Discussion
We investigated the risk of myocardial infarction during national holidays, sports events, and various time periods using data on date and time of symptom onset, documented to the nearest minute in a large nationwide setting with 16 years of data on myocardial infarction. We observed a higher risk of myocardial infarction during Christmas/New Year and Midsummer holidays, but not at Easter. Sports events were not associated with higher risk of myocardial infarction. We observed a prominent peak incidence on calendar week 52, Mondays, and at 8 am. This is to our knowledge the largest study utilising electrocardiographic and biomarker positive myocardial infarction from a well known registry. These results are in line with previous studies using administrative data.
Major holidays
We observed the highest risk of myocardial infarction (37% higher than during the control period) on Christmas Eve. In addition, myocardial infarctions on Christmas Eve peaked at around 10 pm, rather than in the early morning hours. Previous meta-analyses have shown that acute experience of anger, anxiety, sadness, grief, and stress increases the risk of myocardial infarction and thus possibly explains the higher risk observed in our study.813 The association of higher risk at Christmas was more pronounced in people older than 75, those with known diabetes, and those with a history of coronary artery disease. These findings warrant further research to identify the mechanisms behind this phenomenon. Understanding what factors, activities, and emotions precede these myocardial infarctions and how they differ from myocardial infarctions experienced on other days could help develop a strategy to manage and reduce the number of these events. It is possible that family members visiting relatives after a long time apart find them in a poor general condition and decide to admit them to hospital. If this were the case, we would expect to see a decline in the number of myocardial infarctions in the weeks after Christmas compared with the weeks leading up to the holiday. Similarly, patients might delay reporting symptoms and seeking care because of unwillingness to disrupt the celebrations, and we would expect this to result in lower rates of myocardial infarction before Christmas than afterwards. However, the absence of any decline preceding or following Christmas indicates that these behavioural aspects are not the main contributing factors to the Christmas peak in myocardial infarction.
We observed a 20% higher risk of myocardial infarction on New Year’s Day. This could be due to the effects of excess alcohol and food consumption, exposure to cold temperatures at night, or sleep deprivation on New Year’s Eve. The associated risk of myocardial infarction during all holidays was similar between men and women, except for Midsummer, which was associated with a trend towards higher risk in men. It is possible that men are more likely to smoke, consume alcohol, and eat to excess during this holiday than women. Although no sex specific statistics are available to support this, statistics on the sale of alcohol from a government owned monopoly chain of retail stores shows sales peaking at Christmas and Midsummer.27
Time aspects
We confirm previous studies that use administrative data and circadian and circaseptan variation in myocardial infarction.462829 The incidence of myocardial infarction peaked on calendar week 52, on Mondays, and at around 8 am. The rate of STEMI had a normal, lightly skewed distribution, whereas NSTEMI ran a more fluctuating course throughout the day (fig 5). Mondays were associated with the highest risk of myocardial infarction and we observed differences between STEMI and NSTEMI. Risk of NSTEMI was higher on weekdays than weekends, but no other day of week was associated with higher or lower risk of STEMI. To rule out behavioural factors, such as delay in patients seeking care, we used symptom onset and not admission date. Patient delay in seeking medical attention may still confound the results, however, as symptom onset may be less defined in patients with NSTEMI. The decline in incidence of NSTEMI at weekends and at night is supportive of this. By contrast, STEMI usually presents with more pronounced symptoms and is usually treated with minimal delay. Previous studies have shown a higher risk of myocardial infarction in the working population.29 By contrast, we found the pattern to be similar in both retired (≥75 years) and younger patients (<75 years). Previously proposed explanations to the circaseptan peak in myocardial infarction include stressful Mondays and a rise in arterial blood pressure and heart rate.30 The circadian variation has been attributed to peak cortisol levels, increased blood viscosity, and platelet aggregability in addition to a rise in arterial blood pressure and heart rate in the morning hours.5
Sports events
Sports events were not associated with a higher risk of myocardial infarction, which was contrary to our expectations based on previous studies.1731 Wilbert-Lampen et al. presented an increased incidence in myocardial infarction in the Greater München area during the 2006 FIFA World Cup in Germany. Several aspects may contribute to the discrepancy in results. Germany was the host nation for the tournament, which might infer involvement by people who do not habitually follow football, resulting in a greater exposure to the sports event. In addition, the associated increased risk was restricted to days where the German football team was involved in the tournament, and highest during the days of the quarter and semi finals. The risk was neutralised on the day the German team played for third place. The only day associated with a higher risk and which did not involve the German team was the final, played between France and Italy. Together, these factors indicate that a strong emotional stress may be required to trigger myocardial infarction. We tried to address this in our sensitivity analysis of days where the Swedish team was playing. Our analysis did not show any associated risk. Moreover, no subgroup experienced a higher risk of myocardial infarction during any sport periods studied. Viewing sports events can therefore be considered safe.
