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Amanda I Adler a Diabetes Trial Unit, Oxford Centre for
Diabetes, Endocrinology and Metabolism, University of Oxford, Radcliffe
Infirmary, Oxford OX2 6HE, b Division of
Public Health and Primary Care, Institute of Health Sciences,
University of Oxford, OX3 7LF, c University College London
Medical School, Whittington Hospital, London N19 3UA, d Oxford Centre for
Diabetes, Endocrinology and Metabolism, University of Oxford, e Selly Oak Hospital, Birmingham B29 6JD, f Diabetes Research
Laboratories, Oxford Centre for Diabetes, Endocrinology and Metabolism,
University of Oxford
Correspondence to: A Adler amanda.adler{at}dtu.ox.ac.uk
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Abstract |
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Objective:
To determine the relation between systolic blood pressure over time and the risk of macrovascular or microvascular complications in patients with type 2 diabetes.
The UK prospective diabetes study (UKPDS) has shown that a policy
of tight control of blood pressure, which achieved a median blood
pressure of 144/82 mm Hg compared with 154/87 mm Hg over median 8.4 years of follow up, substantially reduced the risk of microvascular
disease, stroke, and deaths related to diabetes,1 but not
myocardial infarction. Complementary information for estimates of the
risk of complications including myocardial infarction at different
levels of blood pressure can be obtained from observational analysis of
the UKPDS data. This information can help to estimate the expected
reduction in the risk of diabetic complications from a given change in
blood pressure. It can also help to assess whether or not thresholds in
blood pressure exist below which the risk of complications is
substantially reduced. Such thresholds would have substantial influence
on the establishment of guidelines on clinical care.
People with type 2 diabetes have a greater incidence of cardiovascular
disease, cerebrovascular disease, and renal disease than the general
population. Epidemiological studies suggest that relative
hyperglycaemia accounts for part but not all of the increased risk.2-7 Raised blood pressure is more common in
people with type 2 diabetes than in the general
population,8-12 and in people without diabetes it is a
major risk factor for myocardial infarction and
stroke.
13 14
Epidemiological studies of the role of blood pressure on the development of cardiovascular disease have categorised people as either hypertensive or normotensive or have measured blood
pressure on a single occasion,
5 6 15-17
whereas repeated measurements of blood pressure over several years should be more informative.
In these analyses, we evaluated the relation between systolic blood
pressure over time and the development of macrovascular and
microvascular complications using data from the UKPDS and looked for
possible thresholds. We compared these results to those of the UKPDS
trial of a policy of tight control of blood pressure.1 When the achieved reduction in risk notably exceeded that expected from
observational data, analyses were performed to evaluate the presence of
a treatment effect beyond that of blood pressure alone.1
Participants recruited to the UKPDS
Participants in epidemiological analyses
Table 1.
Design:
Prospective observational study.
Setting:
23 hospital based clinics in England,
Scotland, and Northern Ireland.
Participants:
4801 white, Asian Indian, and
Afro-Caribbean UKPDS patients, whether randomised or not to treatment,
were included in analyses of incidence; of these, 3642 were included in
analyses of relative risk.
Outcome measures:
Primary predefined aggregate
clinical outcomes: any complications or deaths related to diabetes and
all cause mortality. Secondary aggregate outcomes: myocardial
infarction, stroke, lower extremity amputation (including death from
peripheral vascular disease), and microvascular disease (predominantly
retinal photocoagulation). Single end points: non-fatal heart failure and cataract extraction. Risk reduction associated with a 10 mm Hg
decrease in updated mean systolic blood pressure adjusted for specific
confounders
Results:
The incidence of clinical complications was significantly associated with systolic blood pressure, except for
cataract extraction. Each 10 mm Hg decrease in updated mean systolic
blood pressure was associated with reductions in risk of 12% for any
complication related to diabetes (95% confidence interval 10% to
14%, P<0.0001), 15% for deaths related to diabetes (12% to 18%,
P<0.0001), 11% for myocardial infarction (7% to 14%, P<0.0001),
and 13% for microvascular complications (10% to 16%, P<0.0001). No
threshold of risk was observed for any end point.
