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European Heart Journal (1998) 19, 429–434

The incidence of suspected myocardial infarction in Dutch general practice in the period 1978–1994 K. M. van der Pal-de Bruin*†, H. Verkleij†, J. Jansen†, A. Bartelds* and D. Kromhout† *Netherlands Institute of Primary Health Care, Utrecht; †National Institute of Public Health and the Environment, Bilthoven, The Netherlands

Aim To evaluate how the incidence of suspected myocardial infarction has developed from 1978 to 1994 and to study the incidence of confirmed acute myocardial infarction in Dutch general practices during the period 1991–1994. Methods In three periods (1978, 1983–1985 and 1991– 1994) the incidence of suspected myocardial infarction has been registered by the ‘Dutch Sentinel Practice Network’. This is a registration system that obtains data from general practitioners covering about 150 000 persons. During the period 1991–1994 the incidence of confirmed myocardial infarction was also registered. Results For men, the incidence rate of suspected myocardial infarction decreased by 28% from 4·7 per 1000 in 1978 to 3·4 per 1000 in the period 1991–1994. For women,

Introduction In most European countries, as in the Netherlands, cardiovascular diseases are responsible for about 40% of deaths in both men and women[1]. An important category of cardiovascular disease is acute myocardial infarction, which accounts for about 30% of total cardiovascular mortality. As in other Western European countries, the age-standardized mortality from acute myocardial infarction decreased continuously in the Netherlands during the period 1972–1993, by 46% in men and 40% in women[2]. It is not known whether this decrease in mortality is due to a decreasing incidence, or case fatality rate, or to a combination of the two. This is of great interest because a reduced incidence rate points to effective preventive measures and a reduced case fatality to effective medical care[3–5]. In the Dutch Sentinel Practice Network, the incidence of suspected myocardial infarction was regisRevision submitted 8 August 1997, and accepted 14 August 1997. Correspondence: Dr H. Verkleij, Department for Public Health Forecasting, National Institute of Public Health and the Environment, P.O. Box 1, 3720 BA Bilthoven, The Netherlands. 0195-668X/98/030429+06 $18.00/0 hj970766

the incidence fell by 23% from 2·6 per 1000 in 1978 to 2·0 per 1000 in the period 1983–1985 and stabilized thereafter. The most pronounced decrease in the incidence of suspected myocardial infarction during the period 1978—1994 occurred in both men and women aged 45–64. In the period 1991–1994, the standardized incidence rate of confirmed myocardial infarction is 1·7 per 1000 for men and 0·9 per 1000 for women. Conclusions These data suggest that the incidence of suspected myocardial infarction has declined substantially between 1978 and 1994 both for men and for women; this decline is most pronounced in the age group 45–64. (Eur Heart J 1998; 19: 429–434) Key Words: Myocardial infarction, incidence, trends, general practice, The Netherlands.

tered during three periods (1978, 1983–1985, 1991– 1994). These data are used to study changes in the incidence of suspected myocardial infarction from 1978 to 1994. In the period 1991–1994 also the incidence of confirmed infarction was established.

Methods Data collection The ‘Dutch Sentinel Practice Network’ is a registration system that obtains its data from general practitioners[6]. This network covers circa 150 000 persons (approximately 1% of the Dutch population). Attention is paid to maintaining records of a representative distribution of the population according to age and gender over different regions in the country and degree of urbanization. People choose their general practitioner freely, but seldom shift to another except in case of a change of home address. Every 2 years a census of practice populations is conducted. General practitioners in the Netherlands only serve the non-institutionalized population. Thus, ? 1998 The European Society of Cardiology

