What is a Heart Attack?
A heart attack represents irreversible myocardial necrosis (death of heart muscle). The muscle composing the walls of the heart is called the myocardium. An acute myocardial infarction is usually caused by the sudden interruption of blood flow to the heart that can be caused by a blockage of the coronary arteries (also known as heart disease). The most common cause of a heart attack is coronary artery disease.
How can a Blood Test Diagnose a Heart Attack?
Troponin is a protein that normally is not found in blood. Troponin is only released when myocardial necrosis (death of heart muscle) occurs. Positive troponin levels are considered virtually diagnostic of myocardial infarction (heart attack), as they are without equal in specificity and sensitivity in this diagnosis.
Up to 80% of patients with acute myocardial infarction will have an elevated Troponin level within 6 hours of the onset of the symptoms. Treatment should not be delayed to wait for Troponin results, especially since the sensitivity of Troponin is low in the first 6 hours after symptom onset. In addition to its use in the diagnosis of myocardial infarction, an elevated Troponin level can ideentify patients at high risk for adverse cardiac events.
Before the introduction of Troponin, the biochemical marker of choice for the diagnosis of acute myocardial infarction was the CK-MB isoenzyme. CK-MB first appears 4-6 hours after symptom onset, peaks at 24 hours, and returns to normal in 48-72 hours. The value of CK-MB in the early and late (more than 72 hours) diagnosis of acute myocardial infarction is limited.
For patients presenting more than 6 to 8 hours after the onset of most recent episode of pain, a single negative cardiac marker is often adequate to exclude acute myocadial infarction (but not unstable angina).
How can an EKG Diagnose a Heart Attack?
The identification of myocardial injury may be the most important clinical utility of an EKG. Dead heart tissue is electronically silent so forces from the opposite wall of the heart predominate. When an acute heart injury is present as in an acute myocardial infarction, there can be ST elevation. A deviation of the ST segment from the baseline is the most common use of EKG for the diagnosis of ischemia (lack of blood flow to the heart) or infarction.
On an EKG, ST elevation is an acute myocardial infarction. EKG looks for ST segment elevations in 2 or more contigous leads. ST elevation only occurs in full thickness myocardial infarction. One cause of ST segment elevation is the early repolarization of the heart wall due to transmural necrosis.
ST segment depression is often seen when myocardial ischemia exists. Ischemia is diminished blood flow to the heart.
How can a Cardiac Stress Test Diagnose a Heart Disease?
During a stress test, you are asked to run on a treadmill while a 12-lead EKG monitors the electrical rhythm of your heart. With physical exertion, such as running, your heart requires extra oxygenated blood. By placing added demands for oxygenated blood on your heart, your physician can determine how your heart responds. A stress test may elicit EKG abnormalities--such as an ST elevation or ST depression--that were not elicited while you were at rest.
A cardiac stress test shows how much your heart can manage before an abnormal rhythm begins or blood flow to the heart drops. A stress test can determine whether you have heart disease. A stress test is more sensitive in detecting heart disease than blood tests or an EKG.
Do you have Questions about Heart Attacks?
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