Myocarditis is the name given to inflammation of the heart muscles, or “myocardium”. The condition — which typically results from viral infection — can serve to reduce the heart’s ability to pump blood and result in the development of a rapid or irregular heart rhythm. It often occurs in association with pericarditis, an inflammation of the fibrous sac that surrounds the heart that can trigger sharp chest pain, shortness of breath and palpitations.

https://www.express.co.uk/news/science/1598594/vaccine-news-pfizer-modern-heart-problems-myocarditis-pericarditis

https://jamanetwork.com/journals/jamacardiology/fullarticle/2791253