What is a brand name for a chronotropic drug used to increase heart rate?

Atropine is a well-known chronotropic drug that increases heart rate by blocking the action of the vagus nerve on the heart. It works primarily by inhibiting the parasympathetic nervous system's influence on the heart, which normally slows the heart rate. In situations such as symptomatic bradycardia, atropine is commonly administered to help restore a normal heart rate by increasing conduction through the AV node and enhancing the overall heart rate response.

In contrast, the other drugs listed in the choices have different primary effects. Atenolol is a beta-blocker that decreases heart rate and contractility, Labetalol is primarily used as an antihypertensive and also decreases heart rate, and Digoxin, while it can increase heart contractility, primarily has a role in controlling heart rate in atrial fibrillation rather than directly acting as a chronotropic agent to increase heart rate. Thus, the unique action of atropine aligns it directly with the concept of increasing heart rate, making it the correct answer in this scenario.