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48 000 people living in nursing homes (2·3% of the Dutch population aged 65 years or older) are excluded from the registration system. Each year a number of specified diseases are selected for registration purposes. The sentinel physician fills in a weekly registration form to indicate which of these diseases have occurred in his practice in the current week. The occurrence of suspected myocardial infarction was registered during three periods, namely 1978, 1983–1985 and 1991–1994. A suspected myocardial infarction case was registered if the sentinel practitioner or his replacement had seen the patient and acted as if the condition was a myocardial infarction. Patients who had died before the practitioner arrived and for whom there were strong arguments to suspect a myocardial infarction as cause of death, for instance because of a cardiac history, were included explicitly in the registration protocol in the last, but only very implicitly in the first two registration periods[7,8]. As a result, the percentage of sudden death cases among suspected cases in the last period (25%) was higher than in the first two registration periods (10–15%). This will lead to an under-estimation of the number of cases of suspected myocardial infarction in the first two registration periods. Of the patients with a suspected myocardial infarction, the following variables were collected: deceased/ not deceased, cardiac history and the practitioner who had examined the patient. For follow-up purposes the first three letters of the surname, date of birth and gender of each case were registered. The incidence rates of the three registration periods are standardized to the Dutch population on 1 January 1993. From 1991 to 1994, the sentinel physician was also asked to register each case of confirmed myocardial infarction. This group contains patients who have been registered directly as confirmed cases and patients who have been registered before as having a suspected myocardial infarction. Patients who were not attended by the sentinel physician (e.g. direct hospital admissions or while the patient was on holiday) and for whom confirmation was sent to the physician, were also registered. Of the patients with a confirmed myocardial infarction the following additional variables were collected: practitioner who diagnosed the patient (specialist/general practitioner/other practitioner), method (ECG/enzyme test/obduction) and date of diagnosis. It was not possible to separate first and recurrent myocardial infarctions. However, the patient identifiers made it possible to identify recurrent cases within the study period. The diagnosis was mostly based on an ECG combined with an enzyme test (71·3%). In a number of cases the diagnosis was based on an ECG alone (16·1%) but seldom on an enzyme test alone (3·7%). Sometimes an obduction was done, often combined with an ECG or enzyme test (4·3%). In 34 cases (4·5%) it is not recorded what method was used. Most often it was the specialist who made the diagnosis (72·8%). In an extra 11·6% of the cases the diagnosis was made by the general practitioner and the specialist together. This definition of confirmed myocardial infarction corresponds most to Eur Heart J, Vol. 19, March 1998

the categories NF1 (non-fatal ‘definite’) and F1 (fatal ‘definite’) in the WHO MONICA-project[9]. At the end of the registration period 1991–1994 a questionnaire was sent to the participating practitioners (n=73) to be sure that all cases of confirmed myocardial infarctions had been registered. Of the 73 general practitioners, 68 (93%) answered the questionnaire. Of these 68 general practitioners, 88·2% always registered the confirmed myocardial infarction. Three general practitioners had stopped practising during the registration period 1991–1994 and did not remember if they had always registered the confirmed myocardial infarction. Five general practitioners were not sure; they were asked to verify the diagnosis of all suspected myocardial infarction cases in their records. This resulted in 16 extra confirmations. These were included in the incidence figures. If the sentinel physician did not receive confirmation of the diagnosis, the case is stated as a not confirmed myocardial infarction. In the Netherlands, patients with a possible myocardial infarction are generally admitted to hospital. Therefore we assume that almost everyone of this group had further diagnostic tests and the test results were negative. To determine whether there was a change in incidence of confirmed myocardial infarctions between 1991–1994, the incidence was also calculated per year (not presented here). There did not appear to be any variation between the years. Therefore, to obtain a more reliable estimate, the age-standardized incidence has been computed over the whole registration period (1991–1994) and averaged per year, 95% confidence limits were calculated for each age and gender category.

Results Trend in the incidence of suspected myocardial infarction over the period 1978–1994 For all three periods (1978, 1983–1985, 1991–1994), cases of suspected myocardial infarction were registered. In Table 1 the incidence rates for males and females, standardized to the Dutch population on 1 January 1993 are presented. For males the overall incidence rate of suspected myocardial infarction declined from 4·7 per 1000 in 1978 to 3·4 per 1000 in the period 1991–1994. For females, the incidence declined from 2·6 per 1000 in 1978 to 2·0 per 1000 in the period 1983–1985 and did not decrease further for the period 1991–1994. The most pronounced and continuous decrease in the incidence of suspected myocardial infarction during the period 1978– 1994 occurred in both men and women aged 45–64.

