Myocardial Infarction (MI)

Myocardial infarction (MI)

Myocardial infarction (MI) is the irreversible heart injury occurs due to necrosis (cell death) of significant portion of myocardium (generally >1 cm). It is a medical emergency and usually termed as heart attack. If more than 50% of tissues are damaged, heart generally cannot work. If the damage is comparatively less, then inadequate pumping is observed resulting in heart failure and shock. In this article we will discuss about myocardial infarction (MI), its causes, types, symptoms, diagnosis, risk factors and treatment.

Table of Contents

Causes of myocardial infarction

Reduction in blood supply to the heart due to severe blockage in coronary artery is the most common cause of myocardial infarction. If the blood supply is stopped for few minutes, myocardial cells die because of lack of oxygen and nutrients. The extent of cell necrosis is determined by perfused area due to blocked coronary artery, extent of blockage and duration of blockage. Sometimes a blood clot is formed in the pre-existing blocked coronary artery. Rarely clot is formed in the heart. When this clot enters the coronary artery severe vasospasm occurs leading to MI.

Types of myocardial infarction

Myocardial infarction is classified into five types based on etiology and circumstances.

Type 1 spontaneous MI

This type is associated with plaque disruption such as, rupture, erosion, fissuring or dissection. There may also be intraplaque hemorrhage through disruption surface. It may be complicated by intraluminal thrombosis and distal emboli.

Type 2 ischemic related MI

This type is associated with myocardial ischemia. It occurs due to imbalance between supply and demand of oxygen to the heart muscles. Other causes include anemia with hemoglobin less than 5.0 mmol/L, respiratory failure, bradycardia, arrhythmia, coronary artery spasm or atherosclerosis.

Type 3 sudden unexpected cardiac death without cardiac biomarkers evidence

This type is associated with sudden, unexpected death before elevation of cardiac biomarkers.

Type 4 percutaneous coronary procedure related MI

This type is associated with periprocedural or later complications of stent or thrombosis. This type is further subdivided as type 4a and type 4b. Type 4a results from percutaneous coronary intervention (PCI), while type 4b results from coronary stent replacement. This MI may be diagnosed by angiography or by autopsy.

Type 5 cardiac surgery related MI

This type is associated with coronary artery bypass grafting (CABG). The CABG has comparatively higher mortality rate than percutaneous coronary intervention (PCI).

Symptoms of Myocardial infarction

Prodromal sign (indicating possible onset of attack): few symptoms like shortness of breath, fatigue, chest pain, exertional angina may be seen before a day or weeks of attack.

During attack: severe pain occurs in the middle of the chest, which further radiates to jaw, arm or back. This pain can not be relieved by rest or nitroglycerine. During attack some other symptoms are felt like anxiety, restlessness, sweating, oliguria, nausea, blue lips and hands. About 20% of patients do not experience any of the above-mentioned symptoms. This silent heart attacks can only be identified by ECG.

Risk factors of myocardial infarction

Some of the major risk factors of myocardial infarction are,

Hypertension
Diabetes mellitus
Anthropometric measures
High cholesterol levels
Inflammation
Alcohol intake
Family history

Treatment of myocardial infarction

MI is a medical emergency and hence prompt and proper treatment increases the chances of survival. Treatment for MI is of two types one is immediate treatment and another is subsequent treatment.

Immediate treatment

Anti platelate coagulants: Immediate administration of aspirin can reduce platelate aggregation and hence clot formation.
Beta blockers: Beta blockers reduces the cardiac work load and oxygen demand. E.g. atenolol, metoprolol, propranolol, etc.
Thrombolytic therapy: The clots formed can be dissolved by administering thrombolytic drugs like, streptokinase, urokinase, etc.
Supplement of anticoagulant: These prevents the further clot formation. E.g. heparin.
Analgesics: These are the drugs used to relieve pain. E.g. morphine
Surgical management: Angioplasty or coronary artery bypass graft (CABG)
Oxygen mask: Continuous oxygen supply through face mask.

Summary

Myocardial infarction is a serious medical condition where a portion of heart muscle is deprived of blood, leading to damage. This often results from blockage in the coronary arteries due to blood clot. Prompt and timely treatment is required to minimize the damage and improve outcome. Treatment of MI involves surgeries like angioplasty, medications and lifestyle changes.

Frequently asked questions

What is the main cause of myocardial infarction?

Coronary artery disease (CAD) is the main cause of myocardial infarction. Other causes may be severe spasm, sudden contraction of coronary arteries, etc.

How do you diagnose MI?

Myocardial infarction is diagnosed by ST segment elevation in ECG. Depending on the extent of MI this ST segment elevation is concave downward and frequently overwhelms the T wave.

What is the most common site of MI?

MI affects on the left ventricle (LV) of heart, but damage may extend to the right ventricle (RV) or the atria.

Which artery is blocked in MI?

In myocardial infarction, coronary arteries of the heart get damaged, causing infarction (tissue death) to the heart muscle.

Why is aspirin given for MI?

Aspirin is an anti platelate coagulant drug which prevents coagulation of platelate and hence blood clot is not formed.