Context The long-term risk for developing hypertension is best described by
the lifetime risk statistic. The lifetime risk for hypertension and trends
in this risk over time are unknown.
Objectives To estimate the residual lifetime risk for hypertension in older US
adults and to evaluate temporal trends in this risk.
Design, Setting, and Participants Community-based prospective cohort study of 1298 participants from the
Framingham Heart Study who were aged 55 to 65 years and free of hypertension
at baseline (1976-1998).
Main Outcome Measures Residual lifetime risk (lifetime cumulative incidence not adjusted for
competing causes of mortality) for hypertension, defined as blood pressure
of 140/90 mm Hg or greater or use of antihypertensive medications.
Results The residual lifetime risks for developing hypertension and stage 1
high blood pressure or higher (≥140/90 mm Hg regardless of treatment) were
90% in both 55- and 65-year-old participants. The lifetime probability of
receiving antihypertensive medication was 60%. The risk for hypertension remained
unchanged for women, but it was approximately 60% higher for men in the contemporary
1976-1998 period compared with an earlier 1952-1975 period. In contrast, the
residual lifetime risk for stage 2 high blood pressure or higher (≥160/100
mm Hg regardless of treatment) was considerably lower in both sexes in the
recent period (35%-57% in 1952-1975 vs 35%-44% in 1976-1998), likely due to
a marked increase in treatment of individuals with substantially elevated
blood pressure.
Conclusion The residual lifetime risk for hypertension for middle-aged and elderly
individuals is 90%, indicating a huge public health burden. Although the decline
in lifetime risk for stage 2 high blood pressure or higher represents a major
achievement, efforts should be directed at the primary prevention of hypertension.
High blood pressure is a key risk factor for cardiovascular disease
events.1-3 The
long-term risk for developing hypertension in an individual is best described
by the lifetime risk statistic, ie, the probability
that an individual will develop hypertension over the course of his or her
remaining lifetime. Lifetime risk estimates for disease conditions are more
easily understood by the general public compared with other measures of disease
frequency, such as age-specific prevalence.4-6
For example, the American Cancer Society used the "1 in 9" statistic for the
lifetime risk for breast cancer in the early 1990s to promote mammographic
screening.5 Furthermore, lifetime risk estimates
also may be used to assess temporal trends in long-term disease risk.4 Although lifetime risk estimates are available for
several chronic disease conditions, including breast cancer, dementia, fractures,
and coronary heart disease,5,7-9
the lifetime risk for developing hypertension has not been reported.
Although the prevalence of hypertension is an indicator of burden of
disease in the community, it does not tell us about the risk for developing
hypertension in individuals. Data from the serial National Health and Nutritional
Examination Surveys (NHANES) indicate a decrease in the age-adjusted prevalence
of hypertension in the community in the1976-1991 period compared with surveys
during the 1960-1974 period.10 A trend of decreasing
blood pressure levels has been noted also in regional cross-sectional surveys
of cardiovascular disease risk factors11 and
in longitudinal cohort studies.12,13
Although these prevalence trends are encouraging, these data should not be
interpreted to indicate that the risk for developing hypertension has decreased
in recent times. It is possible that the lifetime risk for hypertension may
have increased in recent years because of an increase in life expectancy14 and an increase in obesity in the United States.15 On the other hand, the lifetime risk for hypertension
may have declined because of preventive efforts directed at patients with
high-normal blood pressure.16 Accordingly,
we estimated the residual lifetime risk for developing hypertension among
Framingham Heart Study participants.
The Framingham Heart Study is a prospective longitudinal cohort study
that began in 1948 with the enrollment of 5209 men and women who were free
of cardiovascular disease and between the ages of 28 and 62 years.17 These participants since have been examined every
2 years. Participants who were free of hypertension in 1975 and who attended
subsequent biennial examinations were eligible for our investigation. We chose
the period from 1976 onward up to 1998 so our estimates would reflect contemporary
experience. We evaluated the residual lifetime risk for hypertension beginning
at the baseline ages of 55 and 65 years because the risk for developing hypertension
increases markedly during and after the sixth decade of life.18
Additionally, sufficient numbers of younger participants were not available
in our cohort to estimate risks for those younger than 55 years.
