Why second dose of suxamethonium cause bradycardia?

The second dose of suxamethonium can cause bradycardia due to its action on the autonomic nervous system, particularly at the level of the vagus nerve. Suxamethonium stimulates nicotinic receptors at the neuromuscular junction, and repeated doses can lead to increased release of acetylcholine, which may activate muscarinic receptors in the heart, resulting in a decrease in heart rate. Additionally, the initial dose may cause a transient tachycardia, but subsequent doses can lead to a rebound effect, increasing vagal tone and thus bradycardia.