I've consulted with 3 app dictionaries, since it's not preferable to post image in this site, I try my best to provide the text for convenient reason.
Palpitations (noun) plural
(1) A physical condition in which your heart beats very quickly and in an irregular way.
• Too much caffeine can cause heart palpitations.
[CALD 4th ed.]
(1) A condition in which your heart beats too quickly or not regularly.
• He ended up in hospital with heart palpitations.
(2) have palpitations (HUMOROUS): to be very shocked:
• My mother will have palpitations when she sees my new boyfriend.
(1) A rhythmic expanding and contracting.
• A palpitation of the blood vessels.
As you can see there, three dictionaries have similarity definition in the first sense. However, the example from Merriam-Webster makes me confuse whether I should use singular or plural.
Some words I've known such as trousers and scissors are always plural and I've read it somewhere, probably from Vocabulary in Use (I don't really remember) that we can't singularize those plural i.e. we have to use the plural form. Then, why does M-W use the singular form? Is it another sense?