Final answer:
In this situation, the nurse would first check the insertion site to assess for any complications that may be causing the hypotension after the cardiac catheterization.
Step-by-step explanation:
In this situation, the nurse would first check the insertion site. Hypotension, or low blood pressure, can be a possible complication following cardiac catheterization. It is important to assess the insertion site for any signs of bleeding, infection, or other issues that may be causing the hypotension.
Checking the toes' capillary refill and rechecking the blood pressure every 15 minutes are also important interventions to monitor the child's condition, but assessing the insertion site takes priority in this scenario.