Joey Votto led off the frame with a double off Marc Rzepczynski (0-1). After Ryan Ludwick was issued an intentional walk, Jay Bruce struck out and Heisey came up to pinch-hit for pitcher Aroldis Chapman.
He smacked a pitch deep into the left-centerfield gap, scoring Votto and handing Cincinnati its first victory in a three-game set with the National League champions.
Votto finished 4-for-5 with an RBI double and two runs scored while Wilson Valdez added three hits and an RBI for Cincinnati, when evened its season's record at 3-3.
Chapman (2-0) earned the victory with two scoreless frames of relief.
Jon Jay belted a two-run homer for the Cardinals, who lost for just the second time in seven contests.