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Aroon D Hingorani Centre for
Clinical Pharmacology, Wolfson Institute for Biomedical Research,
University College London, London W1P 9LN
Correspondence to: Dr Hingorani
a.hingorani{at}ucl.ac.uk
Objective:
To describe, and to test against trial
data, a simple and flexible computer program for calculating
cardiovascular risk in individual patients as an aid to managing risk
factors and prescribing drugs to lower cholesterol concentration and
blood pressure.
Prospective cohort studies have shown that the absolute risk
of cardiovascular disease in any individual is determined by a complex
interplay of several factors, of which age, sex, smoking status, blood
pressure, and serum concentrations of total cholesterol and high
density lipoprotein cholesterol are the more important.1 Recent large randomised controlled trials have shown that reducing serum cholesterol concentration reduces the incidence of coronary heart
disease events in patients with a history of angina or myocardial infarction
2 3
and in middle aged men with a high
cholesterol concentration but without symptomatic coronary artery
disease.4 As with antihypertensive treatment, the absolute
benefit from cholesterol reduction depends on the pretreatment level of
cardiovascular risk The interaction between risk factors is not additive but synergistic.
Calculations of levels of risk and possible benefits of
intervention in any individual are not straightforward and cannot
readily be undertaken during a consultation. Attempts to overcome this
problem by developing risk charts or tables have been
useful,6-9 but these approaches give only broad estimates of risk based on clusters of risk factors. With this approach it is
difficult to quantify precisely the predicted risk or, more importantly, the likely consequences of therapeutic intervention in an
individual patient.
We have developed an interactive computer program that overcomes some
of these limitations. The program, based on standard software,
calculates and displays an individual's absolute and relative risks of
coronary heart disease, stroke, or various other end points of
cardiovascular disease and can be used to estimate the expected benefit
of modifying risk factors. We compare the predictions of the program
with data from recent randomised controlled trials and use case
examples to illustrate how this or other programs might be used in
clinical practice.
Computer program
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Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Design:
Descriptive comparison of actual
cardiovascular risk in randomised controlled trials of cholesterol
reduction with risk predicted by a computer program based on the
Framingham risk equation. Comparison of the program's performance with
that of tables and guidelines by means of hypothetical case examples.
Main outcome measures:
Average risk of coronary heart
disease and myocardial infarction.
Results:
The computer program accurately
predicted baseline absolute risk in a UK population as well as the
relative and absolute reduction in risk from cholesterol lowering for
primary prevention of coronary heart disease. The program also allowed a more refined estimate of absolute risk of coronary heart disease than
some existing tables and enabled the impact of prescribing decisions to
be quantified and costed.
Conclusions:
This simple computer program to estimate
individuals' cardiovascular disease risk and display the benefits of
intervention should help clinicians and patients decide on the most
effective packages of risk reduction and identify those most likely to
benefit from modulation of risk factors.
Key messages
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Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
individuals with high absolute levels of risk
stand to gain the most.5 The problems facing doctors are
how to implement the findings of the many clinical trials and cohort
studies into everyday clinical practice and how to involve patients in
the decision making process.
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Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
The program (based on a Microsoft Excel version 5.0 workbook and
available on a single floppy disk) runs on a personal computer and
provides a graphical and numerical display of the risk of
cardiovascular outcomes for any given combination of clinical
variables. Data on standard risk factors are entered into a simple
screen (fig 1). Risks are calculated with logistic regression equations
derived from the Framingham populations,10-12 a large
cohort in the United States studied prospectively over many years. The
program displays an individual's absolute and relative risk together
with an estimate of the change in risk that might follow therapeutic
intervention or changes in lifestyle (such as reducing blood pressure
or total cholesterol concentration or stopping smoking). In its present
form, for use in a clinic, the program displays a 10 year risk for
coronary heart disease and stroke. It also displays the predicted risk
of coronary heart disease and stroke for the general population
the
average risk for a non-smoking subject matched for the patient's age
and sex with age adjusted population mean levels of total cholesterol, high density lipoprotein cholesterol, and systolic blood pressure. The data and outcome estimates can be printed. The data are stored in a
database, which can be interrogated to provide, for example, information on changes in risk factors over time, success in risk reduction, prescribing policy against risk scores, or the overall burden of cardiovascular disease in a practice
population.

