The most recent infant to receive a new heart at Loma Linda University Medical Center experienced a brief bout of low blood pressure, forcing surgeons to reopen his chest, and was reported in critical but stable condition, hospital officials said. Daniel Jirschefske, born Sept. 26 to a Pasadena couple, was in intensive care after a 5 1/2-hour transplant operation, a hospital spokeswoman said. Several hours after surgery, Baby Daniel developed low blood pressure and physicians reopened his chest to check for bleeding. None was found and the infant’s pressure returned to normal, the spokeswoman said. Baby Daniel’s heart-transplant operation--necessitated because he was born with hydroplastic left heart syndrome--was the 22nd at Loma Linda on an infant under the age of 6 months. Eighteen of the infants survive.