AbstractIntracoronary nitroglycerin is frequently administered during invasive procedures such as coronary angioplasty or thrombolysis which may be associated with pain and sympathetic neural stimulation. Whether sympathetic‐mediated reflex vasoconstriction interferes with nitroglycerin‐induced augmentation of coronary blood flow is unknown. Therefore, coronary and systemic hemodynamics were measured in 19 patients during isometric handgrip exercise (3 min × 25% maximal effort), during intracoronary nitroglycerin, and during handgrip plus intracoronary nitroglycerin. Nine patients had no significant left anterior descending coronary artery stenosis (group 1) and ten patients had>70% left anterior descending coronary artery stenosis (group 2). Handgrip exercise increased heart rate, mean arterial pressure, and coronary sinus and great vein flow 15% while increasing coronary resistance 10%. Intracoronary nitroglycerin (200 μg) reduced mean arterial pressure ‐ 4 ± 6% and increased great cardiac vein flow 35–72%. Anterior left ventricular regional coronary flow responses to intracoronary nitroglycerin were unaffected by sympathetic stimulation for group 1. Group 2 had a greater increase in great vein flow with intracoronary nitroglycerin plus handgrip compared to nitroglycerin alone due to increased mean arterial pressure with no change in the great vein resistance. These data indicate that sympathetic stimulation does not alter the nitroglycerin‐induced augmentation of coronary sinus and great vein blood flow in patients with and without significant left anterior descending coronary artery stenosis. In patients undergoing invasive interventions who may have increased circulating catecholamines, mild sympathetic (isometric) stimulation does not appear to interfere with the coronary vasodilatory responses to intracoronary n