I got excellent news from the cardiologist today. The nuclear stress test showed all healthy heart tissue and vessels, and normal ventricles. That's good news. While there's still no explanation for my recent "event", the next step is watchful waiting rather than surgery. More good news. He told me to go back to the gym (with a couple of restrictions) for some mild exercise and to schedule follow-up in 2 months.
On one hand this all sounds rather tentative, but on the other hand waiting is not all bad. The longer things work OK and remain intact the better off I'll be. All told, it's an excellent early Christmas present.