US songstress Demi Lovato adds "magic" to tracks, says a top music producer.

Jason Evigan worked with the star on her single 'Heart Attack, which wasn't initially intended for her.

However, the producer says Lovato made the track her own when it came to laying down vocals in the studio.

Speaking to MTV News, he explained: "The first version had a dubstep break in it. We took that out for her ... Demi has magic to it where she has so much sass in her voice but yet so much power and all those really high notes at the end, she added that on the spot. [Some of those notes are] all freestyle and like that, I feel like, took the song up notches."

'Heart Attack' was originally planned for singer Nikki Williams, before being offered to former 'American Idol' contestant Pia Toscana.

Evigan says the track was meant for Lovato though.

He said: "A lot of people wanted the song. And then Demi came in to do another song called 'Two Pieces' and then we played her 'Heart Attack' and she freaked out about it. So she came in to record it and then she actually had a couple of lyric changes that were really good. She added that super high note at the end and she added some really cool things to it and she brought that song to life. Now it's taking off."