Introduction

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The UK government policy framework endorses joint British recommendations on preventing coronary heart disease: primary care teams must assess patients' risk of cardiovascular disease every five years and treat eligible patients (see box on bmj.com).¹ The recommendations require many hours of clinical staff time and considerable cost. Part of this commitment will be devoted to assessing patients who ultimately do not need treatment. The recommendations do not quantify the resource implications or the health benefits of this policy. We describe a model for estimating the efficiency (total health service resources invested versus cardiovascular events prevented) of six strategies for primary care based prevention of cardiovascular disease.

	Methods

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The joint British recommendations require patients to undergo five yearly assessments for risk of cardiovascular disease. Therefore our model analysed resource implications and health benefits over a five year period. Resource implications were considered from the perspective of the health service. The data for our model came from several sources (see bmj.com).

Cardiovascular risk estimation
We used the Framingham coronary heart disease risk equation and the cerebrovascular disease risk equation to estimate the number of coronary events prevented and the number of strokes prevented.² The combined Framingham risk equation predicts the risk of all cardiovascular events (stroke, coronary event, peripheral vascular disease, or heart failure). We used it to calculate prior risk estimates for the preselection strategies.

Distribution of risk factors and risk
Using data from the 1998 health survey of England, we estimated the distribution of systolic blood pressure and cholesterol concentration by age, sex, history of diabetes, and smoking history in individuals who neither had prior cardiovascular disease nor were receiving antihypertensives.³ Then we generated a hypothetical population whose distribution for age, sex, and risk factors reflected a typical general practitioner list. Our typical general practitioner list had 939 patients, aged between 30 and 74, eligible for primary prevention services.

Prevention strategies modelled
The joint British recommendations do not indicate how to prioritise assessed patients for treatment. In strategies JBR-1 (joint British recommendations-1), JBR-2, and JBR-3 all patients undergo five yearly assessments for clinical risk. Once assessed the general practitioner treats them in rank order of their risk of coronary heart disease: the total number of patients treated being determined by the total resources available.

In strategies RM-1 (Rouse Marshall-1), RM-2, and RM-3 a two stage assessment is proposed (see box and bmj.com).

	Resource implications

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We assumed that all clinical tasks are carried out by practice nurses. The total health service cost of providing an hour of practice nurse clinic time is £28 ($43; 44).⁴ Costs of blood tests were derived from the Pathology Services costings.⁵ Drug costs were obtained from the British National Formulary.⁶ Dispensing costs were calculated at 87.4p for each prescribed item on the assumption that four prescriptions are issued each year.⁷ Where appropriate, costs have been discounted at 6% a year. We estimated that each risk assessment takes a minimum of 20 minutes of clinical staff time.

Treatment and follow up
Ongoing management of patients receiving treatment requires at least two follow up clinic appointments a year, totalling 20 minutes of clinical staff time.¹ Patients receiving thiazide diuretics require an annual estimation of electrolyte and uric acid, and patients receiving statins require annual liver function tests.

	Health benefits

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We calculated the benefits of treatment as the number of cardiovascular events prevented. As patients may have two cardiovascular events, this differs from the number of patients having a cardiovascular event. For each patient, the absolute risk reduction is the product of the initial risk and the risk reduction with treatment. The assumed risk reductions afforded by treatment are given on bmj.com.

	Results

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Cost effectiveness

Technical efficiency: maximising benefits within total resources
For any given allocation of resources to the primary prevention of cardiovascular disease, more cardiovascular events can be prevented under RM strategies than under the equivalent JBR strategies. A primary care team can prevent 13.5 events for £49 960 under strategy RM-2 or 13.5 events for £15 110 under RM-3. The most efficient strategy for a primary care team with this budget is therefore RM-3. A primary care team can prevent 16.7 events for £119 806 under strategy RM-1 or 13.5 events for £73 716 under RM-2. The most efficient strategy for a primary care team with this budget is therefore RM-2. For a primary care team with over £119 806 the most efficient strategy is RM-1. Figure 1 shows the maximum number of events prevented with increasing amounts of resources under each strategy.

