by Y Tian

Created on: April 10, 2009

Have you ever suddenly stood up and felt dizzy and light-headed, had dimmed vision or even fainted? You probably shrugged it off, claimed that it was a dizzy spell or a head rush and then just forgot all about it until the next episode. These symptoms are all caused by an event clinically known as postural hypotension (also referred to as orthostatic hypotension) caused by a sudden drop in your blood pressure. So why does postural hypotension cause these symptoms? What causes it to occur? And what are the possible lifestyle adjustments to reduce chances of future episodes? To understand the process of postural hypotension, it is important to firstly visualise what normally happens within the human body.

The body's blood pressure is constantly being monitored and adjusted accordingly. Firstly, any changes in blood pressure is picked up by sensors known as baroreceptors situated strategically in both the vessels leading to the brain and in the major arterial trunk before it gives off branches to supply to the rest of the body. That way, the blood pressure throughout the body is adequately and quickly sensed. These signals are sent to an integrating centre, which is a cardiovascular control centre located in the medulla within the brain stem. Based on the signals, the integrating centre then alters the ratio between sympathetic and parasympathetic activity to the heart and blood vessels, thus bringing the blood pressure back to homeostasis.

Under normal circumstances, when a person suddenly stands up and blood pools to the lower legs due to gravity, the drop in blood pressure is immediately sensed by the baroreceptors which triggers the blood vessels to constrict and thus maintain an adequate blood pressure. Consequently, the person would not suffer from any of the symptoms described above. This compensatory mechanism is known as the baroreceptor reflex. In postural hypotension, however, this mechanism fails to fully compensate for the drop in blood pressure. Thus clinically, postural hypotension is adequately defined as "a transient hypotensive condition resulting from insufficient compensatory responses to the gravitational shifts in blood that occur when a person moves from a horizontal to a vertical position, esp. following prolonged bed rest."

The last phrase of the previous sentence accounts for the fact that during prolonged bed rest, the compensatory responses to restore the blood pressure is depressed so that for bedridden patients, it is very common for them to suffer from postural hypotension during their first attempt to stand up. Other risk factors include the elderly, hypovolemia and certain diseases such as Addison's disease or atherosclerosis.

The resultant drop in blood flow to the brain is responsible for the dizziness or actual fainting that occurs. Thus, simply standing up slowly to allow the body to adjust slowly to the gravitational shifts in blood will be able to prevent postural hypotension. Maintain an adequate fluid and electrolyte balance will also help reduce the risks. If there is suspicion of an underlying disease, it is important to seek professional medical advice.

In conclusion, postural hypotension is a common event that happens in most people. While it is mostly harmless, it is good to take precautions to reduce the chances of postural hypotension from occurring.

Reference:

Harrison, T. et al. (2005) Harrison's Principles of Internal Medicine, 16th edition. McGraw-Hill Companies, Inc.

Sherwood, L. (2004) Human Physiology: From Cells to Systems, 5th edition. USA: Thomson Brooks/Cole.

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