Three new studies show cardiac MR helps to predict post-MI outcomes

Reed Miller

May 31, 2010

Chicago, IL - Cardiac magnetic resonance (CMR) measurements of infarct size can identify salvageable myocardium and predict patient outcomes after acute ischemic syndromes, according to three new studies appearing in the June 1, 2010 issue of the Journal of the American College of Cardiology.

Two of the studies examine magnetic resonance of acutely reperfused ST-segment-elevation MI (STEMI), and the other evaluates magnetic resonance in stabilized patients with non-STEMI syndromes scheduled for invasive study. Magnetic resonance identifies myocardium at risk for infarction with T2-weighted imaging, which highlights areas of myocardial edema; the infarct size is quantified with late gadolinium enhancement (LGE) imaging.

CMR predicts late systolic dysfunction after STEMI

Previous studies have shown that LGE measured by cardiovascular magnetic resonance imaging (MRI) identifies necrosis burden in the chronic phase after MI and therefore might predict functional recovery after revascularization. However, some observational studies have found "infarct shrinkage" between the first week after infarction and later follow-up, which raises the possibility that LGE represents a combination of necrosis and edema and therefore overestimates myocardial damage in the very early STEMI period. DrEricLarose (Laval University, Quebec City, QC) and colleagues designed a study to find out whether infarct characteristics evaluated in the early hours of STEMI could improve the prediction of late left ventricular (LV) systolic recovery and poor outcomes over and above that of traditional risk factors [1].

Larose et al examined contrast-enhanced cardiovascular MRIs of 103 patients taken within 12 hours of primary angioplasty for a STEMI and then again six months later. The primary end point of the study was LV dysfunction up to two years later. Poor outcomes were a secondary end point.

The published results show that traditional risk factors were "modest" predictors of late LV dysfunction, but a multivariable logistic regression model showed that LGE volume during STEMI was the best predictor of late LV dysfunction (odds ratio 1.36; p=0.03) and was a stronger predictor of changes in LV ejection fraction than measures of infarct transmurality, microvascular obstruction, or myocardial salvage during STEMI (p=0.02). An LGE volume of >23% of the left ventricle during STEMI accurately predicted late LV dysfunction (sensitivity 89%, specificity 74%).

LGE volume was a strong independent predictor of poor outcomes and LGE of >23% carried a hazard ratio of 6.1 for adverse events (p<0.0001). During the follow-up (average 2.6 years), 23 patients had poor outcomes: one death, two MIs, five malignant arrhythmias, four cases of severe LV dysfunction (LVEF<35%), and 11 heart-failure-related hospital stays.

"Risk stratification based on contrast-enhanced cardiovascular magnetic resonance infarct imaging very early during STEMI might improve tailoring of prognosis-altering therapies in patients who will likely benefit most from aggressive treatment," Larose et al conclude.

CMR-assessed myocardial salvage index predicts prognosis

"Assessment of myocardial salvage after STEMI is crucial in evaluating the efficacy of reperfusion therapy and predicting prognosis, [and] myocardial salvage assessment might enhance the clinician's ability to rapidly and accurately assess risk in infarction and is very likely to be clinically useful in the triage and management of STEMI patients," DrIngoEitel (University of Leipzig, Germany) told heart wire . "This includes decisions regarding the opportunity to select low-risk patients for early hospital discharge, continuous monitoring of high-risk patients in intensive care, and decision making for aggressive treatment" [2].

Eitel and colleagues analyzed 208 consecutive patients undergoing primary angioplasty within 12 hours of the onset of STEMI symptoms. They used T2-weighted and contrast-enhanced CMR imaging to calculate the myocardial salvage index (MSI), which is the area at risk for infarct minus the current infarct size, divided by the area at risk. The area at risk equals the volume of edema showing on the CMR image divided by the total volume of the LV mass. Patients were divided into two groups: those above and those below the median MSI of 48. The primary end point of the study was major adverse cardiovascular events.

Major adverse cardiovascular events, including death, reinfarction, and new congestive heart failure within six months, were significantly lower in the group with an MSI above the median than in the group with an MSI below the median (2.9% vs 22.1%; p<0.001). Furthermore, the stepwise Cox proportional hazards model showed that the MSI was the strongest predictor of major adverse cardiovascular events at the six-month follow-up (p<0.001), and all prognostic clinical, angiographic, and electrocardiographic parameters were significantly correlated with the MSI.

