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Most people can recall feeling suddenly lightheaded, perhaps even fainting, when they suddenly stood up after lying down or sitting for a long time. Perhaps they have had a similar episode brought on by standing still for a long time. This feeling of temporary lightheadedness is called orthostatic hypotension (low blood pressure) or vasovagal syncope (SIN-ko-pee - fainting).
Othostatic hypotension means that the blood pressure in the arteries of the brain has suddenly dropped. This happens because when the body suddenly assumes the vertical stance, gravity forces blood to tend to pool in the legs and lower abdominal veins. The heart works by pumping all the blood that returns to it; if the blood return to the heart is reduced, the pumping output is decreased and thus the blood pressure falls. The brain is exquisitely sensitive to this drop in pressure and begins to malfunction. The cure is for one to assume the horizontal position, which increases the blood return to the heart and improves blood flow to the brain. Voila! Clarity of thought returns!
While it can affect anyone past puberty, this effect is most common in teenagers, whose blood vessels are quite elastic, which promotes pooling of blood. Teens may report this phenomenon happening after they have been lying on the couch watching television for a long time, or upon arising from bed in the morning, or perhaps they may feel faint while standing in the August sun during marching band practices (during which practices there is a lot of standing around in relation to the amount of marching).
While orthostatis, as it is called, is fairly harmless (unless the patient suffers a fall and sustains injury), it is certainly a nuisance. The usual counsel is to simply be careful about jumping up suddenly from a seated or recumbent position. If the patient will sit up first with the feet on the floor for a few seconds, and then arise to standing, there is usually no light-headed feeling. Some cases are not helped much by this manuever, however; salt retaining hormone and extra dietary salt is sometimes prescribed to increase the blood volume and lessen the frequency and severity of the attacks.