SD, my husband had this issue. The called it SVT supraventricular tachycardia. They used an IV medication to bring it down. A couple times they also pushed another med (sorry, can’t remember the names) and waited for a pretty immediate response. They had the crash cart ready to use to shock his heart into normal rhythm if the meds weren’t working (fortunately they did).

The plan was that he would have cardiac ablation down the road. We never got that far because of the Covid though.

My father had cardiac ablation at one point for a different cardiac issue. It did improve things.

I hope your dad responds and they find a solution for the tachycardia. It’s a lot. Keep us updated.