Participants could contribute blood pressure information at more than
1 baseline age provided they reached the next baseline age free of hypertension.
For example, a 55-year-old nonhypertensive subject attending a biennial examination
in 1976 could contribute information over the next 10 years for this baseline
age. If he or she was free of hypertension in 1986, the subject could provide
additional information as a 65-year-old individual for the remainder of the
time between 1986 and 1998.
Measurement of Blood Pressure
At each biennial examination, participants underwent a physical examination
(with a medical history), laboratory assessment of cardiovascular disease
risk factors, and routine electrocardiography. A physician measured the systolic
and diastolic blood pressures of seated study participants using a mercury
column sphygmomanometer and a standard protocol. The first and the fifth Korotkoff
sounds were taken as indicative of systolic and diastolic blood pressure,
respectively. The average of 2 such systolic and diastolic blood pressure
readings was taken as the examination blood pressure. At each examination,
participants were asked if they used antihypertensive medications for the
specific purpose of lowering elevated blood pressure.
Blood Pressure on Follow Up
All eligible participants were followed up from the time of entry into
the study until the end of the observation period, the development of hypertension,
death (in the absence of hypertension), or the last follow-up examination.
We examined separately the development of each of the following 5 blood pressure
outcomes:
Hypertension defined according
to the recommendations of the Joint National Committee on Detection, Evaluation,
and Treatment of High Blood Pressure (JNC VI)19
and the World Health Organization-International Society of Hypertension1 as a systolic blood pressure of 140 mm Hg or higher
and/or a diastolic blood pressure of 90 mm Hg or higher or the use of medications
for the purpose of treating high blood pressure at a qualifying examination;
Use of antihypertensive medication on follow-up
for the treatment of hypertension;
Stage 1 high blood pressure or
higher, defined as blood pressure of 140/90 mm Hg or higher regardless
of the use of antihypertensive medications;
Stage 2 high blood pressure or
higher, defined as blood pressure of 160/100 mm Hg or higher regardless
of the use of antihypertensive medications;
Stage 2 high blood pressure or higher or use of
antihypertensive medication on follow-up.
Residual Lifetime Risk: Definitions. The term residual lifetime risk has been defined
in 2 ways in the literature: (1) unadjusted lifetime cumulative incidence
that indicates the cumulative risk over the remaining lifetime but does not
consider the impact of mortality due to competing causes; and (2) mortality-adjusted
lifetime cumulative incidence.20 We estimated
both statistics in our study.
Residual Lifetime Risk for Developing Hypertension and Other Blood
Pressure Outcomes. The residual lifetime risk for developing the blood pressure-related
outcomes was estimated for study participants who attained the ages of 55
and 65 years free of hypertension during the 1976-1998 period. Sex-specific
analyses were performed separately for these 2 baseline ages. For 55-year-old
participants, we estimated the risk for developing hypertension through age
80 years. For 65-year-old participants, we estimated the risk for developing
hypertension through age 85 years. These follow-up time intervals (25 years
for 55-year-olds and 20 years for 65-year-olds) correspond to the current
mean residual life expectancies for white individuals at these 2 ages in the
United States.21
The residual lifetime risk for each blood pressure-related outcome (including
hypertension) was calculated using the Practical Incidence Estimator macro
detailed elsewhere.20 This method uses survival
age as the time scale, combines information on participants entering the observation
periods at different ages, and accounts for varying durations of follow-up
of individuals. Briefly, a modified Kaplan-Meier method was used with survival
age as the time scale (Figure 1).
