I've had high blood pressure for a couple of years now and when I finally realized I needed to go on medication to control it, I was also just getting into the longer bike rides. My doctor and I tried out several medications and dosages to try to find what worked best -- this took months to work through -- and during that time, I figured out the days that I went on long bike rides, my blood pressure was much lower than on days that I didn't ride. I had to buy a digital blood pressure cuff to monitor my pressure at home every day, sometimes two or three readings a day.

Those really low days, I was having the same symptoms as you, nearly blacking out when I'd stand up. Everything would go a bit yellow and fuzzy for several seconds until it regulated itself.

My doctor told me at one point that on the days I planned a long ride, not to take my meds. That worked for a bit. We finally hit on a good dosage for me and now my blood pressure is in the same range as yours, 100/60.

There are others here who can tell you the particulars, but low blood pressure is just as dangerous as high blood pressure. At least, that's what I understood from conversations with my doctor.

I really think you should check this out with your doctor before it goes on too long to repair whatever damage might be happening.

Roxy