Conclusions:
In patients with type 2 diabetes the risk of diabetic complications was strongly associated with raised blood
pressure. Any reduction in blood pressure is likely to reduce the risk
of complications, with the lowest risk being in those with systolic
blood pressure less than 120 mm Hg.
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Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
![]()
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
We enrolled 5102 of 7416 patients with newly diagnosed type 2 diabetes (defined as fasting plasma glucose concentrations over 6.0 mmol/l on two separate mornings) who were referred to the UKPDS and
were aged 25 to 65 years. Recruitment occurred between 1977 and 1991 at
23 clinical centres in England, Scotland, and Northern Ireland.
Exclusion criteria are presented elsewhere18; the main
reasons were severe vascular disease, myocardial infarction or stroke
within the year before recruitment, or major systemic illness.
We studied the incidence of complications of diabetes in the 4801 white, Asian Indian, and Afro-Caribbean patients who had blood pressure
measured at two and nine months after the diagnosis of diabetes. Of
these, 3642 with complete data for potential confounders were evaluated
in proportional hazards models. Their characteristics are presented in
table 1.
Participants in UKPDS blood pressure control study
The UKPDS clinical trials of blood glucose and blood pressure
control are described elsewhere.
19 20
In summary, 1148 patients with hypertension, defined as previously receiving
antihypertensive treatment and with a blood pressure
150/85 mm Hg or
not previously receiving antihypertensive treatment and a blood
pressure
160/90 mm Hg, were randomised to a policy of tight control
of blood pressure with a
blocker or an angiotensin converting
enzyme inhibitor or to a policy of less tight control. At entry, the
mean duration of known diabetes was 2.6 years, and the patients were
older and heavier than in the whole cohort (table 1). The aim in the
group allocated to tight control was to achieve blood pressure values
<150/<85 mm Hg. If this target was not met with maximal doses of a
blocker or angiotensin converting enzyme inhibitor, additional
agents were prescribed, including a loop diuretic, a calcium channel
blocker, and a vasodilator. The aim in the group allocated to less
tight control was to achieve blood pressure values <180/<105 mm Hg
without the use of a
blocker or an angiotensin converting enzyme
inhibitor but using the same stepwise addition of other treatments.
Blood pressure measurement
Blood pressure was measured with the person in a seated position
after a five minute rest with a Copal UA-251 or a Takeda UA-751
electronic, auscultatory blood pressure reading machine (Andrew
Stephens, Brighouse, West Yorkshire). The first reading was discarded,
and the mean of the next three consecutive readings with a coefficient
of variation below 15% was used. In participants with atrial
fibrillation, examiners used a Hawksley random zero sphygmomanometer.
Blood pressure exposure
Blood pressure was measured firstly at baseline (mean of measures
taken at two and nine months after diagnosis) and secondly as an
updated mean of annual measurement of systolic blood pressure,
calculated for each participant from baseline to each year of follow
up. For example, at one year the updated mean is the average of the
baseline and one year values and at three years it is the average of
baseline, one year, two year, and three year values.
Biochemical methods
The biochemical methods used have been reported previously.21 Biochemical variables are quoted for
measurements after the initial dietary run-in period.
Clinical complications
The clinical end points studied18 and their definitions19 were separated into aggregate and single end
points (see box).
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Aggregate end points
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Statistical analysis
Incidence rates by category of systolic blood pressure
The unadjusted incidence rates were calculated by dividing the
number of people with a given complication by the person years of
follow up for the given complication within each category of updated
mean systolic blood pressure and reported as events per 1000 person
years of follow up. The categories were defined (median values in
parentheses) as: <120 (114), 120-129 (125), 130-139 (135), 140-149 (144), 150-159 (154), and
160 (168) mm Hg over the range of updated
mean systolic blood pressures 85-230 mm Hg. Time of follow up was
calculated from the end of the initial period of dietary treatment to
the first occurrence of that complication or loss to follow up, death
from another cause, or the end of the study on 30 September 1997 for
those who did not have that complication. The median follow up time for
all cause mortality was 10.5 years. For myocardial infarction and
stroke, for participants who had a non-fatal event before a fatal
event, the time to the first event was used.