"Our data clearly demonstrate the critical importance of ischemic time for myocardial salvage," Eitel said. "Based on these results, STEMI treatment protocols and treatment systems need to be improved to reduce ischemic times to salvage as much myocardium at risk as possible, thereby improving clinical outcome." The retrospective assessment of myocardial salvage after intervention can help predict a patient's prognosis and therefore MSI could be a valuable tool for evaluating new reperfusion modalities, the authors conclude.

CMR identifies at-risk myocardium in non-STEMI patients

In addition to predicting outcomes for STEMI patients, CMR can identify reversibly injured myocardium in non-STEMI patients and can help guide early invasive management, according to DrSubhaRaman (Ohio State University, Columbus) and colleagues, the authors of a 100-patient study of CMR in non-STEMI patients [3].

All patients in the study were prospectively enrolled and underwent a 30-minute CMR with T2-weighted edema imaging and LGE prior to an angiography for symptoms of NSTE acute coronary syndrome, but the clinical management, including decisions on revascularization, did not rely on the CMR results.

Of the 88 CMR examinations in the study that yielded adequate images, 57 showed myocardial edema. Obstructive coronary disease requiring revascularization appeared in 87.7% of those edema-positive patients, but in only 25.8% of the edema-negative patients (p<0.001). Multiple regression analysis showed that every one-unit increase in T2 score increased the odds of subsequent coronary revascularization 5.70 times. After adjustment for peak troponin I, patients with edema were about four and a half times more likely to have a cardiovascular event or death in the six months after their NSTE-ACS than those without edema.

"Given that we cannot rush all of them to the cath lab, how do we decide, especially at community hospitals where these patients initially present, who is going to [need it] and who may not need it or even be harmed? Based on our findings that rapid CMR can identify reversibly injured myocardium in NSTE-ACS, one might envision CMR fitting into the decision-making in NSTE-ACS patients without declarative biomarker or ECG abnormalities," Raman told heart wire .

Raman believes rapid CMR (25 to 30 minutes) of these non-STEMI patients will lead to the timely recognition of patients with myocardium at risk and could improve the selection of patients who will benefit from angiography and revascularization.

"Because we found that edema positivity marked increased risk of adverse outcomes, regardless of whether or not they underwent revascularization, this suggests we need more information on how we protect myocardium and improve outcomes once myocardium at risk is identified," Raman said. "We don't know, for example, if cardioprotective strategies specifically targeting edema would help as adjunctive therapies to improve outcomes."

Next studies?

Raman and colleagues are planning to implement a multicenter study that randomizes lower-risk NSTE-ACS patients without an acute indication for coronary angiography to CMR-guided or standard management. He noted that this type of study was recommended by the participants at a recent National Heart, Lung and Blood Institute workshop on outcomes research in cardiovascular imaging and would provide the necessary evidence to better understand how CMR can improve treatment of these patients.

In an accompanying editorial [4], Dr Francis Klocke (Northwestern University, Chicago, IL) suggests that "further studies in large, well-defined subsets of ischemia patients will be needed to define the additive value of T2-weighted imaging in specific settings. Experimental studies that might clarify the relative importance of the various factors capable of influencing T2 signals during and after ischemia also deserve high priority."

Professional guidelines on CMR are likely forthcoming in the next three or four years, according to DrGregHundley (Wake Forest University, Winston-Salem, NC), chair of the committee that wrote the new Expert Consensus Document on Cardiovascular Magnetic Resonance, which was published online May 17 in Circulation[5] . The committee included representatives from the American College of Cardiology Foundation (ACCF), the North American Society for Cardiovascular Imaging, the Society for Cardiovascular Magnetic Resonance, the American Academy of Pediatrics, and the American College of Radiology.

Appropriateness criteria for CMR have already been published, so now that the ACCF committee has created a consensus document summarizing the current peer-reviewed literature in the area, the process of developing formal guidelines can begin.

In the meantime, "for the person who is managing patients on a day-to-day basis, this [consensus] document provides guidance on when this particular technology could be useful," Hundley explained. "It's not guidelines and recommendations that you have to follow, but it certainly goes through all of the evidence to date worldwide and indicates, as you're managing these patients, when you need to be thinking about using CMR vs something else."

Hundley said that, currently, CMR is generally confined to the large academic medical centers, so "these types of documents are part of the road map [to expand the technology to other centers]. It allows for all of those who are practicing cardiovascular medicine to say 'Okay, these are the experts, and they have identified where we need to be using this. I have that percentage of patients in my population like that. We need to figure out how we can get this to them.' "


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