For example, the remaining lifetime risk for hypertension for 55-year-old
participants is simply the cumulative incidence of hypertension over 25 years
(estimated residual life expectancy) thus:
Cumulative Incidence55 = Σi(hiSi−1)
where hi is the conditional probability
of developing hypertension at age i years given survival
beyond age i −1 years, Si −1 is the probability of survival beyond
age i −1 years free of hypertension, and hi Si −1 is the unconditional
probability of developing hypertension at age i years, where
the summation is from i = 55 to i = 80. Thus, a person who attends an examination at age 61 years and
is free of hypertension but who did not attend any examination at age 55 years
(subject B in Figure 1) still can
contribute to the estimation of lifetime risk for baseline age 55 years. If
the subject then attends an examination 4 years hence (at age 65 years) and
is still free of hypertension, he or she would have contributed 4 person-years
of observation as a nonhypertensive individual to this baseline age group.
In similar fashion, person-years of observation on all individuals older than
baseline ages of 55 and 65 years are summated. The 95% confidence intervals
(CIs) of the lifetime risk were calculated as described previously.20 In the calculation of unadjusted cumulative incidence,
participants who die free of hypertension were censored (treated as withdrawals
from the risk sets). Additional analyses were performed in which the lifetime
risk for hypertension was estimated adjusting for competing causes of mortality
using double decrement life table analyses.20
In these analyses, nonhypertensive participants who died due to other competing
causes were treated as "escapees" (ie, they cannot develop hypertension and
do not contribute to the estimation of hypertension incidence).
Residual lifetime risks for developing hypertension for men and women
at a baseline age were compared using a z test and
by fitting Cox proportional hazards regression models22
with left truncation (SAS program PROC PHREG).23
The assumption of proportionality of hazards was met, and there were no time-dependent
covariates in the models. These Cox models intrinsically adjust for age and
compare the risk for developing hypertension among groups over the entire
follow-up period (up to time t assuming a constant
ratio of hazards) while the z tests compared lifetime
risks at a specified point in time.
Short-term Risks for Developing Hypertension
We also estimated the sex- and age-specific 10- and 15-year cumulative
residual risks for developing hypertension (and other blood pressure outcomes)
for both age groups. These short-term estimates are of immediate relevance
to individuals and complement estimates of lifetime risk.
Temporal Trends in Residual Lifetime Risk for Developing Hypertension
We compared the residual lifetime risk for developing hypertension (and
other blood pressure outcomes) in the 1976-1998 period with that in the 1952-1975
period. For this comparison, we focused on unadjusted lifetime cumulative
incidence because this statistic is unaffected by changing rates of mortality
due to other causes over time.24
Subjects in the Framingham Heart Study who were not hypertensive at
their third biennial examination (earliest eligible baseline for that time)
contributed to the estimation of lifetime risk for developing hypertension
during the 1952-1975 period. We compared the sex- and age-specific risks of
developing hypertension and other blood pressure outcomes for the 2 periods
with z tests (for lifetime risk estimates) and with
the Cox models with left truncation. For 65-year-old men, only 15-year risk
estimates were compared because few individuals in this age group contributed
information beyond this time interval in the 1952-1975 period. Two sets of
analyses were performed: (1) models that included information from all eligible
participants in each of the periods; participants who contributed to the 1952-1975
period could provide additional information in the later period (at a different
age) provided they reached the later period free of hypertension and (2) models
that compared incidence of blood pressure outcomes in participants who contributed
to only the 1952-1975 period with that in participants who were eligible only
in the 1976-1998 period.
Additionally, we compared sex-specific trends in body mass index (BMI),
calculated as weight in kilograms divided by the square of height in meters,
across the 2 periods for our study participants using repeated measures analysis
of variance adjusting for age as a covariate. All P
values reported are 2-sided, and a P value <.05
was considered statistically significant.
Residual Lifetime and Short-term Risks for Developing Hypertension
During the 1976-1998 period, 1298 participants (589 men and 709 women)
provided 8469 person-years of observation. The residual lifetime risk for
developing hypertension for study participants was 90% (Table 1). Lifetime risk for hypertension was similar for men and
women and did not differ between participants aged 55 and 65 years (hazards
ratio [HR] for women vs men aged 55 years, 0.91 [95% CI, 0.80-1.04]; for those
aged 65 years, 0.88 [95% CI, 0.76-1.04]). More than half of the 55-year-old
participants and about two thirds of the 65-year-old participants developed
hypertension within 10 years.
