Monday, August 10, 2015

Ruling out acute MI in 2 hours, not 2 days

- Jennifer Middleton, MD, MPH

Since the current troponin assay used in the U.S. may not be positive until several hours after myocardial damage has occurred, many patients presenting to emergency departments (EDs) with chest pain will spend a night in the hospital to obtain serial troponin measurements. A POEM in the current issue of AFP, however, demonstrates that ruling out acute MI can be done in only 2 hours with a new high-sensitivity cardiac troponin assay.

The high-sensitivity cardiac troponin assay has several advantages over the troponin assay currently used in the United States. It detects myocardial damage within 90 to 180 minutes after an acute MI, allowing for both faster diagnosis and treatment along with faster rule out, allowing for quicker discharge home. The high sensitivity assay is also more specific for myocardium cell death compared with the current assay.

The researchers for this POEM's study enrolled approximately 1600 patients from participating hospitals in Europe and Australia who presented to EDs complaining of chest pain. Included participants had normal electrocardiograms (ECG) that did not show ST-segment elevation. They had routine serial troponins followed but also had high-sensitivity troponins drawn at 0 and 2 hours of presentation. Participants were managed independently according to their clinical presentations. After discharge, blinded cardiologists reviewed the lab work and patient courses to determine which patients did and did not have acute MI. Comparing the high-sensitivity assay against the current gold standard troponin test, the researchers found a specificity of 99% for the high-sensitivity assay with a negative predictive value of 99.5%.

This study is predated by several others seeking to improve the efficiency of ED chest pain evaluation. A 2002 study found that a 6 to 12 hour observation in the ED, with stress tests in higher risk patients, was equivalent to care with hospital admission for patients at lower risk of acute MI. A 2007 study suggested that low risk patients with normal ECGs can reasonably be discharged home. A 2009 study found that, in patients with normal ECGs, the presence of active chest pain did not confer additional risk of acute MI. Even the idea of a two-hour window is not new; in 2002, a group of researchers studied an "accelerated evaluation protocol" and found that they could rule out acute MI in 2 hours for low risk patients. Despite these studies, though, an overnight stay to rule out MI remains the standard of care in many hospitals across the U.S. in 2015, perhaps partially due to high rates of malpractice suits against ED physicians for missed MI diagnoses, but perhaps also due to the significant risk of missed MI given the limitations of the current troponin assay.

Challenges exist with the new assay as well, though. The high sensitivity of the test may lead to false positives, especially in patients with a prior history of coronary artery disease. Clinicians currently disagree about what cut-off levels to use with the high-sensitivity assay to rule in MI. And, although cost-effectiveness models in Europe suggest overall savings with use of the new assay, it's unclear how expensive it will be in the United States. It's possible that we won't realize any cost savings if the new assay is significantly more expensive than the current one.

If U.S. EDs adopt this new test, outpatient family doctors will need to complete the evaluations for these patients after they are discharged. This 2013 AFP article discusses outpatient chest pain evaluation. You can also find several resources regarding acute coronary syndrome diagnosis in the AFP By Topic on Coronary Artery Disease/Coronary Heart Disease.