French surgeons tried unsuccessfully for more than two hours to restart the Princess's heart, but the injuries she had suffered - and particularly one to a tiny and fragile, but vital blood vessel - are almost always fatal.
Even with the best medical help, barely anyone survives the necessary surgery, experts said yesterday.
Bruno Riou, head of the intensive-care unit of the Pitie Salpetriere in eastern Paris, about four miles from the crash scene, said the Princess's heart stopped beating on arrival at the hospital, almost 90 minutes after the crash, due to massive blood loss inside her chest.
"Her chest cavity was urgently opened up, revealing a significant wound to her left pulmonary vein. Despite a closure of the wound and an external and internal cardiac massage lasting two hours, no effective circulation could be re-established."
Her death was finally confirmed at 4am (0300 BST), almost three and a half hours after the accident.
One anonymous source who claimed to have seen the body in the hospital said the Princess's face was almost unscathed by the crash. "She wanted to die beautiful ... her face was preserved," he said.
The crash caused serious internal injuries, tearing a hole in one of the four pulmonary veins - thin-walled blood vessels less than an inch long, which bring oxygen-rich blood from the lungs to the heart, to be pumped to the body.
Emergency surgeons rank damage to those as more serious than almost any other wound. The veins lie behind the heart and lungs, in the chest cavity. "Even getting at them is an exceptionally difficult challenge," said Stephen Miles, a specialist in accident and emergency medicine at the Royal London Hospital yesterday.
The seriousness of the Princess's injuries meant that she would have rapidly lost consciousness at the scene - possibly even before the car stopped moving. She never regained consciousness.
Ambulance workers at the scene said they managed to get her heart beating using external heart massage, while she was trapped for an hour in the car's wreckage. A special team meanwhile assembled at the hospital, which specialises in emergency treatment.
When she was taken there the team of more than 10 surgeons discovered she was suffering heart failure from blood loss and immediately opened her chest cavity, where they discovered a wound to the pulmonary vein amidst massive internal bleeding in the chest cavity. While some of the team tried to repair the vessel, the Princess received more than 20 pints of blood, and other doctors used manual and electronic methods to try to shock the heart back into action - including squeezing it to keep blood pumping - because brain damage can rapidly occur without a constant flow of oxygenated blood.
But Mr Miles said that such cases are almost hopeless for surgeons.
"Survival is almost anecdotal," he said. "The problem is not just the haemorrhage that has happened. Because the blood transfusion comes from a blood bank, it doesn't clot, and that makes another haemorrhage more likely."
Alastair Wilson, another expert emergency surgeon, commented: "I don't think we could have done better here. This was the very best care possible, and I think they did all that they could in the circumstances - which were extreme."
Mr Miles added: "With these kind of injuries heroic measures are called for but are very rarely successful.
"These were mortal injuries. With such a large and major vessel leaking, a patient can bleed to death very quickly."
Initial reports mistakenly said that the Princess had suffered concussion, a broken arm and cuts to her leg in the high-speed crash.
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