The head of the cardiothoracic surgery unit at the Naval Medical Center in Bethesda is not allowed to perform any operations or treat patients as a result of an investigation by an internal review team, Navy spokesmen said yesterday.

Cmdr. Donal Billig, a heart surgeon who headed a unit that had performed 170 operations a year, had his clinical privileges suspended by the Navy at the end of November after a "review of his performance," Navy officials said.

Billig is awaiting a hearing, a process that is expected to take two to three months, and is serving in an administrative position at the Naval Medical Command, according to Navy spokesmen.

The doctor was cited during a review by a team of doctors, formed as part of a quality assurance program that has been in effect at the hospital since May 1981, said Lt. Alan Goldstein, special assistant for public affairs.

Goldstein would not say what specific event prompted Billig's suspension or what medical data were used to support the decision. He said the medical review team's investigation is being confined to the hospital and was begun as a routine "ongoing review that is made of each physician."

Billig could not be reached for comment.

The Department of Defense does not require that Navy doctors be registered in the state in which they are practicing. Goldstein said no medical association or group had yet been notified but "if the ruling is adverse, the name of the doctor would be sent on" to the medical board of the state in which he is licensed.