Coronary Artery Disease

The heart is a muscle, and like all muscles it needs a supply of blood to provide it with oxygen and nutrients. Even though large volumes of blood pass through the hearts chambers each time it beats, the blood passing through does not perfuse the myocardium. Instead, the myocardium gets its blood supply from the coronary arteries.

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The inner surface of the artery walls can become damaged. Damage can occur because of various reasons, such as chronic hypertension, high lipid levels in the blood (hyperlipidemia), or from smoking cigareaes. The damaged surface allows the formation of plaque. Plaque is composed of fats and other substances. When plaque forms on the inner surface of the artery wall, it is a condition known as atherosclerosis.

Atherosclerosis can occur in any of the arteries in the body. When atherosclerosis occurs in the arteries in the heart, the condition is called coronary artery disease (CAD). 

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Atherosclerosis causes the arteries to become narrowed, which reduces the amount of blood that is able to pass through. If blood flow is reduced to the myocardium, then oxygen supply is also reduced. This is called myocardial ischemia.

Myocardial Ischemia
The patient with myocardial ischemia typically feels pain in the chest. This pain is called angina pectoris, commonly referred to as angina.

Stable angina is when angina occurs during exertion only, such as during exercise or walking up a flight of stairs. During times of exertion, the myocardium has increased oxygen demands, and so symptoms of atherosclerosis might become apparent during these times. With stable angina, the angina eases with rest then disappears, as the oxygen demands of the myocardium decrease.

Unstable angina is when angina occurs even at times of rest. It also tends to occur more frequently and lasts for a longer duration. This is an indication of more advanced atherosclerosis. It indicates that there is significant narrowing in the coronary artery, resulting in intermiaent periods of inadequate blood supply to the myocardium.

Myocardial Infarction
When the myocardial cells are deprived of oxygen, eventually they die. This is called myocardial infarction (MI). 

The most common cause for this is when some of the surface of the plaque ruptures inside the coronary artery. In response, platelets then gather over the damaged surface. This leads to the formation of a thrombus. The thrombus causes further narrowing, and can even cause complete obstruction of the artery.

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Obstruction of the coronary artery prohibits blood flow through it, and therefore prohibits oxygen delivery to the myocardium that it serves. Without adequate oxygen the myocardial cells die. Myocardial infarction is categorized into two groups according to the presentation on the electrocardiogram (ECG).

  • Non-ST elevation myocardial infarction (NSTEMI) – this is where there has been death of myocardial cells, but there is no elevation of the ST segment on the ECG.
  • ST elevation myocardial infarction (STEMI) – this is where there has been
    death of myocardial cells, and there is elevation of the ST segment on the ECG. 

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Signs and Symptoms
Angina is usually experienced as central chest pain, oMen described as having a crushing, tight or heavy nature. It can sometimes radiate to the arm, or sometimes to the neck or jaw. The patient often has shortness of breath, nausea, and diaphoresis.

It is important to bear in mind that sometimes diabetics and the elderly may experience a myocardial infarction without chest pain, which is often referred to as a “silent myocardial infarction”.

Risk Factors
The risk factors for developing coronary artery disease include cigareae smoking, eating a diet high in fats, sedimentary lifestyle, obesity, untreated hypertension or untreated diabetes. These risk factors are modifiable. Unmodifiable risk factors include advanced age, and having a family history of coronary artery disease.

Investigations
The electrocardiogram (ECG) should be examined for ST elevation. Identification of ST elevation enables quick diagnosis of myocardial infarction. Furthermore, the location of the myocardial infarction can be determined by identifying the location of the ST elevation on the ECG. 

ST elevation in leads V1, V2, V3, V4, indicates an anterior MI; ST elevation in leads I, aVL, V5, V6 indicates a lateral MI; and ST elevation in leads II, III, aVF indicates an inferior MI.

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Treatment
Oxygen should be administered so that supply is increased to the myocardial cells.

Aspirin should be administered. Aspirin is an antiplatelet, which means it inhibits platelet aggregation. This prevents the thrombus from increasing in size, and so should be given as early as possible.

Analgesia should be given for the chest pain. An opiate (such as morphine) is advised as it also reduces the oxygen demand of the myocardium, as well as causing a small degree of vasodilation.

A nitrate (such as sublingual trinitrine) causes vasodilation, thereby increasing the blood supply through the coronary arteries to the myocardium. However, nitrates should not be given to patients who are hypotensive, as vasodilation will further lower the blood pressure.

A beta-blocker (such as atenolol) can be administered to reduce the heart rate and blood pressure, thereby reducing the oxygen demand of the myocardium. Beta- blockers however should not be given to the bradycardic or hypotensive patient. 

Long term daily medication might be required post MI, such as aspirin, beta-blockers, ACE inhibitors and statins. It is important that diseases such as hypertension and diabetes are well managed. Lifestyle modifications might also be required, such as cessation of smoking and eating a low fat diet.

Advanced Management
A blood test for cardiac enzymes (such as troponin level) can help diagnose myocardial infarction. This is particularly useful to diagnose a non-ST elevation myocardial infarction (NSTEMI).

Clopidogrel is another antiplatelet medication that can be administered. This can be administered concurrently with aspirin.

Anticoagulation therapy is beneficial, and can be given concurrently with antiplatelet therapy. Subcutaneous injections of enoxaparin can be administered. Alternatively an intravenous heparin infusion can be commenced, although the blood APTT level needs to be monitored when infusing intravenous heparin.

An intravenous nitrate infusion (such as nitroglycerin) can be administered, and titrated according to the patient’s symptoms and blood pressure.

A coronary angiogram can be performed to investigate for coronary artery disease. A coronary angioplasty can be performed as treatment for coronary artery disease. Less commonly, coronary artery bypass graft (CABG) can be performed as treatment for coronary artery disease.