Katie Burke underwent open-heart surgery hours after she was born, weighing slightly more than 4lb.
Doctors said that the operation on the girl, from Stetchford, Birmingham, had been a success.
Katie, who is now three weeks old, was born with a hole in the heart. She was moved to the Diana Princess of Wales Children's Hospital, Birmingham, a day after she was born for an immediate operation to repair it. Surgeons fitted her pacemaker eight days later.
Consultant cardiologist Joe De Giovanni said: "It is unique because it is the first time a pacemaker has been made for a baby so small. It had to be specially configured to take a very small wire into Katie's heart because her veins are so tiny.
"Usually the wires are much bigger and, although small pacemakers have been produced before, one which could work from such a small wire had never been developed."
He said he expected her to be able to live a normal life thanks to the technology behind the pacemaker.
Her mother, Sue, 36, who was told her baby had a heart defect after scans while pregnant, said: "We're just so relieved she seems to be doing so well. She's a lovely colour."
Pacemakers, which are usually fitted in a relatively minor operation under general anaesthetic, regulate the heart beat by sending an electrical charge from a tiny lithium battery down a wire to the heart muscle.
They are often placed in a pocket under the skin of either the abdomen or the collarbone.
Dr De Giovanni said: "Usually pacemakers are inserted into the front of the chest in older patients, but in Katie's case there wasn't enough room or fat to take the machine, so we had to hide it under her arm." The first pacemakers, which were introduced almost 40 years ago, did vary the rate of beat, making exercise or exertion difficult. But modern variants monitor body functions with a microchip, and increase the heart rate where appropriate.
The beauty of the pacemaker is that, given the relative ease with which it can be fitted, it is suitable for patients of almost any age.
While Katie is certainly one of the youngest to be helped by the device, David Henderson, from Grampian, had one fitted in February 1997, at the grand old age of 107.Reuse content