On adjustment for competing causes of mortality, the mortality-adjusted
lifetime risk remained unchanged in women (90% and 86%, respectively, for
baseline ages 55 and 65 years) but decreased marginally in men (83% and 81%,
respectively, for baseline ages 55 and 65 years).
Lifetime Probability of Using Antihypertensive Medications
Nearly 60% of men and women received blood pressure–lowering drugs
on follow-up (Table 2). Estimates
were similar for both sexes (HR for women vs men aged 55 years, 0.98 [95%
CI, 0.83-1.16]; for those aged 65 years, 0.92 [95% CI, 0.76-1.11]), and for
the 2 baseline ages. A third of 55-year-old and nearly half of 65-year-old
participants were likely to receive antihypertensive medication within 15
years.
Residual Lifetime Risk for High Blood Pressure
The lifetime risk for high blood pressure in our study participants
is shown in Table 3. Nearly 85%
of our study participants developed stage 1 high blood pressure or higher
(≥140 mm Hg systolic or ≥90 mm Hg diastolic) during a follow-up period
of 20 to 25 years. Lifetime risk estimates did not vary for the 2 sexes (HR
for women vs men aged 55 years, 0.91 [95% CI, 0.80-1.04]; for those aged 65
years 0.88 [95% CI, 0.76-1.04] ) or between the 2 baseline ages.
The lifetime risk for experiencing stage 2 high blood pressure or higher
(≥160 mm Hg systolic or ≥100 mm Hg diastolic) in the 1976-1998 period
varied from 35% to 44% for different age-sex groups. The lifetime risk for
developing stage 2 high blood pressure or higher or receiving antihypertensive
medications was about 70% regardless of baseline age or sex.
Trends in Blood Pressure Outcomes Over Time
During the 1952-1975 period, 1740 participants (785 men, 955 women)
contributed 12 338 person-years of observation for the estimation of
the residual lifetime risk for hypertension. In women, the residual lifetime
risk for hypertension did not differ between the 2 periods; risk estimates
for the 1952-1975 period were 92% vs 91% for the 1976-1998 periods for baseline
age 55 years (P = .68); and 93% vs 89% for the baseline
age 65 years, respectively (P = .51; Figure 2A). However, for 55-year-old men, the residual lifetime
risk for hypertension was 93% in the 1976-1998 period vs 82% in the 1952-1975
period (P<.001). Similarly, the 15-year risk for
hypertension in 65-year-old men increased from 65% in the 1952-1975 period
to 85% in the 1976-1998 period (P<.001; Figure 2B). Cox regression analyses confirmed
these observations (Table 4).
Women in the 1952-1975 period had a slightly higher risk for developing
hypertension than men in that period (HR for women vs men aged 55 years, 1.15
[95% CI, 1.01-1.30]; for those aged 65 years, 1.23 [95% CI, 0.97-1.56]). As
noted previously, point estimates of the HRs suggested a lower risk in women
compared with men in the 1976-1998 period although this was not statistically
significant.
The residual lifetime probability of being treated with antihypertensive
medications ranged between 16% and 36% in different age-sex groups in the
1952-1975 period (data not shown), increasing 2- to 3-fold in both sexes in
the 1976-1998 period (P<.001 for all comparisons).
These findings were supported by the results of Cox models (Table 4). The risk for stage 1 high blood pressure or higher was
92% for women (for both baseline ages) but varied from 65% (age 65 years)
to 82% (age 55 years) for men in the 1952-1975 period. The risk for stage
1 high blood pressure or higher did not change across the 2 periods for women
but increased by about 25% in men (Table
4). The risk for stage 2 high blood pressure or higher was 57% for
women (for both baseline ages) and varied from 35% (age 65 years) to 48% (age
55 years) for men in the 1952-1975 period. The risk for stage 2 high blood
pressure or higher declined in the 1976-1998 period both in women and in men
(Table 4).
