Epidemiology of Myocardial Infarction

1.1. Coronary heart disease

The most common cause of coronary heart disease (CHD) is atherosclerosis of coronary arteries [2]. Atherosclerosis causes an inadequate supply of oxygen for a given a myocardial demand leading to myocardial hypoxia.

1.2. Atherosclerosis

The sequence of endothelial dysfunction, plaque formation consisting of lipids and smooth muscles, and associated inflammation causes atherosclerotic plaque [3]. Over these plaques rupture and thrombosis causing further narrowing of arteries and occlusion of blood flow can occur.

Although beyond the scope of this chapter, atherosclerosis implies disturbances in the coronary circulation as well as the microcirculation dysfunction.

1.3. Burden of coronary heart disease

Globally, cardiovascular diseases (CVDs) are the number one cause of mortality. According to the World Health Organization (WHO), it is estimated that 7.4 million deaths were due to coronary heart disease in 2015. Eighty-two percent of deaths in low- and middle-income countries are accountable for CVD. Figure 1 shows the age-standardized estimate of mortality by cardiovascular diseases and diabetes per 100,000 people. It is estimated that 23.6 million people will die from CVDs by 2030. These are projected to remain the leading cause of mortality.

1.4. Geographic variations in coronary heart disease

Worldwide the prevalence of CHD is increasing albeit there are regional variations due to the influence of economies, industrialization, and advancement in healthcare systems [5]. Data from the USA suggest about 25% of deaths in the USA are associated with heart disease each year [6]. An American dies due to myocardial infarction (MI) every 60 seconds [6]. The incidence of CHD in the western world is decreasing even though the risk factors for CHD such as hypertension, diabetes mellitus, and obesity are increasing. The decline is due to strengthening healthcare systems due to relative advancement in therapeutic and invasive interventions. As a result, CHD costs the USA about $200 billion each year [7]. The total cost includes not only the cost of healthcare services or medications, but it also includes the loss of productivity [7].

As Asia comprises over one-third of the world population, its experience on the prevalence of CHD is significant. In India, CHD may not be explained due to the traditional risk factors [8], whereas in China, CHD remains the second most cause of the deaths. Chinese cardiovascular medicine focusses centrally on prevention by shifting its focus from symptom-based therapy to lifestyle-guided improvement [9]. Trends in mortality from CHD were favorable in European Union countries, whereas in Eastern European countries, mortality from CHD remains exceptionally high [10]. CVD death rates have been significantly decreasing in most of the countries of Latin America despite the disparities in current trends [11].

The WHO has identified very cost-effective interventions that are feasible to be implemented even in low resource settings for averting the global epidemic of CVDs.

2.1. Defining myocardial infarction

The most common form of CHD is the myocardial infarction (MI) [12]. MI occurs when a coronary artery is occluded or almost occluded, which creates a severe reduction in the blood flow, causing some of the heart muscle being supplied by that artery to become infarcted [13]. There are two clinical settings of MI—ST-segment elevation myocardial infarction (STEMI) and non-ST-segment elevation myocardial infarction (NSTEMI). STEMI is recognized by characteristic changes on the electrocardiography (ECG) [14]. One of those ECG changes is the typical elevation in the “ST segment” which is called as STEMI. On the contrary the absence of ST-segment elevation and the presence of positive cardiac biomarker such as troponin are called NSTEMI [15].

In the following sections, prevalence and incidence of myocardial infarction are elaborated. Prevalence is defined as the number of diseased individuals present in the population at a specific time. The incidence rate is determined as the number of new cases of a disease that occur during a specific time. The estimates are reported from various data sources such as general practice registries and self-reported by the patient through the national survey. The reliability of the data is based on the data source.

2.2. Prevalence of myocardial infarction

According to 2014, based on the self-reported national survey of the UK, the prevalence of MI was reported as 640,000 in men and 275,000 in women; this represents about 915,000 people that have suffered an MI in the UK. In 2013, the prevalence of MI in men was about three times higher than for women in the UK [16]. As shown in Figure 2, the prevalence of age-specific MI extends from 0.06% of men <45 years of age to 2.46% of those ≥75 years old.

In contrast to these developed countries, South Asian countries (India, Pakistan, Sri Lanka, Bangladesh, and Nepal) have the highest prevalence of MI seen in younger than 45 years of age compared to those older than 60 years.

2.3. Incidence of myocardial infarction

Prevalence reflects first (acute) MI and MI in patients who had a previous MI. The incidence of MI only reflects the former. The incidence of MI has been declining in developed countries, including the USA and the UK.

The recent estimates of the incidence of MI in the USA are about 525,000 based on AHA data [18]. The Mozaffarian Study reported data comparing and contrasting the incidence of myocardial infarction in white men and women versus and black men and women. The study concludes that the incidence of myocardial infarction was more significant among black men (12.9/100,000 males) who are in the age group 75–84 years of age than whites for both men (9.1/100,000 males) and women (7.8/100,000 females). Similar trends exist in other age groups and their counterparts (Figure 3) [19]. It is essential to assess the effectiveness of public health strategies to fight MI.

Regarding the clinical type of MI, it has been estimated that incidence rates (per 100,000) of STEMI decreased appreciably (121 to 77), whereas those incidence rates of NSTEMI declined slightly (126 to 132) [20]. In a landmark study, no variation was seen in all-cause mortality for both STEMI and NSTEMI between 6 months and 4 years of follow-up. But, STEMI patients have a worse long-term prognosis matched to NSTEMI patients [21]. Other studies have shown a worse 7-year mortality rate for NSTEMI patients than STEMI patients [22].

2.4. Risk factors

The INTERHEART study evaluated the prevalence of nine potentially modifiable risk factors in more than 15,000 cases with the first acute MI and matched with about 15,000 asymptomatic age- and sex-matched controls [23]. Nine risk factors were strongly associated with acute MI in the 52 countries included in the trial. The modifiable risk factors represent over 90% of the risk for acute MI. Diabetes mellitus is a significant predictor of adverse cardiac outcomes, especially in women. It is considered to be a coronary heart disease equivalent (Tables 1–4) [24, 25].

2.5. Summary

It is evident that MI is the leading cause of morbidity and mortality worldwide. It is responsible for over 15% of mortality each year, among the vast majority of people suffering from NSTEMI than STEMI. The prevalence of MI is higher among men in all age-specific groups than women. Although the incidence of MI is decreased in the industrialized nations partly because of improved health systems and implementation of effective public health strategies, nevertheless the rates are surging in the developing countries such as South Asia, parts of Latin America, and Eastern Europe. The modifiable risk factors account for more than 90% of the risk for acute MI. Nine risk factors such as dyslipidemia, smoking, psychosocial stressors, diabetes mellitus, hypertension, obesity, alcohol consumption, physical inactivity, and a diet low in fruits and vegetables were strongly associated with acute MI in the 52 countries (Chart 1).