CPR and ACLS should not be performed if a valid written "DNR" order is present. In any cases of doubt, emergency medical technicians, paramedics and other medical workers will perform as if a DNR order did not exist, as is required by law.
The exact rules for obtaining and for emergency medical personnel accepting the validity of a DNR order vary widely according to jurisdiction. For example, in the state of Maryland, only Maryland-state DNR orders are acceptable, and they require much verification, witnesses and doctor's signatures in order to be valid. In contrast, the state of West Virginia allows patients to receive a DNR order with relative ease, and will accept them from most other jurisdictions.
See cardiac arrest, durable power of attorney, medical power of attorney