Final answer:
The priority intervention for a nurse caring for a patient with a hematoma post-cardiac catheterization is to apply pressure to the site to control bleeding, followed by notifying the healthcare provider.
Step-by-step explanation:
When a nurse observes the formation of a hematoma at the insertion site and a decreased pulse rate in the affected extremity after a cardiac catheterization, the priority intervention is to apply pressure to the hematoma site. This can help control bleeding and reduce the size of the hematoma. The second priority would be to notify the healthcare provider of the change in the patient's condition, as they may need to take further action or give specific orders. Elevating the affected extremity and administering pain medication are important but are not the immediate priorities in this scenario.