Make sure you talk to the anesthesiologist prior to the surgery and stress that neither arm should be rotated away from the body (the usual position, particularly if it is abdominal surgery is to rotate the arm out about 90 degrees). Some even position it over the head, which is a real "no no" for us. Ask the anesthesiologist to tuck the arm to the side.
Also if there is Horner's Syndrome present, this is also critical, as one pupil will be smaller than the other which could lead them to think that there is something else happening (like brain damage) when it is not. Make sure they look carefully prior to the surgery to determine any differential in pupil size and iris color.
Do not let them take blood pressure or put the IV into the bpi arm under any circumstance. False readings can happen and the arm can be injured further.