Myocardial infarction

Myocardial infarction or acute myocardial infarction (AMI) is the medical term for an event commonly known as a heart attack. An MI occurs when blood stops flowing properly to a part of the heart, and the heart muscle is injured because it is not receiving enough oxygen. Usually this is because one of the coronary arteries that supplies blood to the heart develops a blockage due to an unstable build-up of white blood cells, cholesterol and fat. The event is called "acute" if it is sudden and serious.

A person having an acute MI usually has sudden chest pain that is felt behind the sternum and sometimes travels to the left arm or the left side of the neck. Additionally, the person may have shortness of breath, sweating, nausea, vomiting, abnormal heartbeats, and anxiety.In many cases, in some estimates as high as 64%, the person does not have chest pain or other symptoms. These are called "silent" myocardial infarctions.

The main way to determine if a person has had a myocardial infarction are electrocardiograms (ECGs) that trace the electrical signals in the heart and testing the blood for substances associated with damage to the heart muscle. Common blood tests are troponin and creatinekinase (CK-MB). ECG testing is used to differentiate between two types of myocardial infarctions based on the shape of the tracing. An ST section of the tracing higher than the baseline is called an ST elevation MI (STEMI) which usually requires more aggressive treatment.

Classification

Myocardial infarctions are generally classified into ST elevation MI (STEMI) and non-ST elevation MI (NSTEMI).

MI is classified into five main types:

  • Type 1 – spontaneous MI related to ischemia due to a primary coronary event such as plaque erosion and/or rupture, fissuring, or dissection.
  • Type 2 – MI secondary to ischemia due to either increased oxygen demand or decreased supply, e.g. coronary artery spasm, coronary embolism, anaemia, arrhythmias, hypertension or hypotension
  • Type 3 – sudden unexpected cardiac death, including cardiac arrest, often with symptoms suggestive of myocardial ischemia, accompanied by new ST elevation, or new left bundle branch block (LBBB), or evidence of fresh thrombus in a coronary artery by angiography and/or at autopsy, but death occurring before blood samples could be obtained, or at a time before the appearance of cardiac biomarkers in the blood
  • Type 4 – associated with coronary angioplasty or stents:

Type 4a – MI associated with Percutaneous coronary intervention (PCI)

Type 4b – MI associated with stent thrombosis as documented by angiography or at autopsy

  • Type 5 – MI associated with bypass surgery.

Pathophysiology

The most common triggering event is the disruption of an atherosclerotic plaque in an epicardial coronary artery, sometimes resulting in total occlusion of the artery. Atherosclerosis is the gradual build-up of cholesterol and fibrous tissue in plaques in the wall of arteries. Plaques can become unstable, rupture, and additionally promote the formation of a blood clot that occludes the artery; this can occur in minutes. When a severe enough plaque rupture occurs in the coronary vasculature, it leads to MI. If impaired blood flow to the heart lasts long enough, it triggers a process called the ischemic cascade; the heart cells in the territory of the occluded coronary artery die and does not grow back. A collagen scar forms in their place.As a result, the patient's heart will be permanently damaged and puts the patient at risk for potentially life-threatening arrhythmias. Injured heart tissue conducts electrical impulses more slowly or faster than normal heart tissue.

Calcium deposition is another part of atherosclerotic plaque formation.

Hyperhomocysteinemia (high blood levels of the amino acid homocysteine) in homocysteinuria is associated with premature atherosclerosis; whether elevated homocysteine in the normal range is causal is controversial.

Signs and symptoms

Patients with typical myocardial infarction may have the following symptoms:

  • Fatigue
  • Chest discomfort
  • Malaise

Typical chest pain in acute myocardial infarction has the following characteristics:

  • Intense and constant for 30-60 minutes
  • Retrosternal and often radiates up to the neck, shoulder, and jaw and down to the ulnar aspect of the left arm
  • Usually described as a substernal pressure sensation that also may be characterized as squeezing, aching, burning, or even sharp
  • In some patients, the symptom is epigastric, with a feeling of indigestion or of fullness and gas

The patient’s vital signs may demonstrate the following in myocardial infarction:

  • The patient’s heart rate is often increased
  • The pulse may be irregular
  • In general, the patient's blood pressure is initially elevated
  • However, with right ventricular myocardial infarction or severe left ventricular dysfunction, hypotension is seen
  • The respiratory rate may be increased in response to pulmonary congestion or anxiety
  • Coughing, wheezing, and the production of frothy sputum may occur
  • Fever is usually present within 24-48 hours, body temperature may occasionally exceed 102°F