View larger version (27K):
[in a new window]
Fig 1.
Data entry screen (below)
and individual risk profile (right). The graph shows predicted 10 year
risk for coronary heart disease and stroke (blue columns); risk for an
individual matched for age and sex with population mean levels,
adjusted for age, of cholesterol and systolic blood pressure (yellow
columns); and the effect of stopping smoking on risk (red columns)
Comparison of program's predictions
The Framingham coronary heart disease risk function is derived by
observing the cumulative number of events of coronary heart disease and
cardiovascular disease among the original Framingham cohort and the
subjects of the Framingham offspring study, who were followed for 4-12 years.10-12 The Framingham analysis is therefore designed
for use in the setting of primary prevention and forms the basis
of some commonly used guidelines and risk tables such as the Sheffield
and New Zealand tables.
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Results |
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Baseline absolute risk
Table 1 shows the five year risk of various end points of
cardiovascular disease calculated by our program for a 55 year old man
with clinical variables corresponding to the average baseline
characteristics of a member of the placebo arm of the West of Scotland
coronary prevention study (WOSCOPS), a primary prevention
trial.4 Risks for a smoker and a non-smoker are considered
separately. These data are compared with the five year event rates in
the placebo arm of the WOSCOPS trial itself, which included both
smokers (44%) and non-smokers (56%). For each comparison, we used the
closest equivalent end point from the WOSCOPS trial. For all end
points, the event rates in the WOSCOPS trial lay between those
predicted by the program for smokers and for non-smokers. When applied
to individuals with clinical profiles compiled by using average
baseline data from the placebo arms of the Scandinavian simvastatin
survival study (4S)2 and the cholesterol and recurrent
events trial (CARE)3 (trials of secondary prevention) the
Framingham risk equation underestimated absolute baseline risk about twofold.
Effects of lowering serum cholesterol concentration
In the WOSCOPS trial, treatment with pravastatin resulted in a
20% reduction in total serum cholesterol concentration and a 5%
increase in high density lipoprotein cholesterol, with the full effect
on lipid concentration being achieved within a few months of starting
treatment. Table 2 shows the program's predicted effects of these
changes along with the observed risk reductions in the trial itself.
For all the end points considered, both the relative and absolute risk
reductions predicted by the program were similar to those observed in
the trial and lay well within the reported confidence intervals,
although these were wide. The predicted relative risk reduction for
subjects with pre-existing cardiovascular disease (secondary
prevention) was less accurate, with the program overestimating the
benefit compared with that seen in the 4S and CARE trials (table
3).
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Comparison with Sheffield tables
The Sheffield tables and our risk program are formulated from the
same original data. Table 4 shows how treatment decisions might vary
depending on which system was used to determine whether a patient
should receive a statin (using a 55 year old smoker with a cholesterol
concentration of 7.4 mmol/l as an example). Because the Sheffield
tables treat hypertension as a dichotomous variable (present or absent)
and (as commonly used) assume an average concentration of high density
lipoprotein cholesterol, they predict an artificially high risk for
some patients and an artificially low risk for
others.
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Modulation of individual risk factors
If the aim of treatment is to reduce risk, it is clear that this
might be achieved in several different ways. Figure 2 shows the risk
for a 69 year old male smoker with a blood pressure of 170/90 mm Hg,
total cholesterol concentration of 7.4 mmol/l, and high density
lipoprotein cholesterol of 1.0 mmol/l. His 10 year absolute risk of
coronary heart disease is 45%, and his 10 year absolute risk of stroke
is 18%. A series of options to reduce overall risk is presented and
costed.
