Final answer:
The first action a nurse should take when finding an IV insertion site red, warm, and edematous is to assess the patient's vital signs, then follow with additional appropriate measures such as removing the IV and notifying the physician.
Step-by-step explanation:
When a nurse finds a patient's IV insertion site to be red, warm, and slightly edematous, the first action that should generally be performed is to assess the patient's vital signs. This is to ensure that there are no signs of systemic infection or other immediate concerns that would require urgent intervention. After assessing and ensuring the patient's vital signs are stable, the nurse should then follow proper protocol, which could include notifying the physician and removing the IV catheter to prevent further irritation or infection. Applying a warm compress may also be advised after the IV has been removed, as it can help to alleviate discomfort and reduce inflammation.