Discussion
In this study, we observed a significant decline in the incidence of acute myocardial infarction during the period 2013–21. Despite the decrease in case fatality, acute myocardial infarction remains a serious condition. In 2021, nearly one in five patients died within 30 days. Many deaths occur in out-of-hospital settings. The falling incidence is due to fewer patients being treated in hospitals for STEMI and NSTEMI and a reduction in out-of-hospital deaths.
Previous studies from Norway (4, 13) and other countries (14) have reported a decreasing incidence of myocardial infarction. In some countries, this decline began as early as the 1980s (14). Our data suggest that the decline continued into the 2000s and applied to both sexes and all age groups. Data from the Swedish National Board of Health and Welfare for the period 2013–21 (15) show remarkable similarities between Swedish and Norwegian trends. In Sweden, the incidence fell by 3.2 % per year to 298/100 000 in 2021, while in Norway, the decrease was 3.8 % per year to 271/100 000 in the same year.
National data on the incidence and case fatality of STEMI and NSTEMI events have not been published previously. Our findings align with results from a study in Tromsø, which found that the incidence of myocardial infarction decreased by 3.0 % per year in the period 1995–2010 (13). In both studies, a marked decline was observed in the incidence of STEMI events and out-of-hospital deaths. This supports the hypothesis that a key driver for the reduced mortality from myocardial infarction in Norway is the reduction in the number of people who experience myocardial infarction, as opposed to a reduction in case fatality among patients admitted to hospital. Studies in other countries have reported similar findings. In the international MONICA study conducted in the 1980s and 1990s, it was found that approximately three-quarters of the decrease in mortality from myocardial infarction could be attributed to reduced incidence and one-quarter to reduced case fatality (14).
More favourable levels of the three classic risk factors – cholesterol, smoking and blood pressure – are likely the main reason for the drop in the number of myocardial infarctions. Studies have shown that these three risk factors account for most of the changes in incidence within a population (16, 17). In the Tromsø Study, it was found that 66 % (95 % CI 48 to 97) of the decline in ischemic heart disease incidence from 1995 to 2010 could be attributed to changes in modifiable risk factors such as cholesterol, blood pressure, smoking and inactivity (13).
Surveys of cardiovascular health in three Norwegian counties showed that the decline in cholesterol, blood pressure and smoking began in the early 1970s (18). More recent data suggests that this favourable trend continues (19). The reduction in cholesterol is primarily attributed to dietary changes (20, 21). Even after statins were introduced in the 1990s, these account for less than 30 % of the cholesterol reduction in the population (22). Lower risk factor levels may also have contributed to the decline in case fatality (23).
National data from Sweden shows that 28-day case fatality after all myocardial infarctions was 22 % in 2021 (15), while the 30-day case fatality in Norway was 17.5 % in 2021. A Danish study covering the period 2004–08 shows a slightly higher 30-day case fatality in hospital settings (14.8 %) (24) than what we found for 2012 (10.1 %). This suggests that case fatality after myocardial infarction may be somewhat lower in Norway than in other Scandinavian countries.
It is striking that during the period 2013–2021 there was no change in case fatality for STEMI, in contrast to NSTEMI. Previous studies from the United States (25) and Norway (13) also found no change in STEMI case fatality between 1995 and 2008, despite the introduction of new treatment methods during this period, such as percutaneous coronary intervention. In both studies, and in our study, markers of myocardial injury decreased in NSTEMI cases, but not in STEMI cases. This may indicate that an increasing proportion of NSTEMI patients had less extensive myocardial injuries, potentially leading to lower case fatality.
It is well-known that men have approximately twice the risk of myocardial infarction than women, and this increased risk is lifelong (26). However, studies on gender differences in case fatality have yielded inconsistent findings (27, 28). We found that case fatality was higher in women than men in STEMI cases. This has also been observed in other studies and may be due to longer delays between symptom onset and hospital admission (27) and/or fewer women receiving reperfusion therapy within the recommended timeframe (29).
A strength of our study is that we used information from all three national registries containing data on myocardial infarction. Using data from both the Norwegian Myocardial Infarction Registry and the Norwegian Patient Registry was found to have a positive predictive value of 97 % and a sensitivity of 92 %, measured against a reference standard (30).
The study also has several weaknesses. Concerns have been raised about the quality of data in the Norwegian Cause of Death Registry, particularly for ischemic heart disease (10, 31). The proportion of misdiagnoses is likely higher for out-of-hospital deaths than for patients admitted to hospital. However, we are not aware of any changes in coding practices during the study period that could have affected our incidence calculations. Coding errors have been identified in the Norwegian Patient Registry (32). A validation study nevertheless showed a high positive predictive value and completeness (sensitivity) for acute myocardial infarction (30). The five-year look-back period means that a number of myocardial infarction cases in our study (perhaps 5–7 %) are not first-time infarctions, but recurrent events (12). This will slightly reduce the observed decrease in incidence (12), but does not weaken our findings.