University of Utah officials described artificial heart patient Barney B. Clark as "semiconscious" but "stable" today as he began to "respond very, very slowly" to treatment after suffering a series of seizures Tuesday morning.

"He is responding to spoken words," said Dr. Chase Peterson, University of Utah vice president for health sciences. "He clearly understands what's said to him."

Officials said he was moving his arms, legs and head, and had opened his eyes, moving them from side to side. With both respirator and feeding tubes in his throat, however, he was unable to speak.

At an afternoon briefing, Peterson said most of Clark's vital signs -- temperature, blood pressure and pulse -- remained in the normal range.

"He spent a quiet day," Peterson said. He said Clark's recovery is proceeding much like that of any "seriously ill post-operative surgery patient." The mechanical heart, the world's first permanent artificial heart, "has worked beautifully. It hasn't seemed to miss a beat."

Peterson suggested that Clark's mechanical heart may even be assisting in his recovery. It is normally a "laborious process," he said, to rid a heart patient's lungs of excess fluid. In Clark's case, however, hospital staff members were able to rid his lungs of water in 15 minutes simply by adjusting the dials on the external heart-monitoring equipment. They decreased pumping on the side that sends blood to the lungs and increased pumping on the side that receives it.

Earlier, spokesman John Dwan said that the 61-year-old retired dentist remained in critical condition, but appeared to be coming "out from under the influence" of drugs given to control the seizures.

"He's beginning to improve," Dwan said, but he stressed that little immediate dramatic change is expected.

Dwan said brain X-rays showed no evidence that Clark had suffered a stroke or damage of the sort that might have been caused by a serious brain hemorrhage.

The seizures occurred shortly after dawn Tuesday as Clark awoke after an "excellent" night. He asked his doctor, "How am I doing?"

"You're doing just fine," said Dr. William DeVries, the doctor who implanted the artificial heart in a seven-hour operation last Thursday. Just then, Clark began to shake.

A generalized seizure -- often described as a "grand mal" seizure -- usually lasts for two to five minutes, leading to loss of consciousness and motor control, with contractions of muscles throughout the body. "We usually turn the head so they don't swallow their tongue," said Helen Kee, the nurse supervisor here.

This attack was followed by periodic, localized seizures in Clark's left leg that lasted two to three hours. Such localized or "focal" seizures often reflect electrical changes in parts of the brain.

Doctors now believe all Clark's seizures stemmed from a chemical or metabolic imbalance in his body that worsened following implantation of the artificial heart.

Such imbalances in the body's essential salts, or electrolytes, as well as in other chemicals, can cause temporary short circuits in the sensitive electrical pathways to the brain and lead to seizures. If this were the case, said Peterson, no permanent harm would be expected.

The seizures may be followed by a period of stupor or deep sleep, which may also help account for Clark's reduced state of consciousness, according to the medical staff members.

After the seizures, Peterson said, Clark was given an intravenous injection of diazepam, or Valium, and phenytoin, or Dilantin. All sedation has now been discontinued, Peterson said.

To prevent further seizures, doctors are trying to get Clark's body chemistry back in balance, particularly by pumping nutrients into his stomach. He is receiving a balanced hospital diet of proteins, fats, carbohydrates and minerals -- the medical equivalent of chicken soup and carrots, Peterson said -- through a tube inserted in his nose.

Peterson said doctors were unsure of what might have knocked Clark's body chemistry out of balance. One possibility is the "minor kidney failure" that Clark experienced before the implant surgery and that grew worse Monday. Use of the respirator could also have disrupted the balance of gases in his blood, Peterson said.

In addition, drugs, such as the antibiotics Clark has been receiving to ward off infection, or the diuretics used to flush the excess fluid from his body, may have caused a chemical disruption. Dwan said today that the antibiotics had been discontinued.

Peterson said the goal is for Clark to have "five or six days without any complications" so that his body can regain strength. The doctor expressed optimism that Clark may be able to return home in several weeks.