Acute Myocardial Infarction

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Definition Aetiology Pathophysiology Risk factor Sign and Symptoms Investigations Diagnosis Management

Myocardial infarction (MI), also known as a heart attack, occurs when the blood supply to a part of the heart muscle is blocked, usually by a build-up of plaque in the coronary arteries. This can damage or kill the heart muscle tissue. Symptoms include chest pain, shortness of breath, and fatigue.

The most common cause of myocardial infarction is the buildup of plaque in the coronary arteries, a condition known as atherosclerosis. This plaque can narrow or block the arteries, reducing or stopping the flow of blood to the heart muscle. In rare cases, myocardial infarction can be caused by a clot or embolism that forms in another part of the body and travels to the coronary artery.

The most common cause of this is a clot (thrombus) that forms in the coronary artery, obstructing blood flow and depriving the heart muscle of oxygen and nutrients.

When the blood flow to the heart muscle is blocked, the heart muscle cells begin to die. The longer the blood flow is blocked, the more heart muscle is damaged. As the heart muscle dies, enzymes and other chemicals are released into the bloodstream, which can be detected with blood tests.

The area of the heart muscle that is damaged or killed during a myocardial infarction is known as the infarct. The infarcted muscle is replaced by scar tissue, which does not contract and cannot pump blood. This can lead to heart failure and other complications.

The pathophysiology of myocardial infarction can also cause electrical instability in the heart, leading to arrhythmias.

Some of the most common risk factors include:

• High blood pressure: hypertension increases the workload on the heart and can damage the blood vessels over time.

• High cholesterol: high levels of LDL cholesterol in the blood can contribute to the buildup of plaque in the coronary arteries.

• Smoking: smoking damages the blood vessels and increases the risk of blood clots.

• Diabetes: people with diabetes have an increased risk of developing heart disease.

• Family history: a family history of heart disease increases the risk of myocardial infarction.

• Age: the risk of myocardial infarction increases as people get older.

• Obesity: being overweight or obese can increase the risk of myocardial infarction.

• Sedentary lifestyle: lack of physical activity can contribute to the development of heart disease.

• Stress: chronic stress can increase the risk of myocardial infarction by raising blood pressure and heart rate.

The most common symptom of a myocardial infarction is chest pain or discomfort. This pain can feel like a squeezing, fullness, or pressure in the chest and can spread to the arms, shoulders, jaw, neck, or back. Other symptoms of a myocardial infarction can include:

• Shortness of breath: this can be a sign that the heart is not pumping enough blood.

• Nausea and vomiting: some people may feel sick to their stomach or even vomit.

• Sweating: a cold sweat can be a symptom of a myocardial infarction.

• Fatigue: a person may feel weak or very tired.

• Dizziness or lightheadedness: this can be caused by a lack of blood flow to the brain.

• Rapid or irregular heartbeat: this can be caused by the electrical instability in the heart caused by the myocardial infarction.

There are several investigations that can be used to diagnose a myocardial infarction. Some common tests include:

• Electrocardiogram (ECG): This test records the electrical activity of the heart and can show if an area of the heart muscle has been damaged or is not getting enough blood flow.

• Blood tests/ biomarkers such as troponin can be used to confirm a myocardial infarction. They are very sensitive markers of myocardial injury and their levels will rise within the first 6 hours of a heart attack and remain elevated for up to 10 days.

• Coronary angiography: This is an invasive test that uses X-ray and a special dye to create pictures of the coronary arteries. It can show if there is a blockage in one or more of the coronary arteries.

• Cardiac MRI (CMR) or CT scan: These imaging test can provide detailed images of the heart and blood vessels and can show if there is any damage to the heart muscle.

• Echo-cardiogram: This test uses sound waves to create images of the heart and its chambers, valves, and walls. It can help show if the heart is enlarged, if there is any fluid around the heart, or if the heart's pumping action is abnormal.

If an acute MI is suspected, the individual should be admitted to the hospital immediately. An ECG can be performed if an MI is suspected. 

There are two main types of myocardial infarctions:

• ST-segment elevation myocardial infarction (STEMI) (see figure 1)

• Non-ST-segment elevation myocardial infarction (NSTEMI) (see figure 2)

 Treatment for myocardial infarction (heart attack) will vary depending on the severity of the attack and the individual's condition, but some common treatments include:

• Medications: such as Aspirin, which is often given immediately to help prevent blood clots from forming. Other medications include thrombolytics, which can dissolve clots, antiplatelets, anticoagulants, beta-blockers, ACE inhibitors, and statins to help manage symptoms, reduce the risk of further heart attacks, and prevent heart failure.

• Coronary angioplasty and stenting: This is a procedure where a balloon catheter is inserted into the blocked coronary artery, the balloon is then inflated to open the blocked artery and a stent, a small metal mesh tube, is inserted to keep the artery open.

• Coronary artery bypass surgery: This is a procedure where a healthy blood vessel is taken from another part of the body and used to bypass the blocked coronary artery, improving blood flow to the heart muscle.

• Rehabilitation: After a heart attack, a cardiac rehabilitation program can help to improve cardiovascular fitness and reduce the risk of future heart attacks.

• Lifestyle changes: Such as quitting smoking, eating a healthy diet, regular physical activity, and managing stress can help to reduce the risk of future heart attacks and improve overall health.