Hazard ratios and risk reduction
To assess potential associations between updated mean
systolic blood pressure and complications we used proportional hazards
(Cox) models. The hazard ratio was used to estimate the relative risk.
Potential confounding risk factors included in all Cox models were sex,
ethnic group, age, and smoking (current/ever/never) at diagnosis of
diabetes and baseline concentrations of high density lipoprotein
cholesterol, low density lipoprotein cholesterol, triglyceride,
albuminuria (>50 mg/l measured on a single morning urine sample), and
haemoglobin A1c. At each event time, the updated mean systolic blood pressure for a person with an event was compared with the updated mean systolic blood pressure of those who had not had
an event by that time. The updated mean systolic blood pressure was
included as a time dependent covariate to evaluate systolic blood
pressure during follow up. It was included as a categorical variable in
the categories of blood pressure listed above, with the lowest category
(<120 mm Hg) as the reference category assigned a hazard ratio of 1.0. (This is reflected in the point estimates shown below in figures 3 and
4.) In the analyses for stroke, heart failure, and lower extremity
amputation or deaths from peripheral vascular disease, the two lowest
categories were combined
that is, <130 mm Hg (median 120 mm
Hg)
to increase the reliability of the results as there were
few of these end points in this range. A separate model with updated
mean systolic blood pressure as a continuous variable was used to
determine risk reduction associated with a 10 mm Hg reduction in blood
pressure. (This is reflected in the regression line in figures 3 and
4.) The 95% confidence intervals were calculated on the basis of
absolute floating risk.22 Log linear relations are
reported by convention.
1 18
The risk reduction associated
with a reduction of 10 mm Hg in updated mean systolic blood pressure
was calculated as 100% minus the reciprocal of the hazard ratio
expressed as a percentage.
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Results |
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The risk of each of the macrovascular and microvascular
complications of type 2 diabetes evaluated was strongly associated with
blood pressure, as measured by updated mean systolic blood pressure.
The incidence of cataract extraction was not associated with blood
pressure. Figure 1 shows the incidence rated by category of updated
mean systolic blood pressure for any end point related to diabetes
adjusted for age, sex, ethnic group, and duration of diabetes. The
increase in risk was monotonic, showing no evidence of a threshold, and
showed a twofold increase over the range of systolic blood pressure
from <120 mm Hg (median 114 mm Hg) to
160 mm Hg (168 mm Hg). The
unadjusted and adjusted rates are shown in table 2. Figure 2 shows the
adjusted incidence rates for myocardial infarction and microvascular
end points, both being strongly associated to a similar degree with
increasing blood pressure. Myocardial infarction, however, occurred
about twice as frequently as microvascular end points at each level of
blood pressure. Thus the incidence of myocardial infarction increased from 18 per 1000 patient years in the group with the lowest
systolic blood pressure to 33 per 1000 patient years in the group
with blood pressure >160 mm Hg, with the comparable data for
microvascular disease being 7 to 21 per 1000 patient
years.
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The estimated hazard ratios associated with each category of updated mean systolic blood pressure, relative to the lowest category, are shown as log linear plots in figures 3 and 4. Mortality related to diabetes and all cause mortality were both strongly associated with blood pressure (P<0.001). The risk of each of the complications evaluated, except cataract extraction, rose with increasing updated mean systolic blood pressure with and without adjustment for baseline variables including age, sex, ethnic group, lipid concentrations, HbA1c, smoking, and albuminuria. The decrease in risk for each 10 mm Hg reduction of updated mean systolic blood pressure was between 12% and 19% for both macrovascular and microvascular complications (table 3 and figures 3 and 4).
