Pancuronium bromide
In the United States, people sentenced to death by lethal injection have usually been executed using three different medications.  Pancuronium bromide is the second medication.  First, a sedative called sodium thiopenthal puts the prisoner into a deep sleep.  Second, pancuronium bromide is given to keep the prisoner from moving and to stop their breathing.  Third, potassium chloride is given to stop the prisoner's heart.