Medical Q&As

Weakness - low blood pressure?

I have become weak and almost fainted a couple of times recently, while standing on a crowded bus early in the morning. A friend commented that I should get this checked out as it may be low blood pressure. Is this anything to worry about? Are there dangers associated with low blood pressure? I felt fine again as soon as I got off the bus and got some fresh air.

I suspect that you are perfectly healthy but if you really are worried about the state of your blood pressure then that concern can be readily addressed by having your blood pressure measured next time you visit your GP. I think that you are getting weak on the bus because you are standing and also because the bus is probably full of passengers and consequently it is very warm. When you are standing for a long period of time blood can pool in your lower legs resulting in a reduced volume of blood being available to flow to the brain. This is simply due to the cardiovascular system being affected by the forces of gravity and is not due to a disease process. You can overcome this effect to some degree by exercising your calf muscles while standing thereby using the muscle pump effect, which means that the squeezing of the calf muscles assists the flow of blood back up the legs. This is an old trick performed by soldiers when standing to attention for extended periods of time. Soldiers will often make their calf muscles quiver when standing to attention and this assists the muscle pump. If you move your legs periodically and donít stand rooted to the one spot this should help avoid the weakness. Heat in the bus may also be a factor and your body may be struggling to maintain normal body temperature in the warm environment. When the body becomes too warm the circulatory system diverts blood from the core of the body to the body surface. This automatic physiological reaction helps to drop the temperature of the blood thereby cooling the body. However, if a large volume of blood is diverted this can create a situation analogous to the pooling of blood in the legs. In other words there is less blood available to supply oxygen and glucose to the brain. Another possibility is that if you miss breakfast and travel to work on an empty stomach this can lead to hypoglycaemia or low blood sugar, which can also trigger faintness.