Cmdr. Donal M. Billig, a former heart surgeon at Bethesda Naval Hospital charged with involuntary manslaughter of five patients, received full operating privileges from hospital officials who needed to fill a vacancy but was informed privately that he could not conduct heart operations alone, according to testimony heard in a Navy courtroom yesterday.

Dr. Robert Cochran, former chief of surgery at Bethesda, testified that Billig was given full credentials -- which usually extend the right to perform operations independently -- "only on paper" because hospital officers needed someone to direct the cardiothoracic unit there.

Cochran's testimony followed an account by an Army surgeon who said he observed Billig's work and described him to Bethesda officials as an incompetent heart surgeon shortly before Billig was made department head.

Billig was still being retrained as a heart surgeon, a specialty he had not practiced for five years before coming to Bethesda, when hospital officials put him in charge of the unit in June 1983, Cochran testified.

"There was some concern how we could have a head of the department without credentials," Cochran said. After some discussion by top hospital doctors, Billig was given the credentials but told that he was to be supervised when he attempted heart surgery, Cochran said.

"He was given full credentials on paper . . . but there was an understanding that what was meant was something else," Cochran said.

Cochran's testimony followed statements made by Army Col. Russ Zajtchuk, who described himself as a consultant during Billig's retraining.

Zajtchuk, who observed Billig through the beginning of the summer of 1983, said he spoke to every top official at Bethesda about the surgeon.

Questioned whether he compiled progress reports for the Navy officials, Zajtchuk said: "Progress reports? There was no progress, so how can you make a report? I told everyone of the progress he was making -- which was literally none."

Zajtchuk and Cochran were witnesses in daylong testimony from a Navy pretrial investigation of the practices of Billig, who has been recommended for court-martial based on a previous investigation.

Billig, 54, has been charged with involuntary manslaughter in the deaths of five heart patients and dereliction of duty in the operations of 24 patients.

The latest charge, which came Monday, cites the case of retired Air Force lieutenant colonel John L. Kas Jr., 67, of Cape Cod, who died Oct. 29, 1984, as the result of improper surgery by Billig, according to the Navy.

All the charges stem from heart operations undertaken by Billig when he was stationed at the hospital from January 1983 through November 1984.

Cochran, who is now retired, said yesterday that as surgery chief he was concerned about the heart doctor's capabilities based on information he had received from Zajtchuk -- although he was not satisfied with Zajtchuk's supervision because he had operated only five times with Billig.

At one point, Zajtchuk told Cochran that Billig "operated at the level of a first-year resident." Cochran said he found that assessment disturbing and it led him to restrict Billig's heart operations during the months Cochran was chief of surgery.

Billig received documentation for full operating privileges in June. But Cochran, who retired from the Navy in July, said he met with Billig and Capt. John Fletcher, who would replace Cochran as chief of surgery, to emphasize that Billig could not undertake heart operations alone.