The condition is called NeuroCardioSyncope where the person experiences fainting. I just had enough time to slow down before fainting and falling on the roadside. A kind young man stopped and gave the bike and me a ride home where I recovered. The combination of heat,humidity and dehydration contributed.
Actually Neuro Cariac Sycope has little to do with blood, ishemia and isn't related to heart attacks and won't show anything on an EKG.
Have you been tested for this? If not see if you can have a tilt table test done. And even though an EKG won't show this specifically one should be done too, might be an arrythmia.
Oh, and the big triggers for this type of syncope are heat, dehydration and alcohol.
What happens is that when the peripheral vasculature relaxes in response to heat whatever the blood pressure drops slightly. Normally the pressure regulation center in the heart will sense this and increase the HR slightly to continue adequate perfusion. In this case the opposite happens and the HR drops. The resulting lack of blood flow can cause dizzyness and fainting.
There are some medications that can help but the key is to stay hydrated, be careful riding in the heat and avoid alcohol.
On a recent ride I did, a woman fainted while ascending the last big climb. It was fairly hot (for this area anyway) and dehydration very likely played a role. I don't know if it was NCS but she said similar to you, she was riding along and felt "faint", pulled off/unclipped just in time to avoid a really bad crash, and passed out. But, the cool thing about this tidbit is, she got SAGged to the top of the hill, recovered a bit, and finished the ride, which I thought was pretty awesome.
I have a bad habit of dozing off when I drive. Totaled a couple of rental cars by drifting off the road and rolling them into wads. Also rear-ended a minivan in stop&go traffice once. Not sure what the problem is yet... I do notice it doesn't happen if I drive FAST...
Thanks. I did get the tilt table test. It took place in a cool,controlled environment. I thought, this will never replicate when my NCS spells usually take place, in heat with strenous exercise. Sure enough, after about 10 minutes my blood pressure dropped then skyrocketed. I thought the doctor had given me something to cause it. I was on a low dose of atenolol with a lot of salt added to my diet. I am learning that I cannot over extend myself in the heat here in Florida. I have to keep hydrated, and not just water. I too had one of these episodes at the 25 mile point of the Gainesville Cycle Club horsefarm 100. I lay on the ground a few minutes till the dizziness passed, rode to the aid station up ahead, drank plenty of fluids and finished the century ride strongly.