Final answer:
After securing the airway in a patient with symptomatic bradycardia from metoprolol overdose, the next appropriate step is administering atropine to counteract the overdose effects.
Step-by-step explanation:
The patient presents with symptomatic bradycardia following an overdose of metoprolol, a β-blocker. β-blockers are commonly used for treating cardiovascular disease and work by blocking the β-receptors associated with vasoconstriction and cardioacceleration, thus maintaining a lower heart rate and reducing blood pressure.
In the case of a metoprolol overdose leading to bradycardia, the most appropriate initial step after securing the airway is the administration of atropine. Atropine is an anticholinergic medication used to increase heart rate by inhibiting the action of the vagus nerve on the heart, thus counteracting the effects of β-blockers.