New MI definition aims to better distinguish infarction from injury
MUNICH – The worldwide cardiology community’s newly revised universal definition of an MI refines the way that cardiologists distinguish between myocardial infarction and myocardial injury, said Joseph S. Alpert, MD, one of the two chairs of the definition-writing panel.
“We had three previous definitions, but there is still a lot of confusion [about distinguishing] between injury and infarction. We definitely hope that this fourth definition will further help people distinguish the two and help people determine whether or not a patient has an MI,” said Dr. Alpert following a session at the annual congress of the European Society of Cardiology that introduced some of the key elements of the new revision.
Days before the ESC congress, a task force formed by the European Society of Cardiology, the American College of Cardiology, the American Heart Association, and the World Heart Federation released the Fourth Universal Definition of Myocardial Infarction (2018) (J Am Coll Cardiol. 2018 Aug 24. doi: 10.1016/j.jacc.2018.08.1038), which follows the series of three prior MI definitions that these groups have issued since the first iteration came out in 2007 (J Am Coll Cardiol. 2007;50[22]:2173-95).
The new revision includes 5 “new concepts,” 14 updated concepts, and 6 new sections since the third universal definition from 2012. The change that topped Dr. Alpert’s list of key messages was the need to determine whether a rise in cardiac troponin, a key biomarker of cardiac damage, resulted from infarction or injury.
These two alternative diagnoses mean “a very different outlook for patients. Treatment is different, and their prognosis is different. It’s important to make the distinction,” said Dr. Alpert, professor of medicine at the University of Arizona in Tucson.
The new changes to making an MI diagnosis will likely help drive a couple of important changes in the way U.S. patients with suspected myocardial injury or infarction get assessed, he said in an interview. The first change will be wide uptake of high sensitivity cardiac troponin (hscTn) assays over the next 5 years or so, as the ability to measure this key diagnostic biomarker progresses from its initial Food and Drug Administration approval for the U.S. market in 2017 to “close to 100% of U.S. hospitals using it,” he predicted. A big issue that is currently slowing even quicker adoption of hscTn is that many hospitals, including the one where Dr. Alpert practices, still have laboratory contracts in place that tether them to older troponin-testing technologies and make it economically unfeasible to change until their contracts expire. The contract in place where Dr. Alpert practices runs out in 2019, and soon after that happens he expects to gain the ability to order a hscTn test.
The new, fourth definition says that hscTn is “recommended for routine clinical use,” but routine U.S. use “won’t be immediate because many hospitals will put in hscTn only when their old contract runs out,” he said.
Another practice-changing impact from the fourth definition may be expanded U.S. availability and use of MR imaging, which the fourth definition identified as the most informative and versatile of the several imaging options used to confirm or rule out an MI.
Cardiac MR “provides both functional and tissue characterization. It’s the technique with the most potential,” able to noninvasively identify “both the nature and extent of myocardial damage,” explained Chiara Bucciarelli-Ducci, MD, a cardiologist and imaging specialist at the Bristol (England) Heart Institute. A single cardiac MR scan “gives many answers,” said Dr. Bucciarelli-Ducci, who also served on the fourth definition task force and spoke at the session about the document’s revised imaging recommendations.
“In the setting of acute MI, cardiac MR can also be used to assess the presence and extent of myocardium at risk (myocardial edema), myocardial salvage, microvascular obstruction, intramyocardial hemorrhage, and infarct size, all markers of myocardial injury that have prognostic value,” according to the fourth definition. “In patients with possible acute MI but unobstructed coronary arteries, cardiac MR can help to diagnose alternative conditions such as myocarditis, Takotsubo syndrome, embolic infarction, or MI with spontaneous recanalization.”
“What’s turning out is that, a large number of patients with chest pain have an infection and not an MI, and cardiac MR can distinguish inflammation and myocarditis from infarction. We’re now doing a lot more MRs,” Dr. Alpert said. Although MR capability is not as widely available today as other imaging methods, like echocardiography and CT, over the next 5 years that will likely change, he said. But Dr. Alpert cautioned that not every patient with a suspected MI needs MR assessment. It’s best focused for selected patients with an uncertain diagnosis based on the core indicators of disease: history, ECG, changes in hscTn levels over time, and a chest x-ray. “MR is for when there are questions,” he said. When patients present with classic MI signs and symptoms the diagnosis can depend just on the basics, perhaps supplemented with a more widely available imaging method like echocardiography to look for wall motion abnormalities, he said. “If echo shows good left ventricular function you probably don’t need MR.” he said.
CT coronary angiography (CTCA) is another useful diagnostic tool, and right now is more widely available than MR. CTCA “may be used to diagnose coronary artery disease in patients with an acute coronary syndrome event in the emergency department or chest pain unit, particularly in low- to intermediate-risk patients with normal hscTn at presentation,” said the fourth definition. But Dr. Alpert cited the radiation dose from CT as a limiting factor. “We have patients who get repeat CT scans, and we know that increases their cancer risk. There is no such thing as a totally safe dose of radiation.” Lack of radiation exposure is another feature that makes MR imaging attractive.
Dr. Alpert had no disclosures. Dr. Bucciarelli-Ducci has had a financial relationship with Circle Cardiovascular Imaging.
