This ECG is from a 36 year old lumberjack who was asymptomatic until one week earlier when be became acutely short of breath and unable to climb trees. He was seen by his local physician who noted atrial fibrillation, heard a systolic murmur and started him on digitalis. His physical examination revealed an irregular cardiac rhythm and a grade 4/6 systolic murmur similar to that reproduced here. It was heard best at the apex and was well heard in the back. How would you interpret this sequence of events?