Diagnosis of Heart Attack
Because heart attack is a process that actually occurs over several hours, diagnosis must be made as quickly as possible to minimize permanent damage. Diagnosis involves physical examination, medical history (including history of symptoms related by the patient or a bystander), and diagnostic tests (e.g., blood tests, chest x-ray, electrocardiogram, echocardiogram).
Blood tests include the following:
- Complete blood count (CBC)
- Chemistry and electrolytes
- Clotting function (coagulation)
- Enzyme levels (elevated levels of certain enzymes are released into the bloodstream during heart attack)
Chest x-ray may be performed to detect abnormalities in the size and shape of the heart and to detect the accumulation of fluid in the lungs, which may indicate decreased blood flow.
An electrocardiogram (ECG or EKG) is a noninvasive test used to measure electrical activity in the heart. Electrical sensors (called leads) are attached to predetermined positions on the arms, legs, and chest to record electrical activity and help diagnose heart attack. ECG is not conclusive and the test is repeated in most cases.
Echocardiogram (cardiac echo) is an ultrasound examination of the heart that produces detailed images of the organ. It can be used to detect abnormalities in the structure of the heart and blood clots that may cause additional damage. Echocardiogram also is used to measure the strength of the heart (called ejection fraction), which is an important factor in determining treatment.
Normally, approximately 60 percent of the blood in the left ventricle is ejected each time the heart beats (contracts). Patients with ejection fractions of approximately 40–45 percent have mildly depressed ejection fractions, which may indicate a relatively minor heart attack; patients with ejection fractions of about 30–40 percent have moderately depressed ejection fractions, which may indicate moderate or severe heart attack; and patients with ejection fractions in the 10–25 percent range have severely depressed ejection fractions, which may indicate a massive heart attack, several mild heart attacks, or other heart conditions.
During an echocardiogram, a microphone-like device (transducer) is used to transmit and receive ultrasonic waves that travel through the chest wall to the heart and are reflected back to the transducer. These reflected sound waves are then translated into video images of the heart. The echocardiogram can produce reasonably good images of the heart.
Another method used to measure ejection fraction and damage to the left ventricle is to inject a small amount of radioactive dye into the bloodstream. A special camera is then used to determine how much of the dye is ejected from the heart with each beat.
Cardiac catheterization is used to produce images of the coronary arteries, enabling the physician to assess the amount of damage that has occurred. In this procedure, a catheter is inserted into the left ventricle. Iodine-based dye (contrast agent) is then injected through the catheter into the left ventricle, and images are taken of ventricle function. The infusion of dye typically produces a characteristic "hot flash" sensation throughout the body that lasts 10 to 15 seconds.