What the title said!

The Substitute by Tionne Rogers or even the noncanon novel Into the lion den by the same author.
Guntram have a serious heart problems in both.

Are you referring to dramatic novels? If so, this one is highly recommended:

https://www.goodreads.com/series/135993-surf-bay

[Weelee]

Palpitations or Love sick ?

Cate Ashwood has a really good one: Broken Hearted (Hope Cove #1). Especially interesting since it deals with an undiagnosed genetic cardiac condition that the protagonist only learns about when his sister (extreme athlete and excellent swimmer) drowns.

@WeeLee

Palpitations.

@sagit61

Thank you so much for the rec, I'll definitely check it out.

[Miss A]

Buzz by E. Davies
It's also a factor in the next book. Collectively part of the Riley Brothers series.