(Stroke. 1995;26:2004-2010.)
© 1995 American Heart Association, Inc.
Articles |
From the Department of Public Health, Royal Free Hospital School of Medicine, London, UK.
Correspondence to Dr Goya Wannamethee, Department of Public Health, Royal Free Hospital School of Medicine, Rowland Hill St, London NW3 2PF, UK.
| Abstract |
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Methods We completed a prospective study of 7735 men aged 40 to 59 years at screening selected at random from one general practice in each of 24 British towns.
Results During the mean follow-up period of 14.8 years, there were 277 major stroke events in the 7650 men with data on forced expiratory volume in 1 second (FEV1). After exclusion of 499 men with definite myocardial infarction, stroke, or atrial fibrillation at screening, 7151 men experienced 239 major stroke events. Lower levels of FEV1 were associated with a significant increase in risk of stroke even after adjustment for age, smoking, social class, physical activity, alcohol intake, systolic blood pressure, antihypertensive treatment, diabetes, and preexisting ischemic heart disease. Relative risk in the low third (<3.10 L) versus high third (>3.65 L) was 1.4 (95% confidence interval, 1.0 to 2.0). The inverse association between FEV1 and stroke was only apparent in older men, current nonsmokers, hypertensive men, and men with preexisting ischemic heart disease. Lower FEV1 was associated with higher rates of stroke in hypertensive men irrespective of smoking status. Inclusion of FEV1 in a risk score for stroke provided only a small increase in the absolute risk or the yield of cases in the top fifth of the score distribution during the follow-up period.
Conclusions Lower levels of FEV1 are associated with an increased risk of stroke in those already at high risk, eg, those with ischemic heart disease or hypertension. However, the association is not strong enough to warrant the use of FEV1 in making clinical decisions regarding the treatment of hypertension as it relates to the prevention of stroke.
Key Words: England lung respiratory function tests risk factors stroke
| Introduction |
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| Subjects and Methods |
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Alcohol consumption was recorded with the use of questions on frequency, quantity, and type, similar to those used in the 1978 General Household Survey. The men were classified into five groups based on weekly intake: none, occasional, light, moderate, and heavy.13 Heavy drinking was defined as drinking more than 6 units (1 UK unit=8 to 10 g alcohol) daily or on most days. The men were asked to indicate their usual pattern of physical activity, and a physical activity (exercise) score was derived for each man based on frequency and type (intensity) of the physical activity.14 The men were grouped into six broad categories based on their total score: inactive, occasional, light, moderate, moderately vigorous, and vigorous.
Lung Function
We measured FEV1 and FVC using a Vitalograph
spirometer (model J49-B2) manufactured by Vitalograph Ltd Medical
Instrumentation, with the subject seated. Two consecutive readings were
made 15 seconds apart, and the greater of these two readings was used.
The FEV1 and FVC values used are height standardized to
1.73 m, the average height of the men in this study. FEV1
and FVC measurements were not available on 85 men. Because of the high
degree of correlation between FEV1 and FVC
(r=.83) and because of the suggestion that FEV1
should be used in assessing subjects with mild hypertension for
treatment,10 the focus of this report will be on
FEV1, with additional comments on FVC and the ratio
of FEV1 to FVC.
Smoking
The men were classified according to their current smoking
status into six groups: those who had never smoked cigarettes
(never-smokers), excigarette smokers, and four groups of
current smokers (1 to 19, 20, 21 to 39, and
40 cigarettes per day).
Those who had only smoked pipes or cigars were classified as
never-smokers. Excigarette smokers who currently smoked a
pipe or cigars were classified as ex-smokers. For ex-smokers,
data were also available on the number of cigarettes smoked previously
and the total duration of smoking.15 In some
analyses all current smokers are combined, and ex-smokers
and those who had never smoked are combined as current nonsmokers.
Measurements of serum cotinine for validation of baseline smoking status were not available. However, blood cadmium, which is considered a strong biological marker of smoking and has been shown to be strongly associated with smoking status,16 17 was measured in this study. Mean levels of blood cadmium were shown to regress markedly within 1 to 2 years of quitting and then converged more slowly to the levels of never-smokers after 10 years of cessation.16
Blood Pressure
The London School of Hygiene sphygmomanometer was used to
measure blood pressure twice in succession with the subjects seated and
with the arm supported on a cushion. The mean of the two readings was
used in the analysis, and all blood pressure readings were
adjusted for observer variation within each town.18 Men
with adjusted systolic blood pressure of 160 mm Hg or greater
or adjusted diastolic blood pressure of 90 mm Hg or greater
or subjects on regular antihypertensive treatment were regarded as
hypertensive.
