A ventricle was the chamber which received blood from the atrium and pumped it out, away from the heart. Ventricular fibrillation, a condition affecting the ventricles, could lead to death. (ENT: "Dead Stop")
In 2369, Ibudan murdered his clone by perforating the lower ventricle of the clone's heart with a knife. (DS9: "A Man Alone")
In 2374, Dejaren reached into B'Elanna Torres' chest and grabbed her heart, perforating her fourth ventricle. (VOY: "Revulsion")
In a deleted scene from the Star Trek: The Next Generation sixth season episode "Descent", Crosis states that a puncture to the lower ventricle of the Bajoran heart would cause instantaneous death. [1]
In a deleted scene from Star Trek Generations, Tolian Soran tells Geordi La Forge he has attached a nanoprobe to La Forge’s left ventricle. [2]