An interesting situation has been brought to our attention which occurred in the USA (where else?!)
An unconscious 70 year old was hospitalised. Efforts were made to restore consciousness where by a mental state may have been achieved so that his doctors could determine his precise wishes. However, he had tattooed on his chest DO NOT RESUSCITATE which was accompanied by his presumed signature (how did they do that?)
Doctors sought ethical consultations whilst they preserved his life with drugs.
The ethics consultants advised the hospital to honour the patient’s wishes and he died without undergoing advanced medical management to resuscitate.
As an alternative, it might be simpler to instruct us to prepare an Advanced Care Directive (ACD) rather than go through the pain and uncertainty of a request through a tattoo (evidently often undertaken when intoxicated and later regretted).
As an independent matter or when instructing us to prepare a Will consideration should be given also to an ACD (often now called a Living Will) to deal with your wishes negotiating medical treatment when incapacitated together with Powers of Attorney to allow a person to handle your financial affairs when incapacitated or when unable to deal with them.