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There was no indication of a threshold for any of the complications examined below which risk no longer decreased nor a level above which risk no longer increased. The updated mean systolic blood pressure showed similar risk relations to baseline systolic blood pressure (table 3).
Table 3 also shows the risk reductions associated with a 10 mm Hg
reduction in blood pressure in the observational analysis compared with
the risk reduction associated with a 10 mm Hg median difference in
blood pressure from the clinical trial of blood pressure
control.1 After exclusion of cataract extractions, the
relation with blood pressure tended to be stronger in the clinical
trial than in the observational analysis, although in the clinical
trial the confidence intervals were wider (table 3), reflecting the
smaller number of people at risk and of events.1 With
adjustment for updated mean systolic blood pressure in the trial,
allocation to tight blood pressure control had a greater effect on
reducing the risk of heart failure (P=0.0054; 17 events in 749 patients
allocated to tight control and 20 events in 384 patients allocated to
less tight control) than expected from the blood pressure reduction per
se. This was also true for stroke (P=0.027), with 29 events in 752 patients allocated to tight control and 26 events in 386 patients
allocated to less tight control, and for all deaths related to diabetes
(P=0.038), with 63 events in 752 patients allocated to tight control
and 49 events in 386 patients allocated to less tight control. There
was no interaction between treatment and updated mean blood pressure.
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Discussion |
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This observational analysis shows an important association between the occurrence of each of the diabetic complications evaluated (except cataract extraction), including all cause mortality, and systolic blood pressure exposure across the range observed in patients with type 2 diabetes. This association persisted after adjustment for other characteristics that are associated with risk of complications (age, sex, ethnic group, glycaemia, lipid concentrations, smoking, and albuminuria). On average, each 10 mm Hg reduction in systolic blood pressure was associated with a 12% decrease in the risk of any end point related to diabetes and a 15% reduction in the risk of death related to diabetes. Myocardial infarction occurred more commonly than microvascular complications, but the relative risk reduction for a 10 mm Hg reduction in systolic blood pressure was similar at 11% and 13%, respectively.
This observational analysis provides an estimate of the reduction in
risk that might be achieved by the therapeutic lowering of blood
pressure. While it is important to realise that epidemiological associations cannot necessarily be transferred to clinical practice, the results are consistent with those achieved by the policy of tight
control of blood pressure in the clinical trial.1 Whereas tight control did not significantly reduce the risk of myocardial infarction in the clinical trial, the effect size was commensurate with
the observational analysis. The risk reductions in the clinical trial
of tight control seemed to be greater than those anticipated from the
epidemiological analyses for any complications or deaths related to
diabetes, stroke, microvascular disease, and heart failure.1 After allowance for differences in blood
pressure between the tight and less tight policies in the clinical
trial, this apparent treatment effect, per se, was significant only for deaths related to diabetes, stroke, and heart failure. The absence of
significant interaction suggests that treatment effect does not differ
by level of blood pressure. The heart outcomes prevention evaluation
studies (HOPE and MICRO-HOPE) that used ramipril can also be
interpreted to have effects beyond those anticipated by changes in
blood pressure alone.
24 25
This suggests the possibility that treatment with angiotensin converting enzyme
inhibitors
26 27
and
blockers
28 29
may
have cardioprotective effects separate from blood pressure reduction.
For example, both are beneficial in heart
failure.
1 28 30 31
The diminished risk of heart failure
may have reduced the risk of embolic stroke, but no direct data are
available. Effects greater than anticipated have also been shown in
studies in the general population, where the risk reduction in odds of
stroke from pooled trials of antihypertensive drug treatment exceeded
the 35-40% expected from epidemiological studies.32 It is
possible that the association between blood pressure and cardiovascular
disease differs in magnitude in diabetic and non-diabetic populations,
which could not be tested in this study. In support of this
possibility, the multiple risk factor intervention trial (MRFIT)
observed that the association of systolic blood pressure and
death from cardiovascular disease was of a lower magnitude in diabetic
compared with non-diabetic men.15 Stroke and heart failure
were the complications least strongly associated with
glycaemia,2 suggesting that for these complications, by
comparison, raised blood pressure is of greater pathogenetic importance.
