Looking for pitfalls of stress cardiac MRI «under the bridge»: a case of an ischemic intramyocardial coronary artery course

Background: Myocardial Bridging (MB), a congenital coronary anomaly, is a cluster of myocardial fibers crossing over epicardial coronary arteries. Although MB can be an incidental finding (angiography or autopsy) even asymptomatic patients may present myocardial ischemia (MI). Over the past decade, cardiovascular magnetic resonance (CMR) has evolved into a cardiac stress testing modality that can be used to diagnose MI using vasodilator perfusion agents such as dipyridamole. CMR has become a highly attractive noninvasive testing modality for those suspected of having MI. Several variables obtained during CardioPulmonary Exercise Test (CPET), including O2 consumption relative to Heart Rate (HR, O2–pulse) and work rate (VO2/Watt) provide quantitative patterns of responses to exercise when left ventricular dysfunction is caused by myocardial ischemia (MI). Therefore it provides a unique approach to assess exercise–induced MI. Patients with or without chest pain or dyspnea can demonstrate pathological responses to exercise with sudden reduction in stroke volume. Case presentation: A 74 years old men, sportsman (ski mountaineering), without risk factors and asymptomatic, underwent a maximal (RQ 1.1) ramp protocol CPET showing metabolic and ECG signs of MI. Peak VO2 was 27.7 ml/min/Kg (112% of the predicted), maximal HR 154 bpm (105%). A successive pharmacological (dypiridamole) stress CMR showed no MI. Rest images showed a normal ventricular function without fibrosis (Panel A). A coronary CT scan showed a monovascular Coronary Artery Disease (CAD) with a 75% calcific stenosis on the medium right coronary artery (RCA). Patient underwent uncomplicated PCI on RCA with good final result. Left descending artery (LDA) MB was also observed (Panel B). At two months follow–up, we repeated a same–ramp protocol maximal CPET, still diagnostic for MI (Panel C). Patients was treated with calcium channel blockers and another follow–up CPET at 6 months was normal (no MI, Panel D). Discussion: Although stress CMR imaging is widely used to assess MI in patients with known or suspected CAD, only few patients with MB have been evaluated with this technique. It has been suggested that in this population, due to a pure "steal–flow" vasodilator effect without a fixed obstruction, dipyridamole stress might underestimate MI compared with exercise stress. In this specific setting, CPET provides a unique approach to assess MI due to direct observation of functional changes during strong physiological exercise. Conclusions: We present a case of MB with exercise induced MI detected by CPET but not by stress CMR. Non-fixed, dynamic, coronary obstruction could be an "Achilles heel" of stress CMR, especially with vasodilators. CPET is a peculiar, safe and non–invasive technique carrying a paramount importance in detect silent exercise induced MI in this group of patients.