Got back from the doc and told him about the tunnel vision and tingling in the hands and arms that happened twice last month during the hard part of my ride.
He said the my bp and pulse are low for a normal person, but not for a trained athlete (which I am not, just a commuter). BP of 117/59 with a pulse of 58. He said that I needed to increase, just slightly, my salt intake which he said should help the water retention which in turn should stabilize my BP during exertion. In other words eat a handful of pretzels or dilute some Gatorade throughout the day.
So just remember ladies and gentlemen that the summer is upon us and hydrate properly with electrolytes, potassium, and other good things lest you find yourself blacked out and under a car tire.
FWIW, I have tried Nuun and Gu electrolyte. I like them both, but Gu has a better flavor and did not upset my stomach. Nuun does not always upset my stomach, but enough that I would go to Gu.