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Heart Attack (Myocardial Infarction)

Heart Attack (Myocardial Infarction) - Causes and Risk Factors


Atherosclerosis is the process that causes gradual buildup in the inner lining of the artery and therefore narrowing of the coronary artery. 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 strong family history.

When the narrowing inside the lumen of the artery is severe, the amount of 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 ischaemic occurs only upon exertion, it is called angina (i.e. chest pain). Stable angina is not life threatening and is usually promptly relieved by rest to reduce heart muscle demand or medications to dilate the coronary artery to increase the blood supply.

Conversely, heart attack is due to sudden, complete blockage of the coronary artery causing permanent damage to part of the heart muscle (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 6 hours, majority of the muscles supplied by that artery 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.

When a heart attack occurs, the patient will usually feel severe chest discomfort.

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

Risk factors

Modifiable Risk Factors

  • Smoking
    Smokers have 2 to 3 times the risk of non-smokers for sudden cardiac death. Almost 40% of patients younger than 65 years 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 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. The tar and cancer-causing substances are deposited in his airways and lungs. Smoking also causes a decrease in HDL-cholesterol.

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

    Hypertension usually occurs without any symptoms. Over time, it can lead to damage of the heart and blood vessels, leading to stroke or heart attack. Occasionally, when the blood pressure is extremely high, headaches, dizziness or alterations in vision can be experienced.

    You should check your 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 drug treatment may be needed. However, once medicine has started, it is essential to continue with the treatment, complemented by a healthy lifestyle. Treatment of hypertension for most people is life-long.

  • High cholesterol
    Low-density lipoproteins (LDL) or “bad” cholesterol will increase the build-up of fats in the arteries. The other is high-density lipoproteins (HDL) or “good” cholesterol that removes 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. Any excess cholesterol in the blood may be deposited in the arteries. This build-up causes hardening of the arteries, such that they become narrowed and blood flow to the heart is reduced or blocked.

    High blood cholesterol itself does not cause symptoms. As such, many people are not aware that their cholesterol level is high. It is important to check your cholesterol levels regularly. If the level is high, it can 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.

  • Diabetes
    Diabetes mellitus is a chronic illness. People with diabetes are 2 to 4 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 - cholesterol levels and obesity.

    The basic treatment strategy is to maintain good control over the amount of glucose in your blood. While maintaining a healthy weight, a balanced diet and a regular exercise routine can prevent 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 raises blood pressure, blood cholesterol and triglyceride levels, and lowers HDL, (“good” cholesterol levels). It is also associated with the development of diabetes mellitus.

    Family history and environment both play a part in determining obesity. 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. Similarly, weight control (fat loss) is possible by decreasing food intake together with increasing physical activity. If you use more calories because of increased physical activity, a gradual decrease in body weight will take place. Diet alone can cause weight loss, 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. Moderate intensity activities help if done regularly and over a period of time. Regular exercise may also lead to improvement in other cardiovascular risk factors, such as weight loss, lower blood pressure, decreased stress and improved cholesterol level. Bearing this in mind, exercise is indeed very beneficial especially since the risks involved are minimal. Exercise programmes should start at a slow pace initially to avoid injury to your muscle 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 when you 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 digestive symptoms. Prolonged stress may contribute to a heart attack. Emotional stress and tension also cause the body to produce adrenaline, which makes the heart pump faster and harder, and may also 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 hormone oestrogen, appear to increase their risks of coronary heart disease and stroke.

  • Gender
    Men are 3 to 5 times more likely to have coronary heart disease than women. However, the risk for women increases after menopause. By about 5 to 10 years following menopause, the risk for coronary heart disease for women increases to almost the same rate as men.

  • Ethnicity
    Risk for coronary heart disease varies with different ethnic groups. A study done in Singapore shows the likelihood for coronary heart disease is highest amongst South Asians. Compared with the Chinese, South Asians are 3 times, and Malays are 2 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 tend to run in some families. If there is a history of heart disease in the family, you should try very hard to control your other risk factors too.

  • Menopause
    Many women before the age of menopause seem to be partly protected from coronary heart disease, heart attack and stroke. A woman’s oestrogen level is highest during her childbearing years and declines during menopause. 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 more adverse biochemical profile, which includes triglyceride or very low-density lipoprotein (VLDL) cholesterol level, and LDL or “bad” cholesterol. These changes make the 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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