Lack of thresholds
We observed no thresholds of systolic blood pressure for any
complication of diabetes. This suggests that there is no specific
target blood pressure to aim for but that the nearer to normal systolic
blood pressure the lower the risk of complications, in accord with
recommendations to reduce systolic blood pressure to less than 130 mm
Hg33-35 or less than 125 mm Hg in the presence of
microalbuminuria.36 Whether these target values can
realistically be achieved depends on an individual's initial blood
pressure and willingness to modify life style or to take several drugs
that may have side effects. Neither our study nor the hypertension
optimal treatment (HOT) study37 found a J or U shaped
association between systolic blood pressure and complications, which is
now thought to reflect coexisting morbidity with low blood pressures
rather than the effect of treatment.38 Unlike the HOT
study the UKPDS did not observe a flattening of the relation at low
levels of blood pressure. There was no indication of a level above
which systolic blood pressure was no longer associated with an
increased risk of complications. Thus any reduction of raised blood
pressure is likely to have benefit.
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What is already known on this topic
People with diabetes who also have hypertension are more likely to develop complications Treatment of blood pressure in these individuals reduces the risk of complications What this study addsThere is a direct relation between the risk of complications of diabetes and systolic blood pressure over time No threshold of systolic blood pressure was observed for a substantive change in risk for any of the clinical outcomes examined The lower the systolic blood pressure the lower the risk of complications There may be additional risk reduction with angiotensin converting
enzyme inhibitors and |
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Acknowledgments |
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The cooperation of the patients and many NHS and non-NHS staff at the centres is much appreciated. Details of participating centres can be found on the BMJ's website.
Contributors: AIA coordinated the writing of the paper and participated in interpretation of results. IMS selected the methodology, carried out the statistical analyses, and participated in interpretation and revision of the paper. HAWN, JSY, DRM, and ADW participated in interpretation and revision of the paper. CAC participated in the preparation of the data and interpretation and revision of the paper. RCT and RRH were the principal investigators, planned and designed the study, and participated in interpretation and revision of the paper. RCT was also responsible for the initial draft of the paper. RRH is guarantor.
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Footnotes |
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Professor Turner died unexpectedly after completing work on this paper
Funding: The major grants for this study were from the UK Medical Research Council, British Diabetic Association, the UK Department of Health, The National Eye Institute and The National Institute of Digestive, Diabetes and Kidney Disease in the National Institutes of Health, United States, The British Heart Foundation, Novo Nordisk, Bayer, Bristol-Myers Squibb, Hoechst, Lilly, Lipha, and Farmitalia Carlo Erba. Details of other funding companies and agencies, the supervising committees, and all participating staff can be found on the BMJ's website.
Competing interests: AIA has received fees for speaking from Bristol-Myers Squibb, SmithKline Beecham, and Pfizer. IMS has received support for attending conferences from Zeneca and Hoechst and fees for speaking from Hoechst. CAC has received support for attending conferences from Bristol-Myers Squibb, Novo Nordisk, and Pfizer and fees for speaking from Bristol-Myers Squibb and Novo Nordisk. JSY has received consultancy fees from SmithKline Beecham. DRM has received fees for speaking from Bristol-Myers Squibb, Novo Nordisk, SmithKline Beecham, and Lilly and research funding from Lilly. RRH has received fees for consulting from Bayer, Boehringer Mannheim, Bristol-Myers Squibb, Hoechst, Lilly, Novo Nordisk, Pfizer, and SmithKline Beecham; support for attending conferences from Bayer, Bristol-Myers Squibb, Hoechst, Lilly, Lipha, Novo Nordisk, and SmithKline Beecham; and research funding from Bayer, Bristol-Myers Squibb, Lilly, Lipha, and Novo Nordisk.
Details of participating centres,
staff, and committees and additional funding agencies are on the BMJ's
website
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(Accepted 20 March 2000)