When a tiny baby’s life hangs in the balance, a heart surgeon’s emotional, physical and intellectual strengths are tested to the limit. In an extract from his new book, surgeon Stephen Westaby talks through a pioneering operation and its far-reaching consequences – not just for the child’s family.

The finest of margins separates life from death, triumph from defeat, hope from despair – a few more muscle cells, a fraction more lactic acid in the blood, a little extra swelling of the brain. The Grim Reaper perches on every surgeon’s shoulder and death is definitive. There are no second chances.

Woody Allen famously said, ‘The brain is my second favourite organ.’ I had the same affinity with the heart. I liked to watch it, stop it, repair it and start it up again, like a mechanic tinkering with an engine beneath the bonnet of a car.

What is so fascinating about the human heart is its movement. The rhythm and efficiency of the thing. The heart beats more than 31 million times in a year, pumping 6,000 litres of blood daily to the body and lungs. Each heart is different. Some fat, some lean, some thick, some thin. Just never the same.