The Nightshift Belongs to the Stars

It's a story about a man and a woman. He is single and alone. She is married. Both are mountaineers, and survivors of heart surgeries. Beating in his chest is the heart of a young woman, while she got a new mitral valve. Fate and a promise have bonded them. To celebrate recovery, they'll go climbing. A short story where the heartbeat gets faster, only to find, at the end, a steady rhythm.