Doctors implanted a second artificial heart into a woman whose transplanted human heart failed hours earlier. Bernadette Chayrez, 40, apparently is the first person to receive two of the devices, hospital officials in Tucson said. The mini-Jarvik implanted during emergency surgery was not the same one that kept Chayrez alive for four days until it was replaced with a human heart on Friday, said Nina Trasoff, spokeswoman for University Medical Center. Chayrez had been improving since she underwent surgery for bleeding problems Saturday, when "serious problems developed suddenly," said Trasoff. Dr. Jack Copeland and the family made the decision to go ahead with the second implant.