Az's heart murmur is quite noisy now, it seems, and he felt that Az having slowed down this year, being more of a trotter than a runner most of the time, is probably due to that rather than more general old age or decay. (Az is possibly 11-12ish: we aren't sure exactly, but he was an adult when he was found.)
I wasn't quite expecting that - I tend to think of Az as 'the healthy one' - so decided to go away and research putting him on the suggested Vetmedin* treatment in detail before making any decisions. The first step would be an ultrasound scan to work out exactly why his heart is making so much noise, and whether what is wrong with it is something that is likely to benefit from treatment. If it is a problem likely to be helped by Vetmedin, then it might extend his life and make him more active**.
One thing the vet did say was that although his heart is noisy, the amount of noise a heart makes isn't really an effective measure of how well it is working. The only thing that tells you that is symptoms. And the only symptoms he has so far are not being quite such a mad chasy loon. He does not cough, vomit or have the runs.
*apparently there have been two recent major trials of heart medication in dogs in the last couple of years, testing the 2 major treatments on the market, one of which is Vetmedin (I think the other might be Frusemide?) Anyway, apparently both companies ran tests, but only Vetmedin published the results.
**it is, undeniably, easier to have a slightly less active Az than a very active one. But is it fair on him...? Hmmm.