white out after extreme exercise

Last year I was a 70 year old male with a 20 year history of moderate to severe mitral regurgitation; no symptoms or other problems diagnosed by echocardiology and my cardiologist.  I was told that there were no limits on my exercising and the better shape I was in the better the outcome when valve surgery would ultimately be required.  I was very physically fit, 5ft 11 inches and 165 lbs.

I began training for a long-distance bike ride by riding up a long hill.  When I got to the top I felt normally fatigued and got off my bike to rest before riding back down the mountain.  Within a minute or two my vision went white, both eyes.  It was as if an atom bomb had gone off turning everything white.  I could see through the whiteness; putting on my sunglasses helped.  It went away gradually after about a half hour. I wasn't (abnormally) out of breath, dizzy or anything but fatigued.  Simultaneously I tried to take my pulse but couldn't because it was very erratic and very weak.  I'm pretty sure I was in paroxysmal atrial fibrillation.  After another half hour I rode my bike down the hill and 5-miles more to my home.  I had no residual symptoms and continued training with my exercise tolerance unchanged.
A few weeks later I rode a 63 mile bike ride that included the same hill near the beginning of the ride.  I was fine during the entire ride but within a few minutes after finishing the 63 miles the same thing happened, only it was more severe and lasted about 3 hours:  the white out and the irregular, weak, uncountable pulse. I was fatigued but that was normal. There were no other symptoms that I was aware of.  Again the next day and after that there was no change in my exercise tolerance and I continued my training.
I had one more minor episode after the 3rd day of the finale, a 3-day bike ride over the passes around Copper Mountain, Leadville,  Vail, etc.  Again there were no residual symptoms and I continued my normal exercise routines, which included: long rides and hill climbing on my bike, climbing 2 14ers (Colorado's mountains over 14,000 feet), inline skating and various other physical activities; there were no more "episodes".  Then, about 3 weeks after the 3-day ride, one morning I woke up unable to climb a flight of stairs without getting out of breath, and was diagnosed with persistent atrial fibrillation.
The afib and mitral regurgitation led to open-heart valve surgery and a maze procedure, followed by persistent problems with the afib.  I've been treated medically and am back to sinus rhythm and skiing downhill and cross country.  Surgery was October 4th.  This is February 12th and I'm 71 and 15 pounds lighter.
My question is what caused the whiteouts.  What are the whiteouts.

[Frankenite]

Frankenite's commentary has nothing to do with eye disease.

[John C Hagan III, MD, FACS, FAAO]

Given the nature of your problem and it affecting both eye likely it was bilateral amaurosis as a form of severe cardiac caused transient ischemic attack.

NOTE: the literature is starting to identify prolonged maximum aerobic exercise as causing arterial damage reflected in higher than normal coronary artery calcium counts. Also atrial fib is more common in endurance athletes.

You might want to change your exercise to shorter duration, less than maxium exertion and greater frequency. Do your own research, ask your cardiologists.

The researchers werre father/son physicians from Minneapolis and both are marathoners or at least were marathoners before the results of their studies became well known.

JCH MD