Pope John Paul II's (search) blood infection is a catastrophic condition for even the fittest people, doctors said Friday.

Septic shock (search) results when bacteria infect the blood vessels, leading to a weakening of the vessels and a catastrophic loss of blood pressure. That puts an ever increasing burden on the heart as it tries to compensate and also harms other organs.

John Paul suffered blood poisoning when a urinary tract infection spread to his bloodstream Thursday afternoon.

The Vatican (search) announced a further deterioration in the 84-year-old pontiff's condition, saying he had slipped into heart and kidney failure, with a further drop in blood pressure and shallow breathing.

"The shallow breathing is totally consistent with severe failure of the blood vessels to provide blood to all the key organs," Dr. Peter Salgo, associate director of the intensive care unit at New York Presbyterian/Columbia University Medical Center, told The Associated Press in a telephone interview.

Experts were divided over whether the decision to keep John Paul at the Vatican indicated doctors had given up on him recovering.

"The chances of an elderly person in this condition with septic shock surviving 24 to 48 hours are slim — about 10-20 percent, but that would be in an intensive care unit with very aggressive treatment," said Dr. Gianni Angelini, a professor of cardiac surgery at Bristol University in England.

"If he is not going back to the hospital, they must realize there's not much point in doing anything more heroic. It indicates they are preparing for him to die peacefully at the Vatican," Angelini said.

However, Dr. Reed Phillips, senior director of Hospice Care Network in New York, said that while the pope's condition was certainly grave, many of the key treatments could be duplicated at the Vatican.

"Not taking him to the hospital is not an indication that they are giving up on him," Phillips said.

Unless blood pressure is restored quickly, the heart and other major organs start failing because of lack of oxygen, said Dr. Peter Weissberg, medical director of the British Heart Foundation.

"If you've got somebody who is otherwise fit and able — and you can get antibiotics into them to kill off the bacteria and you can support their circulation with drugs — then you may get them over it," he said.

However, for a frail and elderly patient like the pope, the chances of dying from septic shock are 80 percent to 90 percent, he said.