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A 62 year old man was admitted because of frequent episodes of palpitation and chest pain. Two dimensional echocardiography showed hypokinesis of the posterolateral wall with preserved global left ventricular function and volumes. Holter monitoring showed several episodes of non-sustained ventricular tachycardia, and the subsequent electrophysiologic study confirmed the presence of non-sustained inducible ventricular tachycardia. Coronary angiography showed slow coronary run-off of contrast medium and a complex eccentric lesion in the proximal portion of the left circumflex coronary artery (1). The intracoronary injection of glyceryl trinitrate (200 μg) caused diffuse coronary vasodilation but it failed to affect both stenosis severity and morphology and contrast medium run-off (2). Conversely, intracoronary injection of adenosine (20 μg) was followed by normalisation of contrast medium run-off and a striking modification of stenosis morphology revealing a mild concentric plaque (3). Soon after adenosine was washed out, the same complex eccentric lesion reappeared (4). However, intracoronary ultrasound confirmed the presence of a mild concentric fibrous plaque only (arrow indicates the site to which the ultrasound image refers). Slow coronary run-off caused by coronary microcirculation dysfunction might be responsible for flow turbulence causing inhomogeneous contrast medium distribution at the site of mild plaques; this, in turn, can simulate the presence of a severe complex stenosis. Accordingly, in our patient glyceryl trinitrate, which causes epicardial coronary artery dilation but only mild microvascular dilation, probably failed to increase coronary blood flow velocity and to affect the distribution of contrast medium at the site of the plaque. By contrast, adenosine, a potent vasodilator of the coronary microcirculation, increased coronary flow velocity, thus unmasking the artefact caused by contrast medium turbulence. In patients with slow coronary run-off and apparently complex lesions, intracoronary injection of adenosine may be useful in order to avoid inappropriate revascularisation procedures.