Jan. 3, 2001 IOWA CITY, Iowa -- The drug sildenafil citrate -- better known as Viagra -- causes a dramatic increase in the nerve activity associated with cardiovascular function, especially during physical and mