A vagal maneuver is an action used to slow down the heart rate by stimulating the vagus nerve. The vagus nerve is the longest nerve of the autonomic nervous system and helps regulate many critical aspects of human physiology, including heart rate, blood pressure, sweating, and digestion. Stimulation of the nerve releases acetylcholine, which slows the pulse. Simple maneuvers may include holding the breath, coughing, and cold water applied to the face. Over-stimulation of the vagus nerve, however, can cause fainting.