MUNICH – A new universal definition of myocardial infarction has been unveiled, sparked by the development of ever more sensitive cardiac biomarker assays and imaging techniques.
These assays, including the new high-sensitivity cardiac troponin (cTn) assays available throughout Europe and awaiting approval in the United States, have created confusion in the diagnosis of myocardial infarction because they detect small cTn elevations associated with many other clinical conditions such as heart failure, arrhythmias, and pulmonary embolism that are not MIs, but rather myocardial injury with necrosis.
"I think there has been a little bit of a problem in the past where we’ve had too many infarctions [diagnosed] ... where there is some damage or injury to the myocardial cells," said document task force cochair Dr. Kristian Thygeysen, who presented the third universal MI definition at the annual meeting of the European Society of Cardiology (ESC).
The expert consensus document, developed by the ESC, American College of Cardiology (ACC), American Heart Association (AHA), and World Heart Federation (WHF), maintains the pathological definition of acute MI as myocardial cell death due to prolonged myocardial ischemia, but goes on to refine the definition of MI in five settings, including the controversial area of MIs associated with revascularization procedures.
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Dr. Kristian Thygesen
MI in the PCI Setting
An MI related to percutaneous coronary intervention (PCI) is defined as an elevation of cTn values more than five times the 99th percentile upper reference limit (URL) in the first 48 hours after a procedure in patients with normal baseline troponin values, or a rise of cTn values of more than 20% in patients with elevated baseline levels that are stable or falling.
It also requires one of the following events: symptoms suggestive of myocardial ischemia, new ischemic ECG changes, angiographic findings consistent with a procedural complication, or imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.
In the previous 2007 document, the troponin threshold had been more than three times the 99th percentile, and was raised based on new prognostic information from long-term follow-up of patients undergoing PCI showing that there is unavoidable injury associated with the procedure, document task force codirector Dr. Joseph Alpert said during the presentation.
CABG-Related MI
Similarly, the 2012 version raises the troponin threshold for MI related to coronary artery bypass graft surgery from five times the 99th percentile URL in the 2007 document to 10 times the 99th percentile in patients with normal cTn baseline values.
It also requires one of the following: new pathological Q waves or new left bundle branch block (LBBB), angiographically documented new graft or new native coronary artery occlusion, or imaging evidence of new loss of viable myocardium or new regional wall motion abnormality.
Once again, the decision to raise the troponin threshold was made because there is unavoidable injury to the heart during CABG from needle sticks, handling of the heart, and the myocardial preservation procedure, said Dr. Alpert, a professor of medicine at the University of Arizona, Tucson.
Many Sources of Heart Injury
"The problem that every clinician – not just cardiologists, but internists and surgeons – is having with these troponin tests, and particularly with the high-sensitivity test, is that it turns out we’re finding that there are lots and lots of people having heart injuries," he said in an interview. "We’ve known for decades that it’s not uncommon for a very sick patient to have liver injuries, but now we’re saying, ‘My goodness, they’re having heart injuries, and these injuries are not MIs, or at least we have no evidence there is ischemia.’ "
The updated guideline points out that novel procedures such as transcatheter aortic valve implantation or mitral clip may also cause myocardial injury with necrosis, and that "it is likely that, similarly to CABG, the more marked the elevation of the biomarker values, the worse the prognosis – but data on that are not available.&qu
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Dr. Joseph Alpert
Although high-sensitivity troponin assays are not yet approved in the United States, it is only a matter of time before they are and the financial battle heats up over the distinction between myocardial injury and MI, according to Dr. Alpert. The reason is that there is currently no reimbursement code for patients with myocardial injury, who require substantial time and resources that currently are not being reimbursed.
"We’re pushing to get that code, because when you have an elevated troponin it means something, and it always means something not good," he said in the interview.