The first recipient of a self-contained artificial heart was placed back on a ventilator Thursday, one day after being removed from the breathing machine.

The patient remained stable and was able to sit in a chair Wednesday night, doctors said Thursday.

"We continue to be pleased with his progress," Dr. Laman Gray, one of the surgeons who implanted the device, told ABC's Good Morning America. "We have a pact: He's going to take me fishing and teach me how to bass fish."

The decision to place him back on a ventilator was a result of pre-existing medical ailments not related to the functioning of his heart, Jewish Hospital spokesman Jeff Polson said.

"This is not a step back," Polson said.

The man, identified only as being in his late- to mid-50s, was removed from the breathing device Wednesday afternoon. He had been awake after the operation and was able to visit family, but could not talk while on the ventilator.

The man has chronic renal and kidney failure and diabetes and had multiple heart attacks, one of his doctors said. He had had previous coronary bypass surgery, around 1992.

"Before he came into the hospital, he was so weak he was basically wheelchair bound," Gray told reporters Wednesday.

The recipient was picked as a candidate for the operation because doctors believed he had an 80 percent chance of dying within 30 days and was ineligible for a natural heart transplant.

The softball-sized titanium and plastic pump known as the AbioCor is intended to give the recipient more mobility than previous devices. It is wireless, and sends an electrical current through the skin to a control device that regulated the pumping of blood.

Like the human heart, it increases blood flow when the body is more active and decreases it when the body is at rest.

While stressing the man is "an extremely sick gentleman," Gray said there was a chance he could live well beyond the 60 days that is the initial goal of the experimental surgery.

But he and fellow surgeon Robert D. Dowling said Wednesday they expect complications to arise from Monday's surgery. The doctors had implanted the devices in 40 healthy baby cows during research, and experienced no real problems, they said.

Gray said the man could be susceptible to lung problems, infection, stroke, liver and kidney failure.

"I can't even predict what the complications could be in the future that we have to deal with, and I am sure we will have them," Gray said.

The AbioCor, manufactured by Abiomed Inc. of Danvers, Mass., is considered a technological leap from the Jarvik-7 mechanical heart used in the 1980s, which was attached by wires and tubes to bulky machinery outside the body.

The hope is the mechanism will one day allow recipients to lead a relatively normal life, said John Watson of the National Heart Lung and Blood Institute, which has given $20 million toward research into self-contained artificial hearts.

The artificial heart raises hope for some that up to 100,000 Americans who require cardiac therapy may have an alternative treatment. Only about half of the 4,200 Americans on waiting lists for donor hearts received one last year.

However, critics such as Dr. Robert K. Jarvik, who invented the Jarvik-7 heart, said science should focus on devices that give the natural heart mechanical help in healing itself.

In response, Gray said devices that assist the heart were good but the replacement artificial heart could be a better option for some patients.