI; Honolulu Department of Veterans Affairs (G.W.R.), Honolulu, HI.
Correspondence to Lenore J. Launer, PhD, Laboratory of Epidemiology, Demography and Biometry, National Institute on Aging, National Institutes of Health, Room 3C-309 Gateway Building, 7201 Wisconsin Ave, Bethesda, Maryland 20892. E-mail launerl{at}nia.nih.gov
| Abstract |
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Methods Data are from the Honolulu-Asia Aging Study, a community-based study of Japanese American men. Midlife BP was measured in 1971 to 1974 and dementia assessment was conducted in late-life. The 2505 men who were dementia free in 1991 and had complete follow-up data were re-examined for incident dementia in 1994 to 1996 and 1997 to 1999. Their age ranged from 71 to 93 years. Survival analysis with age as the time scale was performed to estimate the risk (hazard ratio [HR] and 95% CI) for incident dementia associated with mid- and late-life tertiles of PP and mean arterial BP, as well as SBP and diastolic BP categories.
Results Over a mean of 5.1 years of follow-up, 189 cases (7.5%) of incident Alzheimer disease or vascular dementia were identified. After adjustment for cerebrovascular risk factors, dementia was significantly associated with SBP (HR 1.77; 95% CI, 1.10 to 2.84, for SBP
140 mm Hg compared with SBP <120 mm Hg), but not with PP tertiles. Limiting the analysis to those never treated with antihypertensives, high levels of all 4 BP components were significantly associated with dementia. In models with 2 BP components, only SBP remained significant in both the total sample and the never-treated subgroup (HR 2.29; 95% CI, 1.23 to 4.25, for SBP
140 mm Hg in total sample), whereas PP was not significantly associated with the risk for dementia.
Conclusions Midlife PP is not independently associated with dementia incidence. Midlife SBP is the strongest BP component predicting incident dementia.
Key Words: blood pressure dementia epidemiology
| Introduction |
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High pulse pressure (PP), the difference between systolic BP (SBP) and diastolic BP (DBP), is a measure of the pulsatile component of BP and has been shown to increase the risk of CVD and total mortality.510 Potential physiological mechanisms of the pulsatile effects on vascular disease are currently under investigation. PP is positively correlated with arterial stiffness and therefore might be a marker for CVD.11
In many societies, both average SBP and PP increase with age, whereas average DBP decreases in older individuals.12,13 In the Framingham cohort, it has been shown that the strongest predictors for CVD change across the lifespan: from DBP to SBP and ultimately to PP.14 However, some recent studies did not show any significant additional role of PP beyond SBP alone in predicting CVD outcomes or total mortality.10,15,16
The association of midlife PP with cognitive function and dementia has not yet been examined. In the Kungsholmen Project, both low and high PP measured in late-life were found to be associated with increased incidence of dementia in women but not in men.17
Here, we examine the association of mid- and late-life PP on dementia incidence in a community-based cohort of Japanese American men.
| Methods |
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Dementia Case-Finding
The multistep case-finding procedures to identify dementia cases started in 1991 and has been previously described.19,20 Briefly, the total cohort was screened with the 100-point Cognitive Abilities Screening Instrument (CASI),21 and a subset was selected to undergo further evaluation that included more detailed neuropsychologic testing, a neurologic examination, and a proxy interview. Those diagnosed with dementia received blood tests and brain imaging. Diagnoses were made in a consensus conference attended by the neurologist and 2 other study physicians with expertise in dementia. Dementia was diagnosed according to internationally accepted guidelines.2224
BP Measurements
BP was measured at each examination taking the mean of 3 measurements made 5 minutes apart on the left arm while the subject was seated. Standard sphygmanometers and cuffs were used. DBP was recorded as the fifth phase. The analysis presented here is based on the BP measurements from examination 3 (midlife) and examination 4 (late-life). PP was defined as [SBPDBP] and mean arterial pressure (MAP) as [DBP+
(PP)].
Confounding Variables and Covariates
We controlled for the potentially confounding effects of age and education and for cardiovascular risk factors. These included midlife alcohol consumption (none, <1 drink [13.2 g], 1 to 2 drinks, and
3 drinks/d) and smoking (never, former, current). The ankle brachial index, measured at the fourth examination, was dichotomized at 0.9; values below this point were interpreted as an indicator of generalized atherosclerosis.25 History of antihypertensive treatment was self-reported from exams 1 to 3 and obtained from the drug vials presented at examination 4. Diabetes was defined according to the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus.26 Stroke and coronary heart disease history were assessed at baseline in 1965 and subsequently, via surveillance of hospital records that has been carried out through the entire follow-up period. Finally, we controlled for Apolipoprotein E (ApoE)
4 allele; participants with 1 or 2 copies of the ApoE
4 allele were considered
4 positive, and
4 negative otherwise. For all variables with missing data, a separate category within each variable was defined so that the observations remained in the analysis.
Statistical Analysis
Analytical Sample
There were 226 prevalent dementia cases identified at examination 4 and excluded from this analysis; thus, there were 3508 nondemented individuals for follow-up. Among those, 418 participants died between examination 4 and 5, 462 who declined participation, and 72 subjects with missing examination 3 BP measures. In total, there were 244 cases of incident dementia identified at exams 5 (132 cases) and 6 (112 cases), including 148 (64.2%) cases of Alzheimer disease (AD), 39 (16.0%) cases of vascular dementia (VaD), and 51 other dementia cases, including Parkinson disease and undertermined subtype. Given the heterogeneity of the other cases, we excluded them from the analysis. Therefore, the analytical sample included 2505 subjects who were not demented at examination 4, had at least 1 follow-up examination for dementia status, and had their BP measured at examination 3.
