I have the same problem. The lightheadedness comes before the straining. If able to eliminate then lightheadedness clears up.
It goes like this: when you strain, tonus of your parasymphatetic nerves is increased, what causes arterial dilatation, so your blood pressure falls. Heart reacts on this with increased frequency and power of every beat.
The cause may be over-reactivity of your autonomic nerve system, this could be psycho-organic related.
But other causes are possible:
- low blood pressure.
- dehydration. The sign of appropriate hydratioj is urinating at least twice a day, and you should excrete at least 200 ml of the first morning urine. If you're dehydrated, yopu should drink more - water.
- alcohol may contribute to dehydration, well, probably not beer.
- hypoglicemia - this may be constant or only after the meal or exercise.
You may want to visit endocrinologist to check blood glucose and maybe adrenalin and cortisol and aldosteron levels.
You may also have some cigare-free week. Cigars may lower blood pressure, I'd say.