The former Guns N' Roses star said while performing with Michael Jackson in New York in 2001 he got worried that his newly-fitted pacemaker couldn't take the strain.
He teamed up with the King of Pop to play at his 30th Anniversary Concert at Madison Square Garden celebrating Jackson's solo career.
Slash, who had recently undergone surgery to fit a pacemaker to his heart, said the star-studded show made him ill - but he was determined to finish his performance with the Thriller hitmaker.
He said: "On stage I was suddenly hit in the chest by a shock and my vision flooded with electric blue light. It happened about four times each song.
"I had to stand there and make it look as if everything was normal. I saw it later on TV and you couldn't tell - so I pulled it off."
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