Final answer:
EMS crews need a valid Do Not Resuscitate (DNR) order to withhold resuscitation if a patient has a cardiac arrest and does not wish to be revived.
Step-by-step explanation:
If EMS crews are called to a cardiac arrest situation where the family indicates that the patient does not want resuscitative efforts, the legal document required by the crew to withhold resuscitation is a Do Not Resuscitate (DNR) order. This document is a type of advanced directive that states if a person's breathing stops or their heart ceases to beat, medical personnel are instructed not to perform CPR or other revival techniques. It's important that the DNR be a valid and officially recognized document to ensure that medical professionals follow the wishes of the patient in these critical situations.