There were 786 participants (344 men, 442 women) who contributed information
to both periods although at different ages. Analyses were repeated after excluding
these participants. Estimates of residual lifetime risk for hypertension in
the 1976-1998 period were higher after these exclusions, in part because people
who contributed to the 1952-1975 period and remained eligible in the 1976-1998
period (having reached this period free of hypertension) were likely participants
with a lower risk for developing hypertension. A comparison of participants
contributing to the 1952-1975 period only (n = 954; 513 women, 441 men) with
those providing information in the 1976-1998 period only (n = 512; 267 women,
245 men) confirmed the increased risk for hypertension in men and the increased
use of antihypertensive medications in both men and women in the 1976-1998
period. However, the increased risk for stage 1 high blood pressure or higher
among men in the 1976-1998 period noted in earlier analyses did not achieve
statistical significance, in part due to a decrease in sample size and statistical
power.
The temporal trends we observed may be due, in part, to concomitant
trends in obesity. In women, age-adjusted BMI decreased from 25.8 kg/m2 in the 1952-1975 period to 25.3 kg/m2 in the 1976-1998
period (P = .05). In contrast, age-adjusted BMI increased
in men from 25.8 kg/m2 in the 1952-1975 period to 26.3 kg/m2 in the 1976-1998 period (P = .02).
Our investigation provides estimates of the residual lifetime and the
short-term risks for developing hypertension and examines trends in these
risks over time. Whereas the short-term risk estimates are of more immediate
value to individuals and, therefore, more likely to motivate behavioral changes,
residual lifetime risk estimates permit a comparison of the likelihood of
developing 1 or more disease conditions over a lifetime.
The residual lifetime risk for developing hypertension in our sample
was about 90%. These risk estimates were similar for men and women and for
participants aged 55 and 65 years. It is often assumed that the risk for hypertension
increases with age because the incidence of hypertension increases with age.
Although this is true of short-term age-conditional risk estimates, this may
not be true for lifetime risk. The lack of increase in lifetime risk with
increasing age is consistent with similar observations made for other chronic
diseases such as dementia7 and may be because
the rising age-specific incidence of hypertension is offset by a decreasing
residual life expectancy. Using a higher threshold to define high blood pressure
(stage 2 high blood pressure or higher or use of medications) yielded lifetime
risk estimates of about 70% in both women and men.
A comparison of lifetime risk estimates over the 2 periods showed that
the risk for developing hypertension was unchanged for women but increased
for men. The increase in lifetime risk for hypertension in men may be related
in part to the trend for an increase in BMI over this period among men (but
not women) in our sample. Other investigators have also reported favorable
secular trends in blood pressure of women but less encouraging trends in men.25,26 In contrast, in the 1976-1998 period,
the lifetime probability of experiencing stage 2 high blood pressure or higher
was considerably lower in both sexes, in tandem with and probably as a result
of a 2- to 3-fold increased use of antihypertensive medications. These data
are consistent with prior reports of a decrease in the prevalence of long-term
sustained hypertension in our cohort13,27
and are likely the result of an increasing emphasis on pharmacological treatment
of elevated blood pressure and the progressive lowering of blood pressure
treatment thresholds in national guidelines for hypertension treatment (along
with changing definitions of hypertension).1
However, the residual lifetime risk for experiencing stage 1 high blood pressure
or higher is markedly high at about 85% in the 1976-1998 period.
Trends in Risk for Hypertension: Comparison With Prior Reports
Our findings differ significantly from observations made in serial national
cross-sectional surveys between 1960 and 1991, which suggest that the age-adjusted
prevalence of hypertension in the United States declined from 36% in 1971
to 20% in 1988 through 1991, along with a downward shift in the average blood
pressure of the population.10 A principal reason
our results differ from data from national surveys is that we evaluated trends
in the incidence of hypertension while the NHANES report assessed trends in
the cross-sectional prevalence of hypertension. It is important to note that
our study and the NHANES report evaluated slightly different periods; our
investigation began earlier and extended into 1998. It has been reported that
some of the "gains" noted in the initial phase of the NHANES III (1988-1991)
had leveled off by the time of the second phase of the survey (1991-1994).19 Increases in the mean blood pressure of cohorts in
Iowa and Minnesota in the late 1980s and 1990s, respectively, also have been
reported.28,29 An additional explanation
to be considered is that our sample was white and restricted in its age distribution
while that of NHANES was ethnically more diverse and encompassed a wider age
spectrum.
