I lacked a beating heart for a relatively short time, but it was still a transformative event in my life (naturally!) - just a routine aortic valve replacement surgery, wherein they stopped my ticker, putting the circulatory duties on a heart-lung machine, whilst they opened my chest and sneaked in some new tissue. A nurse assured me this was totally routine but if you ask me it was totally miraculous. Six years later I am still active and vigorous (for a geezer, that is).