Just show him what I'm experiencing. Sometimes it's hard to describe in words; try saying this to your cardiologist: "my heart goes thump-thump and then pauses for a second and then goes thump-thump-thump in rapid succession"
You have to know if you have afib though, which can be tough to detect for people with infrequent and self limiting cases.
Example; I get bigeminy PACs, which apple watch flags as possible afib when there is noise that obscures the p-wave. But my cardiologist has seen the pattern on a more sensitive device and says it’s not afib.