Preexisting IHD and Stroke
The men were asked whether a physician had ever told them that
they had angina or myocardial infarction (heart attack,
coronary thrombosis), stroke, or a number of other disorders.
The WHO (Rose) chest pain questionnaire19 was administered
to all men at the initial examination, and a three-lead ECG
(orthogonal system) was recorded at rest.
Evidence of a previous stroke was determined by the subject's recall of such a diagnosis made by a physician. There were 52 such men in the study.
In regard to IHD, the men were separated into three groups according to the evidence of IHD at screening. Group 1 comprised men with no evidence of IHD on WHO chest pain questionnaire or ECG and no recall of a physician's diagnosis of IHD. Group 2 comprised men with evidence suggesting IHD short of a definite myocardial infarction. This group contained those with ECG evidence of possible or definite myocardial ischemia or possible myocardial infarction (asymptomatic) and those with angina or a possible myocardial infarction on WHO (Rose) chest pain questionnaire or who recalled a physician's diagnosis of angina (symptomatic). Group 3 comprised men with a previous definite myocardial infarction on ECG or who recalled a physician's diagnosis of myocardial infarction ("heart attack").
Arrhythmia
A complex diagnostic tree, which was part of a
larger set of criteria used in interpreting the ECG as a whole, was
used to make the interpretation.20 All ECGs in the study
were reviewed by an experienced electrocardiographer, and if any errors
in the computer-based rhythm interpretation were detected, they
were corrected before results were entered into the database. A normal
rhythm was defined as sinus rhythm, coronary sinus rhythm, or
sinus arrhythmia. All other statements of rhythm were treated
as an arrhythmia, eg, sinus rhythm with ventricular
extrasystoles. As expected, 97.8% of the men in the study were in
sinus rhythm. Only 0.7% were in atrial fibrillation.
Follow-up
All men were followed up for all-cause mortality and for
cardiovascular morbidity.21 All stroke
events occurring in the period up to December 1993 are included in the
study, with an average follow-up of 14.8 years (range, 13.5 to 16.0
years). Follow-up has been achieved for 99% of the cohort.
Information on death was collected through the established
"tagging" procedures provided by the National Health Service
registers in Southport (England and Wales) and Edinburgh (Scotland).
Nonfatal stroke events were those that produced a neurological deficit
that was present for more than 24 hours. Evidence regarding such
episodes was obtained by reports from general
practitioners, from personal questionnaires to surviving
subjects at years 5 and 12 after the initial examination, and by
semiannual reviews of the patients' notes through the end of the study
period. This combination of reporting acute events as they occurred,
personal recall of events in the first 5 years and 12 years later, and
regular scrutiny of the records, which included all correspondence
and reports relating to hospital admissions and attendance, is presumed
to provide information that is as complete and as unbiased as possible
in prospective studies. Fatal stroke episodes were those coded on the
death certificate as International Classification of Diseases codes 430
through 438. All death certificates in which it appeared that coding as
stroke was not appropriate or in which stroke was not the attributed
code when it might have been were explored by correspondence with the
certifying doctor and the hospital concerned. No information on the
type of stroke was available.
Statistical Methods
The Cox proportional hazards model was used to assess the
independent contributions of FEV1 to the risk of stroke and
to obtain the RRs adjusted for age and other risk
factors.22 We assessed tests for trend fitting the
FEV1 in its original continuous form. In the adjustment,
age, blood cholesterol, and systolic blood pressure
were fitted as continuous variables. Alcohol (5 levels), smoking (6
levels), physical activity (6 levels), social class (3 groups),
diabetes (yes/no), and preexisting IHD on WHO (Rose) questionnaire/ECG
(group 2, yes/no) were fitted as categorical variables. In some of
the analyses, all current smokers were combined. Direct
standardization was used to obtain age-adjusted stroke rates per
1000 person-years for FEV1 groups by current smoking
status in hypertensive men, with the whole study population as the
standard. A test for interaction to assess whether the relationship
between FEV1 and stroke differed by current smoking status
was assessed by fitting an FEV1-smoking interaction term in
the model with all current smokers combined. Similar analyses
using interaction terms were performed to assess whether the
association between FEV1 and stroke differed by age,
presence of hypertension, and preexisting IHD.