Strengths and Limitations
Our study is based on the longitudinal surveillance of the same community-based
cohort over a period of 50 years. Over this time, blood pressure measurements
were obtained every 2 years using a standardized protocol, and information
regarding the use of antihypertensive medications was gathered consistently
and routinely. Additionally, we examined trends in the lifetime risk for several
blood pressure outcomes that yielded complementary information.
Nevertheless, we acknowledge certain limitations of our investigation.
Our data indicate the residual lifetime risk of hypertension for 55- and 65-year-old
participants who were free of hypertension at these baseline ages. It is important
to note that a considerable proportion of individuals with hypertension have
onset of the condition before this age. Therefore, the actual lifetime risk
for hypertension for younger individuals may be different. Moreover, these
lifetime risk data are average estimates for our study participants. The risk
for hypertension for a given individual will vary depending on the presence
or absence of risk factors for hypertension (such as obesity, family history
of high blood pressure, dietary sodium and potassium intake, and alcohol consumption).
We did not examine the impact of these covariates. Categorizing people as
hypertensive based on a reading obtained on a single occasion may overestimate
risk for hypertension. For this reason, we assessed risk for several blood
pressure outcomes. The 90% lifetime risk estimate likely represents the upper
estimate of true risk because it is based on a single occasion measurement
and not on the average of multiple visits (required by the JNC VI). The lower
bound of the residual lifetime risk for hypertension is 70%, a figure obtained
using a composite definition of stage 2 high blood pressure or higher or the
use of antihypertensive medications. We may have underestimated the lifetime
probability of experiencing stage 2 high blood pressure or higher because
some participants experiencing this level of blood pressure on seeing a private
physician between biennial Heart Study examinations may have had their blood
pressure lowered with medications by the time of the next examination. Last,
our lifetime risk estimates and trends may not be generalizable to other ethnic
groups. Similar studies in other multiethnic cohorts are warranted to obtain
information regarding risk for hypertension in different racial groups.
Public Health Implications
The finding that 9 out of 10 middle-aged and older adults are likely
to develop elevated blood pressure over their remaining lifetime reemphasizes
that hypertension poses a major public health burden. Furthermore, the lack
of change in the lifetime risk over the 2 periods in women and the modest
increase in risk in men is of concern. With the aging of the US population,14 the societal burden of hypertension will increase
further if the lifetime risk for the condition remains unattenuated.
The marked decline in the lifetime risk for stage 2 high blood pressure
or higher in men and women in our study sample represents a major public health
achievement. In contrast, the risk for stage 1 high blood pressure or higher
remained unaltered and represents the next frontier. Although hypertension
is easily diagnosed and treated,19 prior data
from Framingham and elsewhere suggest that stage 1 hypertension is inadequately
controlled.10,30,31
This is of public health importance because a substantial proportion of cardiovascular
diseases are attributable to stage 1 hypertension.2,32
It is widely accepted that hypertension is preventable through lifestyle
modification.19,33 Our estimates
of the short-term risk for hypertension indicate that more than half of the
residual lifetime risk for hypertension for 55- and 65-year-old participants
is experienced over the initial 10 years of follow-up. The immediacy and the
magnitude of this risk should encourage middle-aged nonhypertensive individuals
to adopt lifestyle-related measures for maintaining optimal blood pressure
and for preventing the development of hypertension. The approach of waiting
for hypertension to develop and only then treating the elevated blood pressure
is injudicious. As articulated by Stamler,34
such a strategy is "late, defensive, reactive, time consuming, associated
with side effects, costly, only partially successful, and endless."
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