Why not use MED 1 or MED 2 as we have in Oz?
MED 1: An aircraft proceeding to pick-up or is carrying a severely ill patient on whom life support measures are taken.
MED 2: An aircraft proceeding to pick-up medical personnel and / or equipment urgently required for the transport of a MED 1 patient, or returning urgently required medical personnel and / or equipment at the termination of a MED 1 flight.