Do you ever leave your pet in the care of a friend, neighbor, or professional pet sitter? In case of a medical emergency, are they authorized to bring your pet to your veterinarian (or an emergency clinic) and to approve treatment?
Anyone who cares for your pet should have written authorization from you, and your vet should have a signed release on file as well.
Use one of the forms specific in the next paragraph (or make up something similar) and give it to your vet for your file. Also post it on your refrigerator, and/or leave it on your kitchen table, or in another prominent place for pet sitters to easily find.
For example, look at this Pet Care Emergency Authorization Form, or this one, or create one of your own. Everyone involved with the care of your pet will be relieved to know this letter exists… as will Fido and Miss Kitty!