| Results |
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FEV1 and Risk of Stroke
Table 1
shows the crude rates per 1000
person-years and the age-adjusted RR of stroke by thirds of the
ranked FEV1 distribution. Men in the highest third were
used as the reference group. A significant inverse association was seen
after adjustment for age. FEV1 is strongly associated with
smoking, which is a strong independent risk factor for
stroke.23 Adjustment for smoking status reduced the trend
slightly, but the association remained significant. Men in the lowest
third showed a 50% increase in risk of stroke compared with men in the
highest third (RR, 1.5; 95% CI, 1.1 to 2.2). Further adjustment for
other factors known to be associated with stroke risk, eg, physical
activity, alcohol intake, systolic blood pressure, diabetes,
preexisting IHD short of myocardial infarction (group 2), social class,
and antihypertensive treatment, reduced the inverse trend further, but
risk was still significantly elevated in the lowest third of the
FEV1 distribution, and the trend was marginally significant
(P=.05).
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Analysis using FVC instead of FEV1 yielded almost identical results, with fully adjusted RRs of 1.4 (95% CI, 1.0 to 2.0), 1.2 (95% CI, 0.8 to 1.7), and 1.0 for the thirds of the FVC distribution (test for trend, P=.03). Since FEV1 and FVC behaved so similarly, no association was seen with the ratio of FEV1 to FVC. The adjusted RRs were 1.1, 1.0, and 1.0, respectively, for the thirds of the FEV1/FVC distribution.
Age, Smoking, and Hypertension
The relationship between FEV1 and stroke was
examined by levels of risk factors, eg, age, smoking, and hypertension
(Table 2
). Because of the small number of
stroke cases (n=28) in men who had never smoked, we divided the smoking
categories into current smokers and noncurrent smokers, the
latter including both excigarette smokers (n=2483) and
never-smokers (n=1738). The inverse association between
FEV1 and risk of stroke was seen only in older men (aged 50
to 59 years), current nonsmokers, and hypertensive subjects. A formal
test for interaction (see "Subjects and Methods") confirmed a
significant difference in the relationship between FEV1 and
stroke by levels of these risk factors.
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Among the current nonsmokers, men with low FEV1 were more
likely to be past heavier smokers (
20 cigarettes per day) or
secondary pipe or cigar smokers than men with high FEV1.
However, further adjustment for past cigarette smoking and secondary
pipe or cigar smoking did not make a major difference to the
significant inverse association seen between FEV1 and
stroke in current nonsmokers. We also examined the relationship
separately in excigarette smokers and never-smokers. In the
never-smokers, no apparent association was seen between
FEV1 and stroke after adjustment, as shown in Table 2
. In
ex-smokers, the inverse association was clearly seen even after we
adjusted for past heavy smoking and secondary pipe or cigar
smoking.
Men With Preexisting IHD
The relationship between FEV1 and stroke was also
examined separately in men with and without preexisting IHD short of a
myocardial infarction (group 2). The inverse association was seen only
in men with evidence of preexisting IHD short of a myocardial
infarction (Table 3
). We further
separated men with preexisting IHD (group 2) into those with
symptomatic (those who reported angina or possible
myocardial infarction on the WHO [Rose] questionnaire or who recalled
a physician's diagnosis of angina irrespective of ECG findings) and
asymptomatic IHD (those with ischemic ECG
abnormalities but who did not report any chest pain). The inverse
association was seen in both groups (data not shown).
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We also examined the relationship between FEV1 and risk of stroke in the small group of men (n=499) with previous definite myocardial infarction/stroke or evidence of atrial fibrillation, who were excluded from all the previous analyses. FEV1 was significantly inversely associated with risk of stroke in this group (test for trend; P=.01; data not shown).
Smoking and Hypertension
Because of the strong influence that smoking has on both
FEV1 and risk of stroke and because the level of
FEV1 has been proposed for use in the management of
hypertension,10 we examined the relationship between
FEV1 and stroke in hypertensive subjects stratified by
current smoking status. Because men in the lower two thirds of the
FEV1 distribution showed similar increased risk in
hypertensive men (Table 2
), these men were combined and
compared with the highest third of the FEV1 distribution
(Table 4
). Lower FEV1 is
associated with higher rates of stroke in hypertensive men irrespective
of smoking status. The difference in age-adjusted absolute rates of
stroke between high and lower FEV1 amounted to 1.4/1000
person-years in currently nonsmoking hypertensive subjects (1.6
versus 3.0/1000 person-years) and to 2.5/1000 person-years in
smoking hypertensive subjects (3.9 versus 7.4/1000
person-years).
