No symbol is more overused in the arts than the human heart. Served cold, golden, or on one’s sleeve, it has been the stuff of countless theatrical metaphors, from the plays of Euripides to Larry Kramer. The Roundabout Theatre Company has, nonetheless, doubled down on the image for its plainly named yet surprisingly complex medical musical, The Heart. Happily, it is a gamble that pays off. Kait Kerrigan’s book mostly avoids sentimentality in favor of scientific facts and hard emotional truths, while the husband and wife team of Ian and Anne Eisendrath deliver a pulsating score, anchored in the realm of electronic dance music.