View larger version (21K): [in this window] [in a new window]   Fig 1.   Total resource costs (assessment, follow up, drugs, and investigations) and health benefits of six strategies for primary prevention of cardiovascular disease

Maximising efficiency within available clinical staff time
For practices allocating one, two, or three clinics a month to the primary prevention of cardiovascular disease, RM strategies dominate JBR strategies (fig 2). At one clinic a month there is not sufficient clinical time to assess all eligible adults, JBR strategies therefore cannot be implemented. Strategy RM-3 prevented 9.1 cardiovascular events at a cost of £863 for each event prevented. RM-2 prevented 1.9 more events at an incremental cost of £10 518 for each event prevented. RM-1 prevented 4.6 more events than RM-2 at an incremental cost of £17 808 for each event prevented.

View larger version (17K): [in this window] [in a new window]   Fig 2.   Resource costs and effectiveness of six strategies for prevention of cardiovascular disease in practice allocating one, two, and three clinics a month (only non-dominated strategies shown)

Sensitivity analysis
Selecting only patients over 50 for primary prevention and altering assumptions about workload did not influence the results of our model. Nor did prior knowledge of patients' blood pressures: RM strategies prevent more cardiovascular disease than JBR strategies. Varying the cost of prescribing statins or angiotensin converting enzyme inhibitors altered our findings only if they cost under £231 for five years' treatment.

	Discussion

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We assumed that all eligible patients accepted and complied fully with treatment. In reality some decline assessment, and some do not comply with prescribed treatment. Reducing the rest period before measurement of blood pressure to under four minutes risks significant overestimation.⁸ Blood tests take longer than 2.5 minutes and, given the variability of cholesterol concentrations, accurate estimation takes more than two measurements.⁹ Calculating the risks of cardiovascular disease and counselling patients probably takes longer than 20 minutes. Follow up may also cost more than we estimated. Patients often visit their general practitioner more frequently than twice yearly, and further investigations may be needed. The cost of staff time may be underestimated, since medical time costs more than nursing time.

Our model assumed no patients leave or join the practice over five years. An annual turnover of 10% in the practice population increases the number of five yearly assessments by 41%. In contrast, periodically recalculating prior risk estimates and re-ranking patients takes only minutes, and the primary care team could fill clinic time left by patient departures by inviting the next highest ranked patients for assessment. The turnover of practice populations therefore strongly favours the RM strategies.

Our model assumed that primary care teams following the joint British recommendations do prioritise patients for treatment on the basis of their risk of coronary heart disease. In fact, patients whose risks are above a threshold are likely to be treated as they are identified. Our model therefore exaggerates the effectiveness of the joint British recommendations. The RM strategies make no such assumption.

Our model used the lowest cost drug for each category of treatment. Non-generic enalapril costs twice as much as generic enalapril. Simvastatin was costed at 10 mg a day, whereas higher doses are usual.¹⁰ Compared with clinical practice, our model therefore exaggerated the cost effectiveness of strategies using simvastatin and enalapril. Finally, we probably exaggerated the benefits of intensive blood pressure lowering.

Implications of model for treatment
Our model raises questions about current treatment recommendations in the context of a population based programme for the prevention of cardiovascular disease. Recent research confirms statins are effective irrespective of initial cholesterol concentrations, across a wide range of risks.¹¹ Our model showed that they are not cost effective even for patients meeting current criteria. More intensive blood pressure lowering may reduce risk but does not justify the additional cost. This casts doubt on the importance of achieving blood pressure targets in a publicly financed, population based programme for the prevention of cardiovascular disease. Aspirin is currently recommended only for patients whose risk of coronary heart disease over five years exceeds 15%.¹ Our model suggests it may be cost effective for patients at much lower risk levels.

Conclusions
We recommend that authors of future guidelines should make explicit statements about the resource implications, health benefits, and efficiency of implementation strategies. Furthermore, the efficiency of prevention of cardiovascular disease in primary care could be greatly enhanced by two innovations: prioritising patients for assessment on the basis of a prior estimate of their cardiovascular risk and avoiding costly drugs such as simvastatin and enalapril.