Pathophysiological aspects
We were able to characterise myocardial infarction to a higher degree than in previous studies and this enabled us to study STEMI and NSTEMI independently, together with a wide range of subgroups. Our results showed a consistently higher risk of myocardial infarction mainly due to higher rates of NSTEMI and a greater number of patients who were elderly, had diabetes, a history of coronary artery disease, or were already taking other medication. This indicates that the “vulnerable patient,” who may have risk factors such as blood vulnerable to thrombosis and myocardium vulnerable to arrhythmias in addition to vulnerable plaques, may be more prone to these precipitators of disease.32
We cannot rule out the suggestion that activities and emotions associated with holidays may result in myocardial infarction secondary to ischaemia, due to an increased oxygen demand in older and sicker patients. This is supported by the subgroup analysis on the Christmas and New Year’s holiday (supplementary fig 2) that showed an incremental risk increase of myocardial infarction with each age quartile.33 Infarct type classification was not introduced in the SWEDEHEART registry until 2010, therefore, in order to explore this area we conducted a post hoc analysis of the risk of myocardial infarction resulting in a coronary angiography between 2004 and 2013. Neither Christmas nor Midsummer holiday were associated with risk of myocardial infarction that resulted in revascularisation; these holidays were rather associated with myocardial infarction in which percutaneous coronary intervention or surgery was not deemed necessary. This suggests that a large proportion of myocardial infarction with non-occlusive coronary arteries may account for the higher risk of myocardial infarction during these holidays. Although this post hoc analysis did not consider chronic total occlusions or distal occlusions that were left untreated by the interventionist, these findings warrant further investigation.
The rationale behind our subgroup analyses of patients using previous medications was to investigate the possible inhibitory mechanisms of certain pharmacotherapies on the short term triggers of myocardial infarction. Drugs lowering heart rate, reducing blood pressure, lipid levels, and platelet aggregation might reduce plaque vulnerability to external triggers. For example, β-adrenergic inhibitors have been shown to blunt the circadian variation in myocardial infarction by reducing heart rate and blood pressure and increasing coronary blood flow by prolonging diastole.34 However, in our study, patients using cardiovascular drugs had a similar or higher risk of myocardial infarction during holidays. Our explanation for this is therefore in line with our main theory—the medications are surrogate measures of a sicker population, more vulnerable to external triggers. However, in the subgroup analyses of circadian variation, the peak at 8 am was less prominent in patients on β -adrenergic inhibitors compared with patients without β -adrenergic inhibitors (supplementary fig 4), a finding in line with previous studies.
Limitations
This is an observational study; therefore, causality cannot be determined as we cannot rule out unobserved confounders. Previous publications have shown a higher incidence of myocardial infarction when ambient temperatures are low. However, it is unlikely that national holidays should be associated with lower temperature to bias the results since we used control periods two weeks before and after the holidays.
Conclusion
In this nationwide real world study covering 16 years of hospital admissions for myocardial infarction with symptom onset documented to the nearest minute, Christmas and Midsummer holidays were associated with a higher risk of myocardial infarction. Consistently, we observed a higher risk in older and sicker patients, suggesting a role of external triggers in vulnerable patients.
What is already known on this topic
In Western countries, cardiac mortality and hospital admission due to myocardial infarction has been observed to peak at the Christmas and New Year holiday
The risk of myocardial infarction has also been linked to football championships, hurricanes, and stock market crashes
It is therefore conjectured that factors associated with emotional stress, physical activity, and lifestyle changes may modulate the onset of myocardial infarction by acting as short term triggers
What this study adds
In this large study covering 16 years of clinical myocardial infarction data, a higher risk of myocardial infarction was observed during Christmas/New Year and Midsummer holidays but not during the Easter holiday
The highest risk was during Christmas Eve and in patients over 75, and those with previous diabetes and coronary artery disease
Sports events were not associated with higher risk of myocardial infarction