Myocardial Infarction
Topic Highlights
l Myocardial infarction (MI) is the irreversible necrosis of heart muscle secondary to prolonged ischemia.
l This usually results from an imbalance of oxygen supply and demand.
l This presentation discusses the factors that lead to atherosclerosis and its complications.
l This visual presentation focuses on myocardial infarction's risk factors, symptoms, diagnosis, first aid, and treatment.
The heart is a hollow muscular organ that pumps blood for circulation to all parts of the body. The coronary arteries branch off the aorta and are located on the surface of the heart. They deliver oxygen-rich blood to the heart muscle.
There are two major coronary arteries, the right and the left. The left splits into two arteries called the circumflex artery (supplying the left and posterior sides of the heart) and the left anterior descending artery (supplying the front of the heart.) The right supplies the posterior side of the heart.
Atherosclerosis occurs as a result of atherogenesis, or accumulation of fatty substances' cholesterol, cellular waste products, calcium and other substances, which may build up in the inner lining of the coronary artery resulting in plaque formation. Atherosclerosis produces an area in the vessel wall, which may act as a focus for atherosclerotic events. Cores with high lipid content tend to rupture, activating clot formation or thrombosis. Although blood flow may not be obstructed at rest or on exercise by a plain plaque, plaque rupture and thrombosis can block the artery at sites where stenosis has occurred.
Some of the risk factors for myocardial infarction are smoking, high blood pressure, a diet high in fat, elevated cholesterol levels, diabetes, heredity and age. Men are more susceptible than women. Women are more likely to experience atypical symptoms of MI compared to men. The commonly exhibited atypical symptoms by women include shoulder and neck pain, shortness of breath, nausea, vomiting, and abdominal pain. Many of the risk factors are linked to obesity.
Thrombosis forms when the surface of a plaque ruptures. When this occurs in the coronary artery it results in a massive decrease or complete obstruction of blood supply to that part of the heart. This leads to the heart muscle or myocardium being starved of oxygen.
Myocardial infarction is characterized by symptoms of tiredness and pressure in the center of the chest that may cause pain. This pain may radiate to the left shoulder and the left arm, though it can be felt in both shoulders, in both arms and in the neck, throat and jaw and occasionally in the upper abdomen as well.
Other symptoms include difficulty in breathing or shortness of breath, feeling sick or vomiting, feeling light-headed or fainting, breaking into a cold sweat or becoming pale. People with long-term diabetes may not feel the chest pain as much, because nerves carrying the pain sensation from the heart may have been damaged by diabetes.
Anyone experiencing these symptoms should be rushed to the hospital immediately. If the patient has pain and has adequate blood pressure, he needs to be administered nitrates or sublingual nitrates, as the case may be. It has been found that many patients do not reach the hospital during these crucial minutes. Reaching the hospital quickly and commencing treatment immediately to unblock the artery can reduce the damage to the heart due to heart attack.
It is normal to give the patient aspirin as soon as possible either before or after the ambulance arrives. If the patient is unresponsive and has stopped breathing, it is important to restore breathing and circulation until medical help arrives. If the patient has collapsed, cardiopulmonary resuscitation (CPR) or chest compression should be started immediately. The heart stops pumping blood when cardiac arrest occurs. CPR can support a small amount of blood flow to the heart and brain until normal heart function is restored.
It's advisable to take a quick medical history followed by checking the pulse, blood pressure, respiration and oxygen saturation. An immediate electrocardiogram (ECG) to record the electrical activity of the heart is performed to identify the areas of heart muscle that are deprived of oxygen. An ECG will determine whether thrombolysis, a clot bursting drug, will be given.
Coronary angiography is a procedure which helps to see the arteries of the heart. It requires a catheter, or wire, to be placed into the arteries in order to image or view the blood vessels. This can recognize thrombosis and confirm the diagnosis.
Blood tests are done to detect the presence of certain proteins called cardiac markers. These proteins are released in the blood circulation as a result of death of cardiac tissue called necrosis. Troponin T and I are two cardio specific markers. These are quite sensitive but their raised quantity is not always indicative of MI, as they can appear due to other causes such as heart failure. Two samples should be collected: one immediately after an attack and the other after 12 hours of the event. The presence of troponins confirms that a myocardial infarction or an attack has occurred. Other markers such as creatinine phosphokinase are measured in special measurement laboratories.
Based on an ECG, report treatment can be commenced. In case of a very serious arrhythmical and cardiac arrest, an electric shock is applied to the heart from the defibrillator. An intravenous line is secured for injecting drugs intravenously if the need arises. Oxygen is administered and aspirin is given immediately.
Aspirin helps to prevent adhesions among platelets and also helps in prevention of further clot formation. An analgesic such as morphine is administered to decrease pain.
Oxygen administration continues and the patient's vital parameters such as pulse, blood pressure, and ECG are continuously monitored. Nitroglycerin drug is administered sub-lingually. It helps by dilating the blood vessels, thereby reducing the workload of the heart. The treatment with these drugs should start as soon as possible.
Angioplasty and thrombolytic therapy are alternate treatments used in myocardial infarction. Thrombolytic treatment dissolves blood clots in blood vessels to improve blood flow. Here clot-bursting drugs are injected into a blood vessel, which flows through the bloodstream and dissolves the clot. In some cases a catheter may be used to deliver medications to the area of clot. Thrombolytic therapy is an alternative to avoid surgery.
Angioplasty is the choice of treatment for myocardial infarction if it's available in emergency care units. Angioplasty is a non-surgical procedure that uses a balloon tipped catheter to widen blocked arteries. Stents may also be combined with angioplasty. Stents are mesh-like tubes that can be expanded inside arteries to hold them open. This prevents an artery from narrowing again.
Cardiogenic shock occurs when damage to the heart muscle is so great that it can not pump adequately. This occurs when the extent of the myocardial infarction is so great that the pumping function of the heart is very severely disturbed and circulation cannot be maintained by the heart. It has a fatality rate in excess of 50%.
The chances of a myocardial infarction can be significantly reduced by leading a healthy lifestyle. Patients should control their blood pressure and cholesterol levels, give up smoking, eat a healthy diet low in fat and high in fruits and vegetables. Managing one's weight reduces the chance of diabetes, which is a significant factor in causing atherosclerosis in the coronary arteries. Regular exercise is important to improve heart fitness.