I watched Sir Magdi Yacoub perform a piggyback heart transplant on a two-and-a-half-year-old girl in 1996, a year after Hannah Clark had her operation. There were more than a dozen gowned and masked figures in the operating theatre when I arrived and I remember being struck by the hush, broken only by Sir Magdi's occasional quiet commands.

Eight hours later, the baby died. Six months of waiting and hoping had ended in tragedy. It was a reminder of the risks involved in the surgery – and makes Hannah Clark's recovery all the more remarkable.

What Hannah's story tells us is that if failing hearts are assisted over a long period, they can revive. The limiting factor is the shortage of donors, especially for babies – because they are growing fast, finding a donor of the appropriate size in the brief time available is a challenge. Piggyback operations overcame this problem. If the donor heart was only required to assist the existing heart, not replace it and take over its function, that widened the potential donor pool.

Today, piggyback operations have been mostly phased out. Instead, children in Hannah Clark's position are supported by "mechanical assist devices" – artificial hearts that can keep them going for a period of time. But the prospect of devices that could be implanted and left in place for years, as Hannah's piggy-back heart was, is still some way off.