We grouped the analytical sample into tertiles of PP: <42 mm Hg, 42 to 51.5 mm Hg and >51.5 mm Hg and MAP (<94 mm Hg, 94 to 105 mm Hg and >105 mm Hg). SBP and DBP were classified according to the seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7): Stage 2 (
160/100 mm Hg) and stage 1 hypertension categories (140 to 159/90 to 99 mm Hg) were combined because of sample size.27 Correlations among the continuous measures of BP were estimated. Cohort characteristics were compared across the PP tertiles with age-adjusted linear models for continuous variables and logistic regression models for dichotomous outcomes. Cox-proportional hazard models using age as the time scale28 estimated the relative risk (95% CI) of incident dementia associated with each of the main BP components. Age of dementia onset was defined as the midpoint between the last examination without and the first examination with a dementia diagnosis. The proportional hazards assumption was tested based on Schoenfeld residuals.
If there was an association between PP and risk of dementia, one would expect an increasing risk with lower DBP, after adjusting for SBP. This was examined in multivariate models, adjusting for demographic and cardiovascular risk factors as described above. Because previous analyses on this cohort have shown differences in outcomes between those who were treated and not treated with antihypertensive medications,3 we conducted stratified analyses by ever (n=1066) and never (n=1439) antihypertensive treatment. We also examined whether the ApoE
4 modified the associations of interest. Finally, we examined whether there was differential survival from examination 3 to examination 4 by BP components; there was no evidence for this. The statistical analysis was performed using Stata 8 statistical software (STATA Corp).
| Results |
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The various BP components were differently correlated, eg, SBP and PP had a correlation coefficient of 0.69 (Figure), and SBP and DBP of 0.58; the correlation between DBP and PP was only 0.08.
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In the age-adjusted analyses of the total sample with single BP components in the model, SBP, DBP and MAP, and not PP, were significantly associated with incident dementia (Table 2). After adjusting for confounders, the association remained significant only for SBP, with a hazard ratio (HR) of 1.77 (95% CI, 1.10 to 2.84) for SBP
140 mm Hg compared with SBP <120 mm Hg. Among those who were never treated for hypertension, all 4 BP components were positively and significantly correlated with incident dementia, an association that did not significantly change after adjusting for cardiovascular confounders. For example, the HR was 2.66 (95% CI, 1.51 to 4.68) for SBP
140 mm Hg compared with SBP <120 mm Hg versus a HR of 1.81 (95% CI, 1.08 to 3.04) for the highest PP tertile compared with the lowest PP tertile (Table 2). There was no differential survival occurring before the assessment of dementia between the high PP and the high SBP group.
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To test further whether there was an association between midlife PP and risk of dementia, we examined in a separate model whether there was an increasing risk for dementia with lower DBP after adjusting for SBP. Across SBP and DBP categories, highest risk was associated with higher SBP, but not lower DBP. Finally, in a separate model with 2 BP components, only midlife SBP remained significant after adjusting for PP, in both the total sample and in the subset of participants never treated for hypertension. There was no significant trend for higher or lower PP.
In models with a single BP component, results were similar for VaD and AD cases, but the relatively small number of VaD cases did not allow us to perform models with 2 BP components. Adding the baseline CASI score did not significantly change the results. There was no significant interaction between high SBP or high PP and the presence of the ApoE
4 allele. None of the analyses were significant when we performed them on late-life instead of midlife BP components.
| Discussion |
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Correlations between the 4 BP components were similar to those found in the Established Populations for Epidemiologic Studies of the Elderly.30 However, when controlling for SBP in our study, a higher PP was not significantly associated with an increased risk for dementia.
It has been argued that PP is a marker of arterial stiffness and could even be a precursor of hypertension.31 The augmentation of central PP might lead to poor perfusion of the coronary vasculature during diastole, ventricular hypertrophy and dysfunction, increased ventricular oxygen consumption and ultimately myocardial infarction.5 However, it is unknown whether PP has significant direct or indirect effects on the cerebrovascular system and the brain. Secondary to brain vascular autoregulation and the distance to the aorta, it could be less affected by central pulsatile stress.
Advantages of our study are large sample size, exposure assessment in midlife, long follow-up time, and clinical dementia assessment. Several limitations should be acknowledged. First, PP was measured peripherally at the brachial artery. Especially in elderly subjects, this leads to an underestimation of central PP, which might be more closely related to vascular damage and disease.32 Our study was limited to old Japanese American men, and associations might be different in women, other ethnic groups or younger subjects. In this sample, the majority of participants had normal BP, and most individuals with severe hypertension were treated. Our analysis was limited to those individuals who survived and remained nondemented until examination 4 and continued follow-up until examination 5 or 6. Given the higher mortality of individuals with high BP leading to an increased loss to follow-up before and after the assessment of dementia, we cannot extrapolate our results to this high-risk group. However, loss of this group would lead to an underestimation of risk in our analysis, but given the nonselective survival between the high SBP and the high PP group it is unlikely to have changed the relation between SBP and PP.
Conclusion
In this study, the association of midlife PP with dementia incidence was mainly explained by the strong correlation of midlife PP and SBP; the addition of PP to SBP did not provide significant information beyond SBP alone. Late-life BP components were not associated with dementia.
| Acknowledgments |
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Received September 6, 2005; revision received October 19, 2005; accepted October 31, 2005.
| References |
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