Issue: August 2018
Perspective from Christopher M. Kramer, MD
April 06, 2018
5 min read
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Cardiac MRI identifies nonobstructive disease, diagnoses microvascular angina

Issue: August 2018
Perspective from Christopher M. Kramer, MD
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Cardiac MRI stress T1 mapping, which does not use contrast agents or radiation, adequately differentiated obstructive epicardial CAD from microvascular dysfunction, researchers reported.

In a separate study by the same team, cardiac MRI noninvasively correctly diagnosed microvascular angina in patients with nonobstructive CAD.

Distinguishing CAD from nonobstructive CAD

The researchers conducted a prospective validation of stress T1 mapping against invasive coronary measurements compared with first-pass perfusion imaging. Obstructive epicardial CAD was defined as fractional flow reserve less than 0.8, whereas coronary microvascular dysfunction was defined as FFR of at least 0.8 and index of microcirculatory resistance of at least 25 U.

Sixty patients with angina and 30 controls underwent cardiac MRI 1.5 T and 3 T to determine left ventricular function, ischemia and infarction. FFR and index of microcirculatory resistance were assessed within 7 days after cardiac MRI.

Normal myocardial T1 reactivity was 6.2% on 1.5 T and 3 T cardiac MRI, Alexander Liu, MBBS, from the Oxford Centre for Clinical Magnetic Resonance Research, division of cardiovascular medicine, Radcliffe Department of Medicine, University of Oxford, and colleagues wrote.

Ischemic viable myocardium downstream of obstructive CAD was associated with virtually no T1 reactivity (0.7%), which was not the case for myocardium downstream of nonobstructed coronary arteries with microvascular dysfunction (3%), the researchers wrote.

Stress T1 mapping outperformed gadolinium-based first-pass perfusion imaging for detecting obstructive CAD (area under the receiver-operating characteristic curve [AUROC], 0.97 vs. 0.91; P < .001), according to the researchers.

Myocardial T1 reactivity of 1.5% was an accurate detector of obstructive CAD (sensitivity = 93%; specificity = 95%; P < .001), whereas myocardial T1 reactivity of 4% was an accurate detector of microvascular dysfunction (AUROC = 0.95; sensitivity = 94%; specificity = 94%; P < .001), Liu and colleagues wrote.

Diagnosing microvascular angina

In the second study, Liu and colleagues validated cardiac MRI for diagnosing microvascular angina in patients with nonobstructive CAD, compared with patients with obstructive CAD, correlated to index of microcirculatory resistance during invasive coronary angiography.

Adenosine stress cardiac MRI 1.5 T and 3 T was administered to 50 patients with angina (mean age, 65 years) and 20 age-matched healthy controls to assess LV function, inducible ischemia, myocardial blood flow and infarction. Of the group with angina, 28 were found to have obstructive CAD and 22 were found to have nonobstructive CAD after angiography within 7 days of cardiac MRI.

Among those with nonobstructive CAD, myocardium with index of microcirculatory resistance less than 25 U was associated with normal myocardial perfusion reserve index (1.9 vs. 2 for controls; P = .49), according to the researchers.

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Theodoros D. Karamitsos

However, those with nonobstructive CAD who had myocardium with index of microcirculatory resistance of at least 25 U had impaired myocardial perfusion reserve index (1.2 vs. 1.2 for obstructive CAD; P = .61).

A myocardial perfusion reserve index of 1.4 accurately detected impaired perfusion as a result of coronary microvascular dysfunction (AUC = 0.9; specificity = 95%; sensitivity = 89%; P < .001), according to the researchers.

The main driver behind impaired myocardial perfusion reserve index in patients with nonobstructive CAD was impaired augmentation of myocardial blood flow during stress, Liu and colleagues wrote.

Patients with FFR greater than 0.8 and index of microcirculatory resistance less than 25 U also had blunted stress myocardial blood flow, and this mild coronary microvascular dysfunction was distinguishable from controls by using a stress myocardial blood flow threshold of 2.3 mL/min/g (100% positive predictive value), the researchers wrote.

Diagnostic potential

In a related editorial, Theodoros D. Karamitsos, MD, PhD, from the first department of cardiology, Aristotle University of Thessaloniki, AHEPA Hospital, Thessaloniki, Greece, wrote that the studies were helped by the inclusion of invasive measurements and “the authors should be commended for performing two important studies that open new frontiers for myocardial ischemia testing.”

“Clinicians frequently face dilemmas as to how to diagnose and manage patients with coronary microvascular dysfunction,” he wrote. “There is no doubt that these novel, noncontrast [cardiac MRI] techniques offer important pathophysiological insights in myocardial ischemia and have a significant diagnostic potential that justifies the conduction of a large-scale study.” – by Erik Swain

Referen ces:

Karamitsos TD. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.12.047.

Liu A, et al. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.11.071.

Liu A, et al. J Am Coll Cardiol. 2018;doi:10.1016/j.jacc.2017.12.046.

 

Disclosures: Liu and Karamitsos report no relevant financial disclosures. Please see the studies for all other authors’ relevant financial disclosures.