The first few minutes a patient with chest pain spends in the emergency department are critical because prompt diagnosis and treatment can limit or even prevent heart damage. However, diagnosis is not always easy, especially for people with non-ST elevation acute coronary syndrome (ACS)—a term that includes both unstable angina and a type of heart attack marked by the absence of ST-segment elevation on an electrocardiogram (ECG).
Delays in Diagnosis of Heart Attack
About 1.5 million of the 6 million Americans who arrive at hospital emergency departments each year complaining of chest pain are diagnosed with ACS, and an ECG does not provide a rapid diagnosis in all patients. If ECG findings are normal and ACS is still suspected, the patient’s blood is tested for markers of heart injury, such as the protein troponin or the enzyme creatine kinase.
Unfortunately, these markers do not appear until several hours after a heart attack. Frequently, serial tests of these heart attack markers and serial ECGs must be performed, and patients may have to remain in the hospital for up to 24 hours before they receive a definitive diagnosis. In addition, because troponin and creatine kinase are released only after the death of heart muscle cells, they cannot detect unstable angina.
Because of such difficulties in heart attack diagnosis, about 2% of heart attacks are missed initially, and many patients with unstable angina are discharged without a diagnosis. Now, research has identified several new imaging studies and blood tests that may soon help emergency department doctors spot a heart attack in progress or recognize patients who are in imminent danger of a heart attack due to unstable angina.
The usefulness of such heart attack diagnosis tests is bolstered by several large trials that have shown the effectiveness of early invasive therapy (angioplasty or bypass surgery) followed by combination drug therapy.
Better Imaging Techniques for Heart Attack and Unstable Angina
High-resolution magnetic resonance imaging (MRI) can detect heart attacks or unstable angina in people with chest pain more accurately and faster than traditional methods, according to a study published in the February 2003 issue of Circulation.
Researchers evaluated the ability of MRI scans to detect ACS in 161 emergency department patients with chest pain suggestive of myocardial ischemia (inadequate supply of oxygen to the heart muscle). The findings on MRI were compared with those for three standard diagnostic tests: ECG, troponin level, and Thrombolysis in Myocardial Infarction (TIMI) risk score (an assessment of cardiovascular risk based on several clinical characteristics).
The MRI detected all of the patients’ heart attacks, including those in patients who had normal initial ECG results. It also identified more cases of unstable angina than the other tests.
A high-resolution computed tomography (CT) scan of the coronary arteries with 3D reconstruction is another new imaging technology that appears highly promising for diagnosing a heart attack. It is more rapid and more readily available than an MRI scan and typically takes about 40 minutes to perform.
Two Blood Tests for Heart Attacks
In October 2003, researchers reported in The New England Journal of Medicine on a blood test that can provide answers quickly in cases of suspected ACS. This test measures the blood level of myeloperoxidase, an enzyme produced by activated white blood cells. Myeloperoxidase levels are elevated in the presence of unstable atherosclerotic plaque, which is the usual mechanism responsible for a cardiac event. The FDA approved the myeloperoxidase test in 2005 as "CardioMPO."
Researchers measured myeloperoxidase levels in 604 patients who came to an emergency department with chest pain. Almost 40% of patients with the highest levels of myeloperoxidase had a heart attack within 16 hours, compared with only 14% of those with the lowest levels of the enzyme. Patients with the highest myeloperoxidase levels also had much higher rates of major cardiac events 30 days and 6 months later.
Another new blood test, the albumin cobalt binding test, which was approved by the U.S. Food and Drug Administration in February 2004, can be coupled with existing diagnostic techniques to help detect myocardial ischemia and assist in determining whether a heart attack is the underlying cause of chest pain.
In one study, the combination of ECG and initial troponin measurement was only 50% accurate in ruling out a heart attack. The addition of the albumin cobalt binding test to the ECG and troponin test increased the accuracy to 70%.
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Posted in Heart Health on February 7, 2006
Reviewed May 2007
