I am so sorry to hear this. But I would not become desperate. First: let's see whether his pulse is as bad at home as it was at the vet's. I know that MY blood pressure is higher when I'm at the doctor's and I am sure deep inside every kitty the vet is time for panic.
Second: I know at least one kitty here who gets beta blockers since about 5 years and is just fine.
Again: all my best wishes for you and Phinehas.