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We determined the RR of stroke in hypertensive men adjusting for age
and other potential confounders (as in Table 2
), using nonsmoking
hypertensive men in the highest third of the FEV1
distribution as the reference group (Table 4
). In
nonsmokers, hypertension accompanied by low FEV1 was
associated with a twofold increase in risk of stroke (RR, 2.1; 95% CI,
1.0 to 4.0). In smokers, hypertensive men with high FEV1
showed more than a twofold increase in risk, and this increased to more
than fourfold in hypertensive men with lower FEV1. The
higher rates of stroke seen in men with lower FEV1 who were
hypertensive were seen even when men with evidence of IHD (group 2)
were excluded.
Risk Score for Stroke
A scoring system derived from logistic regression in this cohort
using age, smoking, systolic blood pressure, and evidence of
anginal chest pain (on WHO [Rose] questionnaire) has been produced to
identify men at high risk of stroke.24 We used a model
based on these factors and examined the stroke rate per 1000
person-years and the percent yield of strokes occurring in the top
20% of the risk score distribution, with and without FEV1
in the model (Table 5
). In the whole
cohort, inclusion of FEV1 in the model provided only a
small increase in yield and stroke rate per 1000 person-years. If
the model is confined to hypertensive subjects (ie, men with
systolic blood pressure
160 mm Hg or diastolic
blood pressure
90 mm Hg or those on treatment for hypertension), the
yield and stroke rate per 1000 person-years was also only slightly
increased. Thus, even restricting the method to hypertensive subjects
would only increase the rate of strokes in the high-risk fifth by
1/1000 person-years.
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| Discussion |
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Another possibility as to why the relationship is only seen in
ex-smokers may be due to the problem of residual confounding as a
result of inaccurate measurements of past smoking habits. However,
blood cadmium levels, which are considered to be a strong biological
marker of smoking, have been shown in this cohort to relate strongly to
smoking levels and to converge to the levels of never-smokers after
10 years of smoking cessation.16 17 It is recognized that
cessation of smoking is associated with a reduction in risk of stroke,
and ex-smokers in this study were shown to experience a reduction
of risk in stroke.17 On the basis of recall of past
smoking habits, we demonstrated that the benefit was dependent on
quantity of cigarettes smoked in the past and not the number of years
smoked. Exheavier smokers retained some increased risk compared
with those who had never smoked, while exlight smokers reverted
to the level of never-smokers.17 However, the inverse
association in ex-smokers persisted even when the degree of past
smoking was taken into account, suggesting that the relationship seen
in ex-smokers was not due to residual confounding. Residual
confounding is also an unlikely explanation of the findings in
hypertensive men, in whom a relationship between FEV1 and
stroke was found in both current smokers and nonsmokers, and it is also
unlikely to explain the findings in those with prevalent IHD.
Furthermore, adjustment for smoking in the crude comparisons made only
a modest difference in the relationships (Table 1
),
suggesting that any residual confounding would be small and unlikely to
explain the large differences observed in hypertensive and older
subjects and those with prevalent IHD.
Previous Studies
In the Whitehall Study of male civil servants aged 40 to 64 years
at entry, there were 262 deaths due to stroke in 18 403 men during an
average follow-up of 16.6 years. Men with FEV1 levels
lower than 3.0 L were almost twice as likely to die of stroke as those
with FEV1 levels of 3.5 L or greater, irrespective of blood
pressure and apparently independent of smoking status and other risk
factors, including IHD. We have identified five other cohort studies
that examined the specific relationship between respiratory function
and risk of stroke.6 7 8 9 11 In three of thesethe
Framingham Study (United States), the Gothenburg study (Sweden), and
the Italian cohort of the Seven Countries Studythe relationship
between lung function and stroke, although inverse, was not
statistically significant at the 5% level after adjustment for the
presence of confounding risk factors. The Framingham Study did not find
FVC to be significantly related to stroke in either men or women in the
multivariate analysis. In the present study
FVC is strongly correlated with FEV1 (r=.83) and
showed similar significant inverse relationships with stroke. In the
Gothenburg study the relationship between FEV1 and stroke
was of marginal significance (P=.08) after adjustment for
smoking and other confounders. The lack of significance in both the
male Gothenburg and the Italian cohorts may be due in part to the small
number of stroke cases involved (57 and 68 cases, respectively).
