ICD for heart failure

My husband is 74 years old, exercises vigorously, and has doneso for his entire life.  He eats well and has good cholesterol numbers, and is a healthy weight/nonsmoker.  Takes medication for blood pressure only; bp is under control.May 2011 had normal EF rate, but b/c of persistent low heart rate (30-ish) had dual pacemaker installed.Had no contact with doctors until October 2011 when undergoing heavy physical labor, felt tightness in chest.  Heart was racing at top speed.  EF was 25.  Pacemaker was turned down or adjusted, he was given beta blocker, told things would go back to normal within a few months.Last week EF still 25.  Doctors recommend CRT with defibrillator, say he can continue vigorous exercise.  Say not to do the heavy, heavy stuff he was doing – like building & repairing sheds, fences by himself – but can keep going walking three 14 minute miles daily.Went to see other doctor for second opinion.  Said not to exercise so vigorously, cut about in half in pace and length, not to lift weights or build or repair fences, sheds – all things he did while maintaining rural property.My questions are two:  What do you think caused the loss of EF?  The exercise?  Could that or heavy building/work/carrying heavy objects have caused the loss of EF?  Or did the pacemaker possibly do it?  It looks like he could have fallen through the cracks with the re-checks – we didn’t know he should have any until much later – are NOT interested in ANYTHING but health improvement, and not repeating past mistakes – just want to prevent further EF loss, and to regain function.  Do you think he should continue same exercise regimen or cut in half length and pace like second doctor recommended?#2 question – at this point, should he have the recommended CRT with defibrillator?