Incidence of confirmed myocardial infarction in the Netherlands in the period 1991–1994 In the period 1991–1994, out of 1483 cases of suspected myocardial infarction 750 cases (51%) were confirmed

MI in Holland in 1978–1994

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Table 1 Incidence rates (per 1000 per year) of suspected myocardial infarction for men and women in three registration periods standardized to the Dutch population on 1 January 1993 Registration period age

1978

1983–1985

1991–1994

1978

1983–1985

1991–1994

64

0·6 9·8 20·0

0·5 8·3 19·0

0·5 6·6 15·0

0·1 2·8 12·4

0·1 2·5 9·3

0·1 1·9 9·8

Total

4·7

4·2

3·4

2·6

2·0

2·0

Men

Women

Table 2 Incidence rate (per 1000 per year) of confirmed myocardial infarction by age and gender, standardized to the Dutch population on 1 January 1993 Men Age

Women

Person years

Cases*

Standardized incidence rate†

Person years

Cases

Standardized incidence rate†

0–24 25–34 35–44 45–54 55–64 65–74 75–84 85+

101 876 51 991 45 992 35 044 27 111 20 449 9333 2266

0 4 28 94 159 133 63 18

0 0·1 (0–0·2) 0·6 (0·4–0·9) 2·7 (2·1–3·2) 5·9 (5·0–6·8) 6·5 (5·4–7·6) 6·8 (5·1–8·4) 7·9 (4·4–12·6)

106 574 50 662 45 569 34 628 28 507 21 233 12 710 5028

0 2 7 16 41 82 73 25

0 0 0·2 (0·1–0·3) 0·5 (0·3–0·8) 1·4 (1·0–2·0) 3·9 (3·0–4·7) 5·7 (4·4–7·0) 5·0 (3·2–7·3)

Total

294 112

498

1·7 (1·6–1·9)

304 911

244

0·9 (0·8–1·0)

*Five cases are not included because of unknown age. †(95% confidence limits); if the number of cases is less than 50, the 95% CL is calculated on the assumption of a Poisson distribution, otherwise on the assumption of a normal distribution.

(503 men, 244 women and three with unregistered gender). The number of confirmed re-infarctions between 1991 and 1994 was 23 (3·1% during an average follow-up period of 2 years). Based on these figures, the incidence rate (per 1000 persons per year and standardized to the Dutch population of 1993) for confirmed myocardial infarction was estimated (see Table 2). From Table 2 it can be seen that the incidence of confirmed myocardial infarction increases with age for men and women. For men the increase is most prominent from the age of 45; for women a strong increase starts ten years later. Standardized for the Dutch population of 1 January 1993 the estimated absolute incidence of confirmed myocardial infarction cases in the Netherlands is 12 595 (1·7 per 1000) for men and 7242 (0·9 per 1000) for women per year. In total, the incidence of confirmed myocardial infarction is 19 837 cases per year in a population of 15·2 million inhabitants.

Analysis of unconfirmed myocardial infarctions From the 1483 reports of possible myocardial infarction, 49% could not be confirmed (461 men and 272 women).

The occurrence of sudden death among unconfirmed cases was 28% (n=130) in men and 29% (n=79) in women. Sixty-two percent of these men (n=80) and women (n=49) had presented before with cardiac symptoms. It is possible that there are some definite myocardial infarctions in this group, but the exact number is unknown. Table 3 presents the age and gender distribution of the confirmed and unconfirmed cases in the non-fatal category. There is no difference between the percentage of confirmation among men and women. For men there seems to be a difference in age distribution between the two groups (P(chi-square)=0·017); in the older age groups the suspicion of myocardial infarction is confirmed less frequently than in the younger age groups. For women, there was no difference in confirmation percentages between age groups (P(chi-square)=0·586).

