Which drug is useful in treating sinus bradycardia?

Atropine is effective in treating sinus bradycardia due to its role as an anticholinergic agent that inhibits the action of the vagus nerve on the heart. The vagus nerve releases acetylcholine, which slows down the heart rate. By blocking this action, atropine reduces vagal tone, resulting in an increased heart rate. This mechanism makes atropine particularly valuable in emergency situations where a rapid increase in heart rate is necessary to restore adequate circulation.

The other medications listed do not have a direct role in managing bradycardia. Cisatracurium, for instance, is a neuromuscular blocker used mainly during anesthesia to facilitate intubation and muscle relaxation but does not influence heart rate. Neostigmine is a cholinesterase inhibitor that increases acetylcholine levels, which could worsen bradycardia rather than alleviate it. Succinylcholine is also a neuromuscular blocker used primarily for rapid sequence intubation and does not have effects on heart rate. Therefore, atropine is the clear choice for addressing sinus bradycardia effectively.