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Discussion |
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Cardiovascular disease is a major cause of mortality and morbidity in Britain. Prevention of coronary heart disease and stroke is an important public health challenge and one that doctors and patients consider a priority. Preventive measures of proved efficacy include reducing hypertension,14-16 lowering blood cholesterol concentration,2-4 and stopping smoking.17 Many doctors and their patients will have discussed the various treatment options and tried to assess which would be most appropriate. However, the interaction between risk factors is complex, and the quantitative benefits on health of treatments or lifestyle changes in any individual are not intuitive and cannot easily be presented to patients in an accessible form. This is likely to become more of a problem as new risk factors, such as genetic predisposition, are identified. To overcome this problem we have devised a simple computer program to run on a personal computer or local network. It is based on the Framingham data and can be used to calculate an individual's level of risk and show the predicted effects of intervention. The predictions of the program are consistent with the published results of major studies and, as such, offer simple guidance on treatment according to best available evidence.
Tables and guidelines
A major driving force behind the development of risk tables and
treatment guidelines has been the high cost of
statins.18-20 The Standing Medical Advisory Committee has
issued recommendations for the use of statins for secondary prevention in Britain13 and has advocated their use in people without
overt vascular disease who have an annual risk of coronary heart
disease risk of 3% or higher. To assess when this risk level has been crossed, as many risk factors as possible should be considered. This
presents a problem for tables and paper guidelines. For example, the
failure of such guides to present blood pressure, total cholesterol concentration, and high density lipoprotein cholesterol concentration as continuous variables may lead to discrepancy with the original Framingham data on which the tables are based. This could lead to
undertreatment of some individuals and overtreatment of others (table 4).
Advantages of computer based system
The advantages of the program described here are that
Limitations of computer program
The fact that we used the Framingham data to predict successfully
the event rate in the placebo arm of the WOSCOPS study22
suggests that the Framingham database is directly applicable to a UK
population without clinically evident atherosclerotic disease. In
predicting the effects of an intervention, the program assumes that, on
lowering blood pressure, adjusting cholesterol, stopping smoking, etc,
individuals adopt the new level of risk predicted from the cohort
study. The evidence supports this assumption,
23 24
but
the program may overestimate the benefits of reducing blood pressure on
coronary heart disease14 and, possibly, underestimate the
benefits of lowering total cholesterol concentration with statins.22
Conclusions
Our simple computer program, based on the Framingham data,
can be used to determine levels of absolute and relative risk of
cardiovascular disease in individuals and to estimate the effect on
risk of proposed interventions. The results are consistent with the
results of the major intervention studies and could be used as an
individualised evidence based guide to prescribing policy, and to
involve patients in the decision making process. The program would
focus attention away from single risk factors towards a more integrated
approach to preventing cardiovascular disease. If it was used in
primary care its implementation might also allow creation of a unified
database of local and possibly national value.
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Acknowledgments |
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Those involved in developing the program and the Vascular Risk Clinic at University College Hospital include Professors John Martin and John Deanfield; Drs Raymond MacAllister, Kiran Bhagat, and Ian Day; and Mike Gahan and Ryan Pervan.
Contributors: PV conceived the idea of a computer program for use in an integrated cardiovascular risk clinic and worked with John Martin and John Deanfield to develop the idea and the clinic. The program was written by Mike Gahan and modified by Ryan Pervan, guided by a team comprising Kiran Bhagat, Ian Day, ADH, John Martin, Raymond MacAllister, and PV. ADH and PV undertook the analysis to assess the program's performance and wrote the paper. PV is guarantor for the paper.
Funding: ADH is supported by a Gerry Turner Intermediate fellowship of the British Heart Foundation.
Potential conflict of interest: Copyright for the program is currently held by John Martin and PV.
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References |
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(Accepted 27 August 1998)
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