MIAMI, Nov 19 (Reuters) - An American teen-ager survived for nearly four months without a heart, kept alive by a custom-built artificial blood-pumping device, until she was able to have a heart transplant, doctors in Miami said on Wednesday.

The doctors said they knew of another case in which an adult had been kept alive in Germany for nine months without a heart but said they believed this was the first time a child had survived in this manner for so long.

The patient, D’Zhana Simmons of South Carolina, said the experience of living for so long with a machine pumping her blood was “scary.”

“You never knew when it would malfunction,” she said, her voice barely above a whisper, at a news conference at the University of Miami/Jackson Memorial Medical Center.

“It was like I was a fake person, like I didn’t really exist. I was just here,” she said of living without a heart.

Simmons, 14, suffered from dilated cardiomyopathy, a condition in which the patient’s heart becomes weakened and enlarged and does not pump blood efficiently.

She had a heart transplant on July 2 at Miami’s Holtz Children’s Hospital but the new heart failed to function properly and was quickly removed.

Two heart pumps made by Thoratec Corp THOR.O of Pleasanton, California, were implanted to keep her blood flowing while she fought a host of ailments and recovered her strength. Doctors implanted another heart on Oct. 29.

“She essentially lived for 118 days without a heart, with her circulation supported only by the two blood pumps,” said Dr. Marco Ricci, the hospital’s director of pediatric cardiac surgery. During that time, Simmons was mobile but remained hospitalized.

When an artificial heart is used to sustain a patient, the patient’s own heart is usually left in the body, doctors said.

In some cases, adult patients have been kept alive that way for more than a year, they said.

“This, we believe, is the first pediatric patient who has received such a device in this configuration without the heart, and possibly one of the youngest that has ... been bridged to transplantation without her native heart,” Ricci said.

Simmons also suffered renal failure and had a kidney transplant the day after the second heart transplant.

Ricci said her prognosis was good. But doctors said there is a 50 percent chance that a heart transplant patient will need a new heart 12 or 13 years after the first surgery. (Editing by Cynthia Osterman)