Low blood pressure is a condition in which the force of the blood pushing against the artery walls is too low. It's also called hypotension. Blood pressure is measured in millimeters of mercury (mm Hg). In general, low blood pressure is a reading lower than 90/60 mm Hg. What's considered low blood pressure for one person might be OK for someone else though. Low blood pressure might cause no symptoms that you notice. Or it might cause dizziness and fainting. Sometimes, low blood pressure can be life-threatening. The causes of low blood pressure include dehydration and other serious medical conditions. It's important to find out what's causing low blood pressure so that it can be treated, if needed.TypesTypes of low blood pressure include: Orthostatic hypotension, also called postural hypotension. This is a sudden drop in blood pressure when standing up after sitting or lying down. Causes include dehydration, long-term bed rest, pregnancy, certain medical conditions and some medicines. This type of low blood pressure is common in older adults. Postprandial hypotension. This drop in blood pressure occurs 1 to 2 hours after eating. It's most likely to affect older adults, especially those with high blood pressure. The risk also is higher for older adults with Parkinson's disease or other conditions that affect the part of the nervous system that controls automatic functions such as breathing and heart rate. This is called the autonomic nervous system. Certain lifestyle changes might help ease symptoms of postprandial hypotension. Eat small, low-carbohydrate meals, drink more water, and stay away from alcohol. Neurally mediated hypotension. This is a blood pressure drop that happens after standing for long amounts of time. This type of low blood pressure mostly affects young adults and children. It might result from communication trouble between the heart and the brain. Multiple system atrophy with orthostatic hypotension. This rare condition affects the autonomic nervous system. Multiple system atrophy with orthostatic hypotension is linked with having very high blood pressure while lying down. It also used to be called Shy-Drager syndrome.