Indeed, in a recent report based on the same Italian cohort but with
longer follow-up, FEV1 was shown to be significantly
and independently (inversely) associated with stroke.11
Lung function expressed as peak expiratory flow has also been shown to
be independently associated (inversely) with stroke in a Swedish cohort
of women.7 Although FEV1 has been shown to be
independently associated with stroke in multivariate
analyses, the relationship has not been examined by levels of
risk factors and, in particular, separately for normotensive and
hypertensive subjects or separately for men with and without
preexisting IHD. The Whitehall Study showed an inverse relationship
between FEV1 and stroke in both smokers and nonsmokers at
different levels of blood pressure. However, the Whitehall Study used
the mean value of the two highest measures of these FEV1
estimations and did not use height-standardized FEV1
for individuals. This makes direct comparison with the present
study difficult.
Other Potential Confounders
Reduced lung function may be a marker of
physiological abnormalities such as polycythemia,
which may predispose individuals to stroke.10 Although
FEV1 was significantly associated with hematocrit in the
present study (r=-.10), risk of stroke was only
elevated in men with hematocrit levels of 51% or
greater,25 constituting 2% of the cohort, and adjustment
for elevated hematocrit made little difference to the significant
inverse association seen between FEV1 and stroke. It has
also been suggested that the increased risk may be associated with
alcohol intake, which has been shown to influence both lung
function26 and risk of stroke.23 However, the
findings in this study were independent of alcohol intake.
Mechanisms
Both FEV1 and FVC were significantly and inversely
related to the risk of stroke, suggesting effects on both airway
function and lung volume. Reduced lung volume may reflect impaired
cardiac function due to occult coronary disease,1
and it has been shown that cardiac impairment is associated with an
increased risk of stroke.27 28 The strong association seen
in ex-smokers, hypertensive subjects, and men with preexisting IHD
suggests that low FEV1 may be an early marker of
ischemic myocardial disease, resulting in greater
susceptibility to stroke, particularly in older men. This concept of
FEV1 as a measure of physiological
reserve capacity to withstand pathological insults to the
cerebrovascular circulation seems tenable, particularly in view of the
stronger inverse association between FEV1 and stroke in
older and hypertensive subjects and in those with preexisting IHD. The
pathophysiological mechanism(s) by which lower
FEV1 may be associated with stroke is not clear. Myocardial
ischemia may be associated with bronchial wall edema, which can
lead to bronchial hyperresponsiveness and airway
obstruction.29 30 Impaired lung function is associated
with the development of hypertension,31 suggesting that
the observed association between stroke incidence and lower
FEV1 and FVC measured at baseline in our study might be due
to strokes occurring in those who developed hypertension during the
course of follow-up.
Risk Assessment in Hypertensive Subjects
In the study of Whitehall male civil servants, increased risk of
stroke was associated with low FEV1 (<3.0 L) in each third
of the systolic blood pressure distribution (<126, 126 to 142,
and
143 mm Hg). It was suggested that the measurement of ventilatory
function might be more useful than many conventional
cardiovascular risk factors in guiding decisions about
the management of mild hypertension (undefined). At present, it is
widely recommended that the decision to treat subjects with elevated
blood pressure with drugs should depend not only on the levels of blood
pressure but on the estimated absolute risk of developing
cardiovascular disease, taking other risk factors into
account.32 33 34 35 Such recommendations do not always carry
with them clear methods for estimating this risk, but they are
available and could readily be used in both clinical practice and
clinical trials, although they are not used for this purpose at
present.24 36 37 When FEV1 is added as an
additional factor to the scoring system already available for
estimating risk of major stroke events based on smoking,
systolic blood pressure, and presence of angina,24
there is only a small increase in absolute risk or yield of total
events in the follow-up period. Despite the role that diminished
lung function apparently plays in the risk of stroke, there would
appear to be little value in adding it to the major risk factors
already used in the various systems of risk assessment.
Conclusions
Since a higher FEV1 appears to be associated with
lower risk of stroke in high-risk subjects, eg, those with
preexisting IHD or hypertension, it would seem advantageous to maintain
higher levels of FEV1. However, there seems a limited
amount that one can do to prevent the progressive decline of
FEV1 with increasing age, although the rate of decline of
FEV1 will be less in those who never smoke or give up
smoking early in life. Although lower levels of FEV1 are
associated with increased risk of stroke, there is no indication that
measurement of FEV1 is a useful adjunct in deciding whether
to use drug treatment in a hypertensive subject, as proposed.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received June 13, 1995; revision received August 2, 1995; accepted August 16, 1995.
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