Diagnosing a Myocardial Infarction – Is it a heart attack?

A myocardial infarction (MI) or heart attack occurs when there is a blockage in one of the arteries supplying the heart (coronary arteries). This may lead to death or permanent damage to the heart muscles (myocardium). A heart attack is a very common medical emergency. Recognition of the symptoms early, prompt emergency care, and appropriate medical treatment can save a life as well as limit myocardial damage.

However, the presentation of an MI is sometimes confused with other common conditions that mimic some of the key features of a heart attack. Therefore it is important to differentiate between a heart attack and other, often non-cardiac, disorders. Nevertheless, those who are at a high risk of a heart attack need to be cautious and immediately seek treatment. This includes any person with known coronary artery disease, over the age of 45 years, overweight or obese, cigarette smoking, with a history of high blood lipids (hyperlipidemia) and/or hypertension and other underlying conditions like diabetes mellitus.

Heart Attack vs Gas or Acid Reflux

It is not uncommon for certain gastrointestinal conditions, like excessive gas, gastritis or acid reflux, to be mistaken for cardiac pain. Although there are a few key similarities, overall the clinical presentation is significantly different. However, ignorance about the signs and symptoms of a heart attack often leads to this confusion.

The pain associated with angina or a heart attack is known as ischemic cardiac pain, meaning the the heart muscle is undergoing damage due to an interrupted blood supply. It can vary in presentation from choking, constricting, dull, aching or burning. The latter, burning pain, is similar to pain associated with acid reflux, although reflux can also present with other types of pain. Due to the anatomical position, acid reflux may closely mimic the location of cardiac pain. The similarities and differences are discussed further under cardiac vs non-cardiac chest pain. Patients with a history of angina pectoris should also be aware of the differences between angina and heart attack pain.

The presence of other features like dizziness, breathlessness, excessive sweating and fainting are indicative of cardiac pain, while nausea, vomiting and even anxiety may be seen with both cardiac and gastrointestinal pain.

Diagnosis of Myocardial Infarction

According to the WHO criteria, the diagnosis of a heart attack or myocardial infarction requires at least 2 out of 3 of the following :

• Typical history
• ECG changes
• Cardiac enzyme elevation

Typical History

Typical symptoms may not always be present ad as discussed above a heart attack could be mistaken for a less-serious condition such as GERD or gastritis and not given due importance. This delay may prove fatal in some cases.

The cardinal symptom of a heart attack is pain. Chest pain is typically felt behind the breast bone or sternum (retrosternal pain) but it may radiate to the arms, shoulder, jaw or neck. The pain is in most cases extremely severe and persistent and may be felt as a heaviness, tightness, or constriction in the chest. There may be other signs and symptoms such as breathlessness, anxiety, sweating, nausea, vomiting, extreme pallor, dizziness, and a rapid pulse.

Physical examination may show pallor, sweating, and a rapid pulse (tachycardia). Heart murmurs or abnormal heart sounds may be heard on auscultation (through the stethoscope). Crepitations or abnormal sounds in the lungs may be heard due to pulmonary edema, or a pericardial rub due to pericarditis.

Electrocardiography (ECG) Changes

Although the typical changes may take a few hours to develop, an initial ECG is usually helpful in diagnosing a heart attack. An ECG is of particular importance in identifying the type of myocardial infarction, which will decide the subsequent line of management.

The earliest ECG change is usually ST elevation, which indicates acute myocardial injury. Patients with ST-segment elevation myocardial infarction (STEMI) need immediate reperfusion. The other types of MI based on ECG changes may be non-ST-segment elevation myocardial infarction (NSTEMI), and patients with chest pain but no ECG changes. Abnormal Q waves may appear.

Cardiac Enzyme Rise

Due to death of the myocardial tissue in MI, there is likely to be a rise in certain enzymes that are normally present within the heart muscle cells.

• Creatine kinase (CK). This enzyme starts to rise at 4 to 6 hours, peaks at 12 hours, and returns to normal in 48 to 72 hours. However, CK is also present in skeletal muscles and may be raised in other situations such as after an injury or after prolonged exercise. CK-MB is more specific for heart muscle damage.
• Asparate aminotransferase (AST). This enzyme starts to rise after 12 hours and peaks on the first or second day.
• Lactate dehydrogenase (LD). This enzyme rises after 12 hours, peaks after 2 or 3 days, and may remain elevated for about a week. Although it is not specific for heart muscle cells since it may also be released from disintegrating red blood cells, it is useful when diagnosis is uncertain several days after a possible infarct.
• Troponin I and T. Troponin T is most specific for myocardial damage. Its level increase within 3 to 12 hours, peaks at 12 to 24 hours and may remain elevated for more than a week. If troponin T remains normal after 6 hours of onset of pain and ECG is normal, myocardial infarction is highly unlikely.

Other Tests

• Chest x-ray to look for enlarged heart size (cardiomegaly), pulmonary edema, or signs of aortic dissection.
• Blood tests like a complete blood count (CBC), glucose, and lipids.
• Radionuclide scanning. A pyrophosphate scan may show the site of myocardial damage. Nuclear ventriculography may indicate the extent of ventricular function impairment.
• Echocardiography. Although not a routine test for diagnosis of MI, it may help to indicate the location and extent of myocardial damage. The heart valves, chambers, and heart wall movement may be seen by echocardiography. It is more useful in evaluating a patient’s progress after a MI.
• Coronary angiography. A dye is passed through a catheter inserted in an artery in the groin and leading up to the heart. X-rays are taken to see the movement of the dye through the artery. Coronary angiography can indicate the site and extent of block in the coronary arteries.