The main findings in this study of middle-aged men and women that had suffered a myocardial infarction was that more women than men had hyperlipidemia diagnosed at the cardiac event and that more females had experienced higher stress load including perceived stress, serious life events, strained economy, and depression the year preceding their MI. Females also estimated poorer sleep quality and shorter sleep duration than men. More females than men were unemployed or on sick leave. Blood pressure and heart rate differed significantly between gender at discharge from the hospital. Most of both genders experienced chest pain as a prodromal symptom at the onset of MI. We hypothesized that more men than women had established risk factors at the time of their MI. But in contrast, significant differences between gender were only observed for hyperlipidemia that were on the contrary significantly more frequent diagnosed among females.
In our study 35.4% of women and 18.1% of men had diagnosed hyperlipidemia before the MI (p=0.015). However, there was no statistical significance between the sexes in lipid values. Participants with hyperlipidemia could have both been diagnosed and medicated, which subsequently affects the level of the lipid values and thereby gender differences. As the mean age for women in our study was 57±7 years, this indicates that the majority were postmenopausal. Women have a more favorable lipid profile before the menopausal transition, after which a shift takes place and men have a better lipid profile in older ages [15]. This explanatory model of a lipid shift in connection with the menopausal transition is a reasonable partial explanation of why more women than men had hyperlipidemia in our study. However, it is with caution that an explanatory model for lipid levels between sexes in a normal population translates to our study participants that all had an MI.
In “the VIRGO study”, which has a similar methodology in terms of study design, there was an opposite relationship between gender, where 72.2% of men and 66.4% of women had hyperlipidemia [2]. In the VIRGO study the average age of women was 47 years [2]. Thus, it can be assumed that the reverse relationship depends on that a much smaller proportion had undergone the menopausal transition. The difference could also be explained by the higher prevalence of obesity in females in the VIRGO study compared to our study (55.3% vs 26.8%). In the VIRGO study [2] the percentage points for hyperlipidemia in both sexes were markedly higher than in our study. This is to be expected as hyperlipidemia is a major public health problem in the US [21], and because of lifestyle changes in Sweden that entailed reduced cholesterol levels [22, 23].
Remarkably 59.6% of the females in our population with MI-cases were current smokers and 92% former smoker. This reflects that smoking has a major impact on women's cardiovascular risk, which previously been confirmed in a study that stated that the impact of smoking on the risk of CVD is greater in females than in males [15]. This study is only based on patients with MI and no healthy control group was used in the analyses. Thus, there is no statistical basis for further reflecting on the impact of smoking on the risk of MI, for each gender. In this study we did not demonstrate any significant gender differences in smoking, but the statistical analysis shows a tendency that more middle-aged women than men with MI smoke or have smoked. This suggests that smoking might possibly have a greater impact on women than men in these age-groups.
Our results with significant gender differences for strained economy, sadness/ depression, and serious life events, shows that these factors appear to be stronger potential risk cardiovascular factors for women than for men. Previous studies have shown consistent results, for example, that more women than men have experienced stress the year before MI, such as depression [2, 9] and sleep disorders [9]. Our study demonstrated that women had poorer sleep quality and a shorter subjective sleep duration than men. Whether this is a consequence of stress, or an independent factor is not possible to assess. Previous studies have shown that a higher proportion of those with disturbed sleep has an adverse cardiac risk profile [24]. This suggests that the gender differences in sleep, shown in our study, have some association with the risk of MI. In our study population, females had a higher stress load than men. This differs from a previous published result, that did not show any significant difference for stress level between gender [9]. Stress was not stated likewise in the studies, which may affect the difference in results as our study used a visual analog scale and the compared study [9] used several response alternatives.
The demonstrated statistical significance between the sexes in SBP at hospital admission, and for SBP, DBP, and HR at discharge is not of clinical relevance due to small differences. Based on risk prediction thresholds it can be observed that the measured value (described as mean or median) for triglycerides was within acceptable range, but LDL was higher than clinically desirable for women, and HDL-cholesterol lower than clinically desirable. For men, the median value for HDL and triglycerides, as well as the mean value for LDL was within an acceptable range.
The strengths in this study is that all participants had a validated diagnose of myocardial infarction and that other clinical data was merged with data from the National Swedeheart Register. This has resulted in an opportunity to map factors, at an individual level, that were present before the onset of MI and clinical measurement values present at the time of the disease. We reduced the possible risk of recall bias by merging the variables that were comparable between the Stressheart database and the National Swedeheart Register, which were diabetes, hypertension, and smoking (current and current or former). As we chose to focus on gender differences in a middle-aged population with MI, the results can only be considered as a mapping of possible risk factors, whose significance for the risk of MI is unproven. The basic design in this study, where we compare men and women that all had suffered a myocardial infarction, is that we primary compare their social and general conditions before the onset of MI. This design means that we don´t have included a healthy control group, which from some aspects could be a limitation.