http://www.cms.gov/manuals/downloads/clm104c12.pdf
page 117
K. Anesthesia Claims Modifiers
Physicians report the appropriate anesthesia modifier to denote whether the service was personally performed, medically directed, or medically supervised.
Specific anesthesia modifiers include:
AA - Anesthesia Services performed personally by the anesthesiologist;
AD - Medical Supervision by a physician; more than 4 concurrent anesthesia procedures;
G8 - Monitored anesthesia care (MAC) for deep complex complicated, or markedly invasive surgical procedures;
G9 - Monitored anesthesia care for patient who has a history of severe cardio-pulmonary condition;
QK - Medical direction of two, three or four concurrent anesthesia procedures involving qualified individuals;
QS - Monitored anesthesia care service;
QX - CRNA service; with medical direction by a physician;
QY - Medical direction of one certified registered nurse anesthetist by an anesthesiologist;
QZ - CRNA service: without medical direction by a physician; and
GC - these services have been performed by a resident under the direction of a teaching physician.
The GC modifier is reported by the teaching physician to indicate