first responder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person, such as a police officer, firefighter, or EMT, trained in urgent medical care and other emergency procedures and prepared to move quickly to the scene of an accident or disaster.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A member of an emergency service who is first on the scene at an emergency
• noun A member of an emergency service trained to provide emergency medical care