A new study led by a Canadian research team has identified the reason why prazosin, a drug commonly used to reduce high blood pressure, may cause lightheadedness and possible fainting upon standing in patients with normal blood pressure who take the drug for other reasons, such as the treatment of PTSD and anxiety.
According to University of British Columbia researcher and study team leader Dr. Nia Lewis, the body is in constant motion leading to changes in blood pressure with every activity. For example, when standing, the body copes with the sudden drop in blood pressure by constricting peripheral vessels to concentrate the blood in the areas that help stabilize the body.
This study found that prazosin prevents this process by blocking the α1-adrenoreceptor, a critical pathway that allows the vessels to constrict. This physiological response is dangerous for individuals with normal blood pressure who take prazosin to treat the symptoms of PTSD and anxiety, for the act of standing up can cause light-headedness and/or fainting.
The study, entitled "Initial orthostatic hypotension and cerebral blood flow regulation: effect of α1-adrenoreceptor activity," (http://bit.ly/104sWJE) is published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology.
Eight males and four females, with an average age of 25, and all of whom had normal blood pressure, were enrolled in the cross-over trial. On day one of the study, participants were weighed, measured, and familiarized with the blood pressure monitoring equipment and procedures that would be used.
On the next visit, participants stayed overnight at the research facility in order to control for activity and diet. The following morning they were given either prazosin (1mg/20kg body weight) or a placebo, and instructed to lie down. After 20 minutes, they were told to rise in one smooth motion from the lying-down position to standing, and their blood pressure and cerebral blood flow was continuously monitored. They were required to remain standing for three minutes or until they felt severe lightheadedness and dizziness, or felt as if they were about to faint.
On their third and final visit the participants underwent the same procedure as on the second visit. At this visit, however, they received the placebo if they had previously been given the medication, and vice versa.
The investigators found that:
•All but one of the 12 participants who took the medication experienced temporary dizziness or lightheadedness upon standing.
•All participants who took the placebo were able to complete the three-minute standing test. By contrast, only 2 of the 12 were able to complete the standing test after taking prazosin.
•After taking prazosin, none of the participants were able to attain normal blood pressure levels after standing. As a result, blood flow to the brain was reduced and subjects were unable to stand for 3 minutes as they began to experience the onset of fainting.