WASHINGTON (AP) _ Doctors placed a monitoring tube in Defense Secretary Les Aspin's heart, which was skipping beats occasionally, but they predicted he would recover fully, the Pentagon said in a statement Tuesday.

Aspin was also receiving daily X-rays to assess his fluid balance, the statement said.

The 54-year-old defense secretary was admitted to Georgetown University Hospital late Sunday after complaining of shortness of breath related to an underlying heart condition. The condition, hypertrophic cardiomyopathy, causes a thickening of some of heart muscles.

Aspin's physicians, whose names have not been made public, released a detailed statement of his treatment Tuesday evening following repeated questions earlier in the day at a Pentagon briefing about the secretary's treatment and chances for recovery.

In the statement, Aspin's doctors said his condition was ''stable and improving'' as he spent his third night in the intensive care unit at the hospital.

The secretary has occasional skipped heartbeats but his physicians said they do not think they are dangerous. They are common in patients suffering from hypertrophic cardiomyopathy.

Aspin's physicians said his breathing problems were triggered by a typhoid inoculation, which caused him to run a fever over the weekend.

Aspin had been given a typhoid shot known to cause nausea and fever, rather than a more expensive oral vaccine with fewer side effects, his spokesman Vernon Guidry said Tuesday.

The injection, first reported by The New York Times in Wednesday's editions, costs 35 cents, compared to $1.90 for the oral vaccine. The oral vaccine must be administered in four doses taken every other day.

Guidry said use of the injectable vaccine is being reviewed by the Defense Department.

Pentagon spokesman Bob Hall had said earlier in the day that Aspin was expected to be out of the hospital within a day or so. But the use of the tube, or catheter, could bring that into question.

The statement said Aspin's doctors believe his prognosis is ''for a full recovery'' and that they will have no medical reservations about his resuming duties at the Pentagon once he is released from the hospital.

However, the Pentagon statement disclosed the first detailed descriptions of the treatment Aspin has received for his heart condition, which the physicians say causes the heart to beat too hard. It said he takes medications known as beta blockers to reduce the force of the contractions.

The doctors said Aspin has been ordered to remain in bed during his hospital stay. He was expected to stay at least another 24 hours because of the catheter, called a SWAN-GANZ, that was placed in the heart to monitor its fluid levels.

''The SWAN-GANZ catheter has been in place for 24 hours and probably will remain another 24 hours,'' the statement said.

The catheter is a very common procedure used in intensive care units to help doctors monitor the fluid requirements of people with congestive heart failure. However, the doctors did not state that such was Aspin's condition.

The tube enters under the collarbone, goes through the heart and comes out the pulmonary artery.

In the statement, the physicians said Aspin was receiving daily X-rays to assess his fluid balance and that they had used both an intravenous tube and diuretics to balance Aspin's fluids.

They also said that his heart was not enlarged and that he did not suffer from chest pains but that the thickness of the heart muscle had increased since he had suffered heart problems in 1991.

Aspin had suffered a backup of fluids in his lungs, the Pentagon said Monday. Such a backup can occur when the heart fails to pump properly.

The secretary has met daily with Gen. Colin Powell, chairman of the Joint Chiefs of Staff, and received a phone call from President Clinton on Monday, the Pentagon said.