Health authorities said the heart of a baby who received a successful cross-boundary organ donation is functioning well, with little post-operative bleeding.

Eight month-old Whitney underwent a heart transplant on Sunday and is now in intensive care.

"We have at least a dozen of these monitoring lines attached to Whitney. We have minute-by-minute monitoring for at least a dozen of these parameters, and this is coupled with another list of interval monitoring like echocardiograms to look at the heart function, and radiographers to look at the chest conditions," said Dr Nicholson Yam, the head surgeon of the operation from the Hong Kong Children's Hospital.

"We have a team of dedicated nurses to look at the surgical wound condition, the bleeding conditions from the tubes that we place after surgery, and this will continue at least for the coming weeks."

Yam said this was the baby's third open-heart surgery, presenting challenges such as removing the previously implanted biventricular assist device and minimising ischemia time -- the period during which tissue, such as a heart, is deprived of blood supply.

"A lot of work has been done before to make sure the logistics [are] as smooth as possible, all the transportation, all the cross-boundary action, and then the transport to the Children's Hospital is as fast as possible, as smooth as possible," he said.

"As for Whitney's surgery, the main challenge was it was the third time the chest was opened, so she had two previous chest openings for heart surgery. We have expected quite a bit of scarring tissue inside the chest and that will make the heart removal procedure technically more challenging."

Meanwhile, Simon Tang, director of cluster services at the Hospital Authority, said close communication with mainland authorities -- specifically strengthened collaboration with the centre for the China Organ Transplant Response System -- was essential for ensuring a smooth cross-boundary organ transplant.

He noted the first successful cross-boundary organ donation was in 2022 with a then four-month-old baby girl.

"We established the communication channels and we also established the essential pathway. So in case if we have suitable grafts in mainland China, but there is no suitable recipients in mainland China, then this graft, we will see if it can be matched to a potential recipient in Hong Kong," Tang said.

"The key step is we need good preparations and so in these two years we have already set up various guidelines and protocols, so that we can ensure ... things can go smooth."

Mr Cheung, Whitney's father, expressed his gratitude to the medical team, relevant departments and the donor's family for saving his daughter's life, calling the procedure a miracle.