Discussion Reliable incidence figures for myocardial infarction are scarce. This applies even more to data on trends. In the present study, the trend in incidence rate of suspected myocardial infarction in the Netherlands is based on a Eur Heart J, Vol. 19, March 1998

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Table 3 Number of non-fatal cases of confirmed and not confirmed myocardial infarction in the sentinel practices during the registration period 1991–1994 Confirmed myocardial infarction (non-fatal)

Gender Men Women Age Men 0–24 25–34 35–44 45–54 55–64 65–74 75–84 85+ Women 0–24 25–34 35–44 45–54 55–64 65–74 75–84 85+

Not confirmed myocardial infarction (non-fatal)

Percentage of confirmation %

n

%

n

%

434 203

68·1 31·9

331 193

63·2 36·8

57 51

0 4 28 83 140 110 54 13

0·0 0·9 6·5 19·2 32·4 25·5 12·5 3·0

0 4 19 49 84 93 58 23

0·0 1·2 5·8 9·0 25·5 28·2 17·6 7·0

0 50 60 63 63 54 48 36

0 2 7 16 40 67 51 20

0·0 1·0 3·5 7·9 19·7 33·0 25·1 9·9

0 1 1 16 39 63 52 20

0·0 0·5 0·5 8·3 20·3 32·8 27·1 10·4

0 67 88 50 51 52 50 50

representative registration from general practitioners, the ‘Dutch Sentinel Practice Network’. The advantage of this approach was the inclusion of (non) fatal cases that never reach the hospital and the exclusion of possible duplicate registrations of hospital re-admissions.

Trend in the incidence of suspected myocardial infarction during the period 1978–1994 From the registration period 1978 to the registration period 1991–1994, the age-standardized incidence of suspected myocardial infarction in men decreased by 28% from 4·7 per 1000 in 1978 to 3·4 per 1000 in 1991–1994. For women the incidence fell by 23%, from 2·6 per 1000 in 1978 to 2·0 per 1000 in 1983–1985 and did not decline further. The true decrease in suspected myocardial infarction is probably even more pronounced because, due to a more explicit registration protocol, in the last period a greater number of sudden deaths were included (see also under Methods). It is not possible to make an adequate comparison of incidence rates in the Netherlands and other Western European Countries because of differences in registration methods and case definitions. Some trends seem to be parallel, however. In the present study, the decrease in incidence rates was most pronounced among persons aged 45–64 years (about 32% decline in both Eur Heart J, Vol. 19, March 1998

men and women). In the Fourth General Morbidity Survey carried out in the United Kingdom, the incidence rate of first acute myocardial infarction fell from 2·6 per 1000 in 1981–1982 to 1·5 in 1991–1992. Among people aged 45–64 years, first incidence rates decreased faster and were half those in 1981–1982[10]. In the Netherlands, a decrease in mortality from myocardial infarction has been reported since 1972[1]. It is not known whether this decrease can be attributed to improvements in care of patients with myocardial infarction with favourable effects on survival, to a decline in risk factor prevalence or to a combination of both[3–5,11–13]. In the U.S.A., 43% of the decline in coronary heart disease in the period 1980–1990 has been attributed to improvements in treatment, 29% to reduction of risk factors in patients and 25% to the effects of primary prevention[14]. Similar detailed information for the Netherlands is lacking. Improvements in care have been implemented rather quickly during the (late) 1980s (coronary bypass surgery, PTCA, thrombolysis, and very recently cholesterol-lowering therapy). Information about changes in major risk factors in the population is available from 1974. In the Netherlands, the percentage of smokers decreased by about 1·6% per year in the 1970s and by 0·8% in the 1980s. In the 1990s, the prevalence of cigarette smoking changed little[15]. In the 1970s and 1980s, almost no change in serum total cholesterol has been observed. Between 1992 and 1995, the average serum total cholesterol level decreased by

MI in Holland in 1978–1994 0·3 mmol (5%), and HDL cholesterol increased by 0·1 mmol (7–9%). These cholesterol data apply to men and women in the age category 20–59 years[16] and are relevant for the reported decrease in the incidence of suspected myocardial infarction among men and women in the age category 45–64 years. On the other hand, the prevalence of hypertension increased by 1·2% in the period 1987–1995, possibly because of a rising prevalence of obesity and a decrease in the treatment of hypertensive cases[16]. So far as changes in risk factors are responsible for the decrease in the incidence of suspected myocardial infarction over the period 1978–1994, the decrease in the prevalence of smoking seems the most important factor. Based on diverging trends in risk factors in the early 1990s (improvements in lipid levels, unchanging cigarette smoking prevalence, decrease in hypertension treatment and increase in prevalence of obesity) it is hard to predict the future incidence of myocardial infarction in the Netherlands.

Incidence of confirmed myocardial infarction in the Netherlands in the period 1991–1994 In the period 1991–1994, the incidence of confirmed myocardial infarction was 1·7 per 1000 for men and 0·9 for women. For both men and women the incidence rate increases with age. For men there is a sharp increase in the incidence rate above the age of 45. In women this sharp increase occurs ten years later. This is also reported from the Framingham Study[17]. The number of confirmed myocardial infarction is twice as high in men as in women. Extrapolation of these rates to the national level gives an yearly estimate of 19 837 cases of confirmed myocardial infarction in the period 1991–1994. In 1993, 19 508 men and 9142 women were dismissed from hospital with myocardial infarction as a main diagnosis. The total hospital figure is 44% higher than estimated by the sentinel practices. There are several reasons for this discrepancy. Coding practices differ between sentinel practitioners and hospital registries. Sentinel practitioners may have occasionally failed to register confirmed infarctions. Re-admittance of cases to hospital, for instance because of treatment by PTCA or coronary bypass surgery, may have been recorded as myocardial infarction by the hospital registers. Also, an unknown but presumably small number of myocardial infarctions may take place during a stay in hospital and may therefore be missed by the sentinel practices. Finally, yearly 2300 persons are referred for all possible reasons from a nursing home to a hospital. An unknown percentage of these patients may be referred because of myocardial infarction and are missed by the sentinel practices. The incidence rate can be biased by inaccurate standardization of the diagnosis. The standardization of case definitions went as far as prescribing which

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diagnostic tests were acceptable, but did not stipulate the acceptable test results. Instead, common clinical diagnostic tests were used, leaving the outcome less quantified than in the MONICA project and leaving room for under-estimation as well as over-estimation[9]. Fifty-one percent of cases of suspected myocardial infarction (fatal and non-fatal) could be confirmed. The confirmation was, in most cases, based on ECG and/or enzyme tests. This is comparable with the findings in an older Dutch study; in 1978, van der Does reported that of the cases of suspected myocardial infarction diagnosed by a physician, 59% could be confirmed after afterwards[18]. Under-diagnosis of cardiac symptoms and myocardial infarction is more frequently reported among women than among men[17,19]. In our study, the confirmation percentage among non-fatal cases was 55% and was somewhat lower in women (51%) than in men (57%). This may imply that Dutch general practitioners use somewhat wider criteria for a suspected myocardial infarction among women than among men, which is not in accordance with the literature. A more likely explanation is that myocardial infarctions in women are less often preceded by familiar cardiac symptoms[20]. The incidence figures of confirmed myocardial infarction can also be biased by the way cases of sudden death are coded. In our calculations, cases of sudden death were only included as confirmed myocardial infarction if an autopsy confirmed the suspicion. Most sudden deaths occur at home. The Maastricht Sudden Death Registry noted that 40% of deaths were unwitnessed, indicating that even statistics based on outof-hospital resuscitation clearly under-estimate the size of the sudden death problem. Of the 53% having a cardiac history, two-thirds had suffered from a myocardial infarction in the past[21]. Of the unconfirmed cases in our study that died suddenly, 62% had a cardiac history. In general, sudden death is believed to be caused by ventricular fibrillation or non-cardiac causes[22–27]. Therefore it is assumed that few cases of confirmed myocardial infarction were missed. Although our data are not perfect, it can be concluded that the incidence of suspected myocardial infarction had declined substantially in the Netherlands. Whether this is also true for confirmed myocardial infarctions cannot be stated with certainty, but it seems to be a fair conclusion assuming a stable confirmation rate over time. Every source of registration has its own drawbacks and missed cases. To overcome this ideally a register should be set up that combines data from several sources, such as general practitioner, ambulance, hospital, nursing home and mortality register. Duplications should be avoided and the diagnostic criteria classified. Only in this manner will reliable population-based incidence data be obtained. We thank J. Fracheboud MD, Professor H. J. A. Collette and J. B. Reitsma, MD for their comments on an earlier version of this article. Eur Heart J, Vol. 19, March 1998

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