the question of whether to try to exercise when possible with an ARVC/D diagnosis is a difficult one. There are highly respected experts who are convinced that exercise will progress the disease. Some athletes with the diagnosis find it very difficult to give up something which has been an important part of their life. For me it is a bit of a meaningless question because I am too unstable to give it a go. I am still a little annoyed that the "experts" are not able to put forward a convincing case to support their beliefs. In the mean time they are spreading pseudo-science.
As for the ICD, I am very concerned that you do not have one, especially if you do try to exercise. Please, please, please get a second opinion on this. The ICD will most likely be your least concern and your best friend with this condition.
Matt, I am no expert but, it sounds like you are already meeting the Task Force Criteria for ARVC. I was diagnosed with an RV angiogram and biopsy. My MRI was interpreted both ways (positive by the most reputable place, Johns Hopkins U, JHU.) The good news is that you are in very good hands with prof. McKenna. In the mean time, be extremely careful. Like you, I can also tolerate 240bpm, but I would never bank on that. I can put you in contact with someone who, one day, did not survive it. Obviously he had an ICD. Please, be ultra careful. You would not believe the first hand experiences told in ARVD support groups about needless deaths due to lack of ICD in known ARVD patients. My ICD is my guardian angel, flaws and all.
On a simpler not, at this year ARVD patient seminar at JHU, the recomendation not to exercise was strongly reiterated.
I found this thread and got an account to be able to reply. I was diagnosed with ARVC/D two years ago. I am ~50 yo, and by then, I had been fairly active for a long time. Various sports: run, bike, climb, swim ... My peak training hours at 40 were still at about 25hrs/week (summer.) I was never very good, just good enough to place in my age group, sometimes. I was dumb enough to think that all was well if I could hurt my training buddies. I never knew that I was slowly developing a heart condition. I just thought that whatever slowdown there was was due to natural ageing. Then in a 5k in April 09, I felt awful, and three days later could not complete a 10 miler and ended up in the ER with an HR of 250bpm for a couple of hours. A week later I had an ICD, and a few months later an ARVD diagnosis. I very quickly became so prone to VT (ventricular arrhythmia) that any kind of running or biking was out of the question. A year later I got an ablation which at some positive effects, since it kept me from having to go to the hospital every few weeks. I do not yet know if I am positive for any of the known mutations which are thought to contribute to the condition, and my kids are still running on their own will. I have no family history that I know of. I should point out that there is a strong belief amongst "experts" that engaging in strenuous activity when symptoms are present will accelerate the disease progression.