Final answer:
Assessing the client's blood pressure is the most important action for the nurse to implement prior to the administration of the antiarrhythmic drug adenosine (Adenocard).
Step-by-step explanation:
The most important action for the nurse to implement prior to the administration of the antiarrhythmic drug adenosine (Adenocard) is to assess the client's blood pressure. This is important because adenosine can cause a drop in blood pressure, and the nurse needs to ensure that the client's blood pressure is within a safe range before administering the medication.