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Heart Attack (Myocardial Infarction)

Heart Attack (Myocardial Infarction) - Causes and Risk Factors


Atherosclerosis is the build-up of fatty deposits (plaque) in the inner lining of the artery, resulting in the narrowing and hardening of the coronary artery (see coronary artery disease). Although its exact mechanism is unknown, certain factors in a person increase the likelihood of this process. These include smoking, diabetes mellitus, high blood pressure, cholesterol and family history.

When the lumen of the artery is severely narrowed, the blood supply is unable to meet the demand of the heart muscle, especially when the person is exerting or exercising. The condition whereby the heart muscle is starved of essential nutrients is called myocardial ischaemia. When myocardial ischaemia occurs only upon exertion, it is called angina (also known as chest pain). Stable angina is not life-threatening and is usually promptly relieved by rest to reduce heart muscle demand or medications to dilate or open up the coronary artery to increase the blood supply.

Conversely, a heart attack occurs due to a sudden, complete blockage of the coronary artery, causing permanent damage to part of the heart muscle (also known as myocardial infarction). This is usually due to sudden breakage of the lining of a narrowing (plaque rupture) inside the artery. This causes clot formation at the site of rupture (a process known as atherothrombosis) and subsequent complete blockage of the artery. Myocardial ischaemia occurs at rest and if the artery is not opened promptly, heart muscle cells will die within minutes. If ischaemia persists for more than six hours, the majority of the muscles relying on that artery for their blood supply will be permanently damaged.

blocked coronary artery

Coronary arteries, which supply the heart with oxygenated blood, can slowly narrow due to a build-up of plaque leading to angina. In a heart attack, there is a sudden plaque rupture leading to blood clot formation, blocking the artery.


In spite of the treatment given, there is still a 10% mortality rate in patients with a heart attack. This is usually caused by abnormal heart rhythm (ventricular fibrillation) due to electrical instability of the heart or heart failure due to a massive heart attack. Occasionally, the heart muscle can rupture after a heart attack and this is usually fatal.

Risk factors

Modifiable Risk Factors

  • Smoking
    Smokers face two to three times the risk of non-smokers for sudden cardiac death. Almost 40% of patients below the age of 65 who die of heart disease are smokers. Smoking also leads to stroke, high blood pressure, blood vessel disease, cancer and lung disease.

    When a person smokes, the nicotine in the smoke speeds up his or her heart rate, raises the blood pressure and disturbs the flow of blood and air in the lungs. The carbon monoxide in the smoke lowers the amount of oxygen carried in the blood to the rest of the person’s body, including the heart and the brain. Tar and cancer-causing substances from the cigarettes are deposited in the airways and lungs. Smoking also causes a decrease in HDL-cholesterol (‘good’ cholesterol), increasing risks of heart disease.

  • High blood pressure
    High blood pressure (also known as hypertension) is one of the major risk factors for coronary heart disease and cerebrovascular diseases, such as stroke. Left untreated, hypertension can also cause heart failure or lead to the rupture of blood vessels in the brain.

    Hypertension usually occurs without any symptoms. Over time, it can lead to damage to the heart and blood vessels, resulting in a stroke or heart attack. When one’s blood pressure is extremely high, headaches, dizziness or alterations in vision might be experienced.

    Anyone above the age of 40 should check their blood pressure, at least once a year. Marginally elevated blood pressure may normalise when you lose weight, exercise more and reduce salt intake. If these measures are not successful, then medication may be necessary. However, once the course of medication has started, it is essential to continue with the treatment, while leading a healthy lifestyle. Treatment of hypertension is usually life-long.

    Learn more about managing high blood pressure.

  • High cholesterol
    There are two types of cholesterol. Low-density lipoproteins (LDL) or ’bad’ cholesterol will increase the build-up of fats in the arteries. On the other hand, high-density lipoproteins (HDL) or ’good’ cholesterol removes excess cholesterol from the cells, before they are deposited as plaque in the arteries.

    The goal is to keep your total cholesterol level as low as possible because any excess cholesterol in the blood may be deposited in the arteries. This build-up causes the arteries to harden and narrow, thus reducing or preventing blood flow to the heart.

    Many individuals with high blood cholesterol are not aware of their own cholesterol levels due to the lack of symptoms. It is important to check your cholesterol level regularly. If it is high, it should be lowered to reduce your susceptibility to coronary heart disease. The desirable level of cholesterol depends on your pre-existing risk for coronary heart disease.

    Learn more about managing cholesterol.

  • Diabetes
    Diabetes mellitus is a chronic illness. People with diabetes are two to four times more likely to develop coronary artery disease and stroke. It is often associated with other cardiovascular risk factors, such as high blood pressure, increased total cholesterol and triglyceride levels, decreased HDL (‘good’ cholesterol levels) and obesity.

    Maintaining a healthy weight, a balanced diet and having a regular exercise routine can help regulate one’s blood glucose level, preventing the onset of diabetes mellitus.

  • Obesity
    People who have excess body fat – especially located around the waist – are more prone to developing heart disease and stroke even if they have no other risk factors. Excess weight increases the strain on the heart. It also lowers HDL (‘good’ cholesterol levels), raises blood pressure, blood cholesterol and triglyceride levels. It is also associated with the development of diabetes mellitus.

    While family history does contribute to obesity, one’s environment and lifestyle play crucial roles in this condition as well. Body fat increases when you consume more food calories than you require over a long period of time. Physical inactivity and a high-fat diet also contribute to obesity.

    Weight control (fat loss) is possible by decreasing food intake and increased physical activity. Increased physical activity burns more calories which results in a decrease in body weight. Going on a diet can also reduce body weight, which leads to a decrease in blood pressure, blood glucose and blood cholesterol levels.

  • Lack of exercise
    An inactive lifestyle is a risk factor for coronary heart disease. Regular physical activity helps prevent heart and blood vessel disease, whereby exercise may lead to improvements in other cardiovascular risk factors, such as weight loss, lower blood pressure, decreased stress, and improved cholesterol levels. Moderate intensity activities help if done regularly and in the long run.

    Exercise programmes should start at a slow pace and build up gradually to avoid injuring your muscles and ligaments. People with known coronary artery disease or those above 40 years of age, who have been inactive, should seek medical advice before starting a regular exercise programme.

  • Stress
    Your blood pressure goes up momentarily when you get angry, excited, frightened or are under stress. If you are constantly stressed over a prolonged period, you may be at a higher risk of developing high blood pressure.

    Stress may cause palpitation, headaches, insomnia and problems with digestion. Prolonged stress may contribute to a heart attack. Emotional stress and tension also cause the body to produce adrenaline. This makes the heart pump faster and harder, which may cause the blood vessels to narrow.

Non-Modifiable Risk Factors

  • Age
    Age increases a person’s susceptibility to heart disease. For women, the effects of menopause, including the loss of the hormone oestrogen, appear to increase their risks of coronary heart disease and stroke.

  • Gender
    Men are three to five times more likely to have coronary heart disease than women. However, the risk for women increases after menopause. The risk of coronary heart disease in women increases to almost the same rate as for men approximately five to ten years after menopause.

  • Ethnicity
    The risk for coronary heart disease varies with different ethnic groups. A study done in Singapore shows that the likelihood of coronary heart disease is highest amongst South Asians. Compared with the Chinese, South Asians are three times, and Malays are two times more likely to suffer from coronary heart disease.

  • Hereditary
    You can be at a higher risk of having heart disease if your immediate family members (parents, children, brothers and sisters) have a history of premature heart disease. Certain risk factors (for example high blood pressure) tend to run in some families as well. If there is a history of heart disease in the family, one should place more emphasis on controlling the risk factors for heart disease.

  • Menopause
    Many women before the age of menopause seem to be partially protected from coronary heart disease, heart attack and stroke due to their high levels of oestrogen. However, women’s loss of natural oestrogen as they age may contribute to a higher risk of heart disease and stroke after menopause.

    After menopause, women have a greater disadvantage in their biochemical profile, which includes triglyceride or very low-density lipoprotein (VLDL) cholesterol level, and LDL (‘bad’ cholesterol). These changes make women more susceptible to developing coronary heart disease. Management of these risk factors becomes more important. If menopause is caused by surgery to remove the uterus and ovaries, the risk rises sharply.

    If menopause occurs naturally, the risk rises gradually. However, routine hormone replacement for women who have undergone natural menopause does not prevent heart disease.

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