I found this on the WPS Medicare website:
Anesthesia Documentation Modifiers
Documentation Modifiers direct prompt and correct payment of the anesthesia claims submitted. Documentation modifiers (AA, QK, AD, QY, QX and QZ) must be billed in the first modifier field. If a QS modifier applies, it must be in the second modifier field. Processing delays and denials may occur for claims submitted without the modifiers in the correct position.
Anesthesia documentation modifiers that MUST be billed in the first position:
AA –Anesthesia services performed personally by an anesthesiologist.
QK -Medical direction by a physician of two, three, or four concurrent anesthesia procedures.
AD -Medically supervised by a physician, more than four concurrent anesthesia procedures.
QY -Medical direction of one CRNA/AA (Anesthesiologist's Assistant) by an anesthesiologist.
QX -CRNA/AA (Anesthesiologist's Assistant) service with medical direction by a physician.
QZ -CRNA/AA (Anesthesiologist's Assistant) service without medical direction by a physician.
Here is the link if you want that as well:
http://www.wpsmedicare.com/j5macpartb/resources/modifiers/anesthesiamodifiers.shtml