Abiomed, which manufactures the first self-contained artificial heart, has received Food and Drug Administration approval to enroll an additional five patients in its U.S. clinical trial, it was announced.

The first five patients, each too sick to qualify for a human heart transplant, have already received the mechanical heart, which weighs three pounds and is about the size of a grapefruit.

"We are very pleased that the FDA has given us the green light to continue with the AbioCor clinical trial without delay," Abiomed chief executive Dr. David Lederman said.

"The first five AbioCor patients have demonstrated that we have made significant progress toward the ultimate goal of providing meaningful life extension to end-state heart failure patients with no other alternative. But there is much left to prove and many lessons to be learned as we proceed," Lederman said.

The AbioCor clinical trial began on July 2 with Robert Tools, 59, who received the first implant at Jewish Hospital in Louisville, Kentucky. Implants since have been performed at Jewish Hospital on Sept. 13; at the Texas Heart Institute, Houston, Texas on Sept. 26; at UCLA Medical Center, Los Angeles, Calif. on Oct. 17; and at Hahnemann University Hospital, Philadelphia, Penn., on Nov. 5.

All five patients are still alive and Abiomed says the AbioCor hearts have functioned flawlessly in each case.