DURING INCLEMENT WX ON AN EMS FLIGHT WITH A NURSE AND PATIENT ONBOARD, THE HELICOPTER COLLIDED WITH POWER LINES WHICH WERE SUSPENDED 36 FEET ABOVE A ROAD. CONTROL WAS LOST AND THE HELICOPTER COLLIDED WITH A RETAINING WALL, CLIPPED THE TOPS OFF SEVERAL TREES AND PLUNGED INTO A 70-FOOT DEEP RAVINE. THE MAIN WRECKAGE CAME TO REST ABOUT 220 FEET FROM THE INITIAL POINT OF IMPACT. THE SOLE SURVIVOR WAS THE PATIENT; HE HAD BEEN STRAPPED ONTO A FULL BODY BOARD. DESPITE THE PILOT'S EXTENSIVE MILITARY & CIVILIAN FLIGHT EXPERIENCE & CONCERN FOR FLIGHT SAFETY, THE OVERRIDING LIFE-SAVING MISSION COUPLED WITH JOB PRESSURES CONTRIBUTED TO HIM DELAYING HIS DECISION TO TURN AROUND. ALSO CONTRIBUTING WAS THE DISPATCHER'S/MANAGEMENT'S FAILURE TO READILY PROVIDE THE PILOT WITH ALTERNATE DESTINATION INSTRUCTIONS UPON THE CLOSURE OF THE PRIMARY DESTINATION'S TRAUMA CENTER.