Vascular surgeon David Nott was working 24-hour shifts with medical charity Medecins Sans Frontieres (MSF) in Rutshuru when he came across the 16-year-old.
His left arm had been ripped off, either in an accident or as a result of the fighting between Congolese and rebel troops, and was badly infected and gangrenous.
Mr Nott, 52 and a surgeon at Charing Cross Hospital in west London, said: “He was dying. He had about two or three days to live when I saw him.”
He knew that the teenager’s only hope of survival was a forequarter amputation, a huge operation which requires the surgeon to remove the collarbone and shoulder blade.
He had never performed the operation but knew a colleague back in Britain who had.
“I texted him and he texted back step by step instructions on how to do it. Even then I had to think long and hard about whether it was right to leave a young boy with only one arm in the middle of this fighting.
“But in the end he would have died without it so I took a deep breath and followed the instructions to the letter.
“I knew exactly what my colleague meant because we have operated together many times.”
Such an operation, if done in Britain, would require careful planning with every sort of modern medical product on hand if things went wrong.
But in the Congo Mr Nott had just one pint of blood and an elementary operating theatre.
Despite the hurdles, the operation was a success and the teenager made a full recovery. Mr Nott volunteers with MSF for a month every year.
He said: “It was just luck that I was there and could do it. I don’t think that someone that wasn’t a vascular surgeon would have been able to deal with the large blood vessels involved.
“That is why I volunteer myself so often, I love being able to save someone’s life.”