The main findings of this study are a pronounced decrease in the incidence and case fatality of acute myocardial infarction from 1978 to 1999 in both Denmark and Sweden. Sex influenced the incidence of acute myocardial infarction—this depended on age, such that the excess incidence among men became smaller with increasing age in both Denmark and Sweden. The impact of sex on case fatality also depended on age. Women younger than 55 years had a higher mortality than men of the same age, whereas women aged 55 or above had a comparable or lower risk than men. This interaction between age and sex for both incidence and case fatality did not change over time.
The incidence differed in Denmark and Sweden. Among the younger population, the incidence rate was slightly higher in Denmark than in Sweden. However, the incidence among the older population was notably higher in Sweden. The most likely explanation for this is the greater risk in the Danish population of dying from many competing diseases. Life expectancy in Denmark has been among the lowest in Europe for many years, especially for women. Life expectancy in Sweden is approximately three years longer than in Denmark for both men and women.
The case fatality rate declined clearly and significantly throughout the entire study period in both Denmark and Sweden. Case fatality was much higher in Denmark than in Sweden in the late 1980s and early 1990s. In the late 1990s, the excess mortality in Denmark was less striking and only clinically and statistically significant among patients aged 75–94 years.
There are several possible explanations for the difference in prognosis in the two countries. Differences in the treatment offered to patients and in the general health of the population may influence the probability of surviving an acute myocardial infarct. As mentioned previously, the life expectancy has been considerably higher in Sweden than in Denmark, but in this study we could not adjust for any differences in the severity of the infarct on an individual basis. However, significant differences in lifestyle have been demonstrated. Dietary habits are reported to be healthier in Sweden than in Denmark.10
Further, the WHO MONICA project11
showed differences in the levels of various risk factors. The proportion of daily smokers in Denmark was almost double that in Sweden, though the level of systolic blood pressure was higher in Sweden. The total risk score was similar in Denmark and Sweden. With regard to differences in treatment, Sweden has among the highest use of β blocker treatment in the whole of Europe for patients with acute myocardial infarction, whereas Denmark has been more restrictive in this regard.12
Furthermore, invasive treatment was very infrequent in the early 1990s in Denmark compared with Sweden (and the rest of the world).13
After the Danish Heart Plan was implemented in 1994, invasive treatment in Denmark rose to a level comparable with that of Sweden.13
However, the proportion of patients with acute myocardial infarction treated with primary percutaneous coronary intervention is still very low in both Denmark and Sweden, and this development cannot account for much of the decrease in case fatality.
The decrease in case fatality over the years may be attributable to factors such as improved care and treatment, changes in lifestyle that modify the risk once heart disease is established, and a decrease in severity of cases of acute myocardial infarction.14
This last factor can result from both profound changes in the presentation of the disease and changes in comorbidity, but changes over time in awareness of the disease, both among lay people and professionals, may also play a role.
Previous studies in USA5,15
have reported differences in the effect of sex on case fatality depending on age, as in our study. Ours is the first to show that the interaction between sex and age has been present and unchanged in the past 20 years, which may indicate that this difference is inherent in biological factors. Vaccarino and colleagues termed younger women a high risk group in view of their relative risk.5
However, the absolute crude risk of anyone younger than 55 years is less than half that of anyone older than 55 years, and this is true for both men and women.
The background for the difference in sex effect according to age is not well described. In Sweden, Rosengren and colleagues found a higher frequency of diabetes among women than among men who had had an acute myocardial infarct, and this difference in comorbidity accounted for most of the excess mortality in young women.7
The study from the USA5
found that, even after adjustment for several risk factors from the patient’s history and the current course and treatment of the infarct, female sex was still associated with a poor outcome. An important difference between the two studies is that the one in Sweden also included death from acute myocardial infarction before hospital admission, as in the present study. This takes into account the higher risk of sudden death among men than among women, as demonstrated by the Framingham heart study.16
Explanations other than comorbidity could be differences in treatment and awareness of the disease in men and women. Small differences have been found but cannot fully explain the differences in outcome.5,6
Women tend to present with less characteristic symptoms; they also present later and are treated less aggressively than men.17,18
The problem of potential differences in the accuracy of the diagnosis of acute myocardial infarction in men and women is difficult to account for. We included all patients dying from ischaemic heart disease and also all sudden deaths from unknown causes, but analyses in these subgroups of patients did not alter the presence and magnitude of the interaction between age and sex. Furthermore, the fact that analyses of case fatality after exclusion of patients dying on the day of their infarct showed similar results indicates the robustness of these data.
The incidence and case fatality of acute myocardial infarction decreased significantly in both Denmark and Sweden over the period studied. Case fatality was notably higher in Denmark in 1987 to 1991, but in the later periods the prognosis after acute myocardial infarction was comparable in the two countries. The relatively poor prognosis among younger women was similar in degree throughout the entire study period.