It would take a lot of air very fast in your actual veins to cause an issue (to cause stroke or stop heart). Injecting bubbles in your fat or muscle won’t do much. In fact if you did, it would eventually just absorb. It’s when jt hits the blood stream itself that causes an issue. But even then has to be a lot of air and quickly.
However, don’t do this:
https://www.ncbi.nlm.nih.gov/m/pubmed/11453096/
Basically she died masterbating via carrot by injecting air in her vagina.