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WASHINGTON - Connor Geddes was 13 days old when surgeons gave him a new heart that didn't match his blood type - deliberately.

Carrie Geddes, left, shows the scar on the chest of her 11-month-old son Connor, at their home in Erie, Pa.,Thursday, Feb. 8, 2007. [AP]

Connor, now 11 months old and thriving, is one of several dozen babies around the world to have received mismatched hearts, part of a slowly growing movement to increase these tiniest patients' survival by taking advantage of a lag in their immune systems.

Now the nation's transplant network is expanding that effort, saying youngsters may be candidates for an incompatible heart up to age 2.

It's the first step in a new push by the United Network for Organ Sharing to decrease the number of children who die awaiting an organ transplant, a toll particularly high for infants and toddlers.

"It will not happen overnight," cautioned Dr. Stuart Sweet of Washington University in St. Louis, chairman of UNOS' pediatric transplant committee. But the new heart guidelines "have the potential for significantly impacting the number of patients who die on the waiting list."

If the policy sounds counterintuitive, well, it is: Implant a mismatched heart in an adult, and he or she will die rapidly. That happened in 2003, when surgeons in North Carolina accidentally gave a teenager the wrong-type heart and lungs.

But babies' immune systems must learn to recognize and attack an organ of a different blood type, a process that's turning out to be more gradual than scientists long thought.

Transplant a heart before the baby starts making antibodies that will attack a mismatched organ, and he or she survives as well as babies given matching hearts, says Dr. Lori West, the Canadian surgeon who pioneered incompatible transplants in Toronto in the late 1990s.

Those babies still need immune-suppressing drugs for life - blood type is just one form of organ rejection.

But given the scarcity of tiny hearts, the mismatch option was good news. In 2005, the last count available, 45 children under age 2 died while awaiting a new heart. As of last month, 74 youngsters under 2 were on the waiting list.

About one in 5,000 children are born with a heart defect so bad that they'll need a transplant in the first year or two of life. Yet few babies die of conditions that allow their hearts to be donated.

Still, until recently, US transplant centers were reluctant to try mismatched hearts. UNOS began allowing them as a last resort for infants, under age 1, in 2002; only 19 were performed through 2005. The concern: whether children really fare well years after getting a mismatched heart, or if rejection just sets in later.

But in the last year - with some of West's initial patients now surviving a decade - that worry is fading. Now the question is who's a good candidate for a mismatched heart, says Dr. Steve Webber, cardiology chief at Children's Hospital of Pittsburgh.

"We know we can't do it in adults, but what's the cutoff?" asks Webber. "Nobody knows for sure."

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