LOMA LINDA, Calif. (AP) _ Doctors were maintaining a close watch on a little Texas boy who remained extremely ill after undergoing two heart transplants last week, a hospital spokeswoman said.

''It is still an hour-to-hour vigil,'' Loma Linda University Medical Center spokeswoman Anita Rockwell said Sunday as 3-year-old Nicky Carrizales of San Antonio remained in critical condition.

Meanwhile, in Pittsburgh, a teen-ager who received two heart tranplants last week died, and a man who spent eight days on a Jarvik-7 artificial heart remained in critical condition after recieving a human organ.

Nicky received the heart of a 6-year-old boy Thursday but there was ''still no significant change,'' she said.

Nicky was on a respirator and other support machinery and was being treated for acute kidney failure. Doctors Friday used a balloon catheter to expand a constricted artery that was restricting the flow of blood leaving the heart. Surgery performed on Nicky as an infant failed to completely repair the narrowing of his aorta, doctors discovered.

''This undoubtedly contributed to the failure of the original smaller and somewhat more stressed heart,'' she said, referring to the heart of a 2-year- old that Nicky received in his first transplant, completed early Wednesday. That heart quickly failed.

Nicky, whose own heart muscle had severely degenerated, is the son of Rudy Carrizales, an X-ray technologist at Kelly Air Force Base in Texas, and his wife, Mary Lou.

In Pittsburgh, a man who suffered the same heart disease that killed his twin brother received a second chance at life from a Jarvik-7 that kept him alive while he waited for a donor organ.

Edmund McDermott, 32, of Scranton, Pa., received his new human heart in an operation Saturday at Presbyterian-University Hospital.

McDermott had been waiting for a human donor heart since May 30, said Tom Chakurda, a hospital spokesman.

Surgeons decided to implant the mechanical device on June 13 after McDermott's condition deteriorated to the point where they felt they could no longer wait for a donor heart, officials said.

McDermott and his twin, Edward, were diagnosed in 1979 as having cardiomyopathy, a degenerative disease of the heart muscle, according to their mother, Mrs. Paul McDermott of Scranton. Edward's condition deteriorated badly before he could receive a transplant, and he died in that same year, Mrs. McDermott said.

Artificial heart implants were not performed until 1982.

Edmund and his wife, Kathleen, are expecting their first child, his mother said. McDermott, who has been unable to work, is also attending school in hopes of earning a business degree, Chakurda said.

At Children's Hospital of Pittsburgh, a 17-year-old Cleveland boy died of organ failure less than a week after undergoing two heart transplant operations in two days, hospital officials said.

Tony Steele, who suffered a congenital heart disease, died Saturday, said a hospital spokeswoman who refused to give her name. He had been put on dialysis Friday after his kidneys began to fail, the spokeswoman said.

The teen-ager had been in critical condition since undergoing his second transplant Wednesday. The first transplant was performed Tuesday night, but the donor heart failed to function properly.

It was not immediately clear exactly what organ failure caused his death.