Snape shrugged. "Coronary arteries actually vary quite a lot from person to person." He summoned three other jars - all recently filled courtesy of the Hogsmeade raids - to illustrate his point.
"Usually you see two main coronary arteries, the right and left. In that one," he nodded at the jar that he'd just filled, "the third one you can see is the posterior coronary artery. Myocardial infarction isn't an exact science: constricting any of these arteries is likely to starve the ventricles when the heart is beating hard - as in under stressful situations like combat.
Obviously it's hard to discover the precise anatomy of the heart you're attacking, but fortunately that's not necessary. The more arteries that are closed, the swifter and more devastating the resulting heart attack will be. A prudent caster will simply hit the heart with a wide-area Reducio that will constrict all coronary arteries."
Snape shrugged, extending one hand and closing a fist in an illustrative clutch. "Problem solved." he declared casually.