Try this: Vasovagal syncope, which is accompanied by a fall in BP and often slowing hr, always has warnings which consist of nausea, feeling hot, becoming lightheaded, and visual changes such as darkening. At the first sign, lie down. If possible, elevate your legs. There are two benefits: it will raise your BP and avoid injury if you do faint. Other hints: always eat breakfast. Betablockers help.
Prevention! You can often avoid fainting by listening to your body and sitting or lying down. Isometrics such as making a hard fist or grasping your arms and pulling hard may ward off a faint. Some experts advocate beta blockers taken daily but there is little evidence to support this.