Which of the following emergency medications increases heart rate by blocking vagal nerve stimulation?

Atropine is an anticholinergic medication that works by blocking the action of the vagus nerve on the heart. The vagus nerve, when stimulated, typically lowers heart rate. By inhibiting this stimulation, atropine effectively increases heart rate. This makes it particularly useful in emergency situations where bradycardia (abnormally slow heart rate) needs to be addressed.

Other medications in the options do serve important roles, but they function differently. For instance, lidocaine is primarily an antiarrhythmic agent that stabilizes cardiac membranes, while ephedrine acts as a stimulant and can increase heart rate through a different mechanism, including increasing norepinephrine release. Succinylcholine, on the other hand, is a neuromuscular blocker primarily used for procedures requiring muscle relaxation. Therefore, in the context of increasing heart rate through vagal blockade, atropine is the correct choice.