What Tests Are Used To Diagnose And Further Evaluate A Heart Attack?
29/03/2012 - 21:31
A variety of diagnostic tests can be performed to:
Confirm that a heart attack has occurred
Provide additional information that can guide treatment, and/or
Determine the long-term outcome or prognosis in someone who has suffered a heart attack
Some tests, such as sequential electrocardiograms or blood tests, are performed on all patients suspected of having suffered an acute heart attack. Others, such as coronary angiography, are performed only on selected patients.
Tests performed on an individual with a suspected or confirmed heart attack include:
Routine or standard 12-lead electrocardiography is the most commonly performed test in cardiology. It is also the most important diagnostic test in someone with a suspected recent heart attack. This painless test produces an electrocardiogram (ECG), which is a record of the heart's electrical activity.
In this test, surface electrodes attached to wires (leads) are applied to the skin of the chest and limbs. These leads send the heart's electrical signals to the electrocardiograph machine, which records this information on paper. The ECG shows a series of waves (P waves, QRS complex, T waves) that represent the electrical events of heart chambers and conduction pathways.
Among other things, a standard 12-lead ECG can demonstrate disturbances in the heart rate or rhythm as well as evidence of damage to the heart muscle. For example, an ECG in someone with an acute heart attack affecting the entire wall of heart muscle may show
ST segment elevation (an ST segment above the baseline)
T wave inversion (upside-down T waves)
And later, deep Q waves
These ECG signs are all consistent with a Q-wave heart attack.
Conversely, the ECG of someone with a non-Q-wave heart attack may show
ST segment depression (an ST segment below the baseline)
T wave inversion
No abnormally deep Q waves
This diagnostic information is important as treatments for these two types of heart attack differ.
Need To Know:
Sequential or multiple ECGs are important because abnormalities are not always present on a single ECG or may be difficult to interpret. Patients in a hospital for a heart attack usually have a daily ECG. Sequential ECGs are also helpful in determining if someone has not suffered a heart attack if all tests appear normal.
Blood tests are performed to evaluate organ function in a patient with a heart attack. A particularly important diagnostic test is the measurement of cardiac or heart enzymes. When heart muscle is damaged, these enzymes are released into the blood. Their measurement is one of the best ways to confirm a heart attack has occurred.
The first blood sample for this test is usually taken in the emergency room. Additional blood samples are obtained every six to eight hours for the first 24 hours after hospital admission.
The chest X-ray uses a beam of ionizing radiation to obtain images of the heart, lungs, and ribs. It allows a physician to view the size of the heart, and may reveal heart enlargement due to muscle damage from a heart attack. Alternatively, a chest X-ray may show fluid in lung airways due to heart failure caused by the heart attack.
In addition to diagnosing complications of a heart attack, a chest X-ray also may help with making a diagnosis. For example, it can reveal evidence of a lung problem, such as pneumonia or pneumothorax or rib fracture that is causing the chest pain.
Echocardiography is a noninvasive diagnostic test that uses ultrasound (high-frequency sound waves) to
Visualize the structures of the heart
Assess how well the heart is working
Obtain information about blood flow within the heart
An ultrasound probe is placed on the chest and sound waves are directed through the probe to the heart. A computer shows images of the heart on a video screen by "reading" echoes of the sound waves as they bounce off the heart. This procedure does not involve any exposure to radiation and is painless.
Echocardiography is particularly useful for evaluating for
Potential complications of a heart attack including heart wall or valve damage or blood clot formation in heart chambers
Assessing a patient's long-term prognosis if he or she is unable to exercise and cannot undergo a stress test
Heart function can be assessed by injecting radioactively labeled substances called "tracers" into veins. The distribution of this radioactive material is tracked with special gamma-camera detectors. The resulting images, called scans, show the distribution of the radioactive tracer within the cardiovascular system. Two commonly used substances are thallium-201 and technetium-99m MIBI.
Nuclear imaging is useful for:
Assessing blood flow to heart muscle, called "myocardial perfusion imaging"
Identifying regions where blood flow to heart muscle is insufficient, called myocardial ischemia. This results in damage to heart muscle cells, called myocardial infarction.
Thallium-201 is sometimes injected intravenously while a person is exercising on a treadmill or bicycle during the exercise tolerance test. This improves the sensitivity and specificity of the stress test.
The thallium travels through the bloodstream to the heart where it is taken up by heart muscle cells. Parts of the heart that are scarred due to a heart attack or don't have as much blood flow during exercise do not accumulate as much thallium as normal tissue. These areas appear as cold spots - areas where heart muscle cells take up less thallium on scans.
Technetium-99m MIBI (also called technetium-99m sestamibi) may be used to image blood flow in a process called blood pool imaging. In this procedure, the technetium is attached to blood cells or blood protein before being injected into a vein. This technique allows imaging of blood flowing through the heart and major blood vessels, which is useful in evaluating the heart's pumping ability.
Exercise Tolerance Or Stress Test
The exercise tolerance test, also known as the "stress test" or "stress ECG," measures the heart's response to exercise. An electrocardiograph machine records the heart's electrical activity while the patient increases his or heart rate by walking on a treadmill or riding a stationary bicycle.
During this activity, the physician or other trained personnel asks if the person is experiencing any symptoms such as chest pain and closely monitors the ECG to make sure the person does not overwork his or her heart.
In addition, the person may receive an injection of thallium isotope (a radioactive material), which makes the heart and its vessels visible to a special, computer-linked camera. The camera records how the heart moves and which parts of the heart muscle are short of blood during the exercise (see Nuclear imaging above).
A stress test can show if the arteries that supply blood to the heart are partially blocked, as may occur with coronary artery disease. It cannot, however, identify exactly where or how severely the coronary arteries are blocked. In these cases, coronary angiography may be necessary.
Need To Know:
A stress test can be performed 14 to 21 days after the heart attack to assess a patient's ability to perform routine tasks and his or her long-term prognosis.
Cardiac Catheterization And Coronary Angiography
Cardiac catheterization and coronary angiography provide information about
The anatomic structures of the heart
Blood pressure inside the heart chambers
The size and location of blockages in the coronary arteries
A thin tube called a catheter is inserted into an artery in the forearm or groin and "snaked" through blood vessel until it reaches the coronary arteries. Dye is then pumped through the catheter, so that the heart and coronary arteries can be visualized on a special video screen.
Coronary angiography can pinpoint narrowing, obstruction, and other abnormalities of the coronary arteries and is an essential test if the doctor is considering angioplasty or coronary artery bypass surgery. This procedure usually takes about 60 to 90 minutes. Patients receive an anesthetic or numbing agent to avoid discomfort.
Because coronary angiography is invasive, it is not routinely performed in all individuals who have had a heart attack. It should not be performed in individuals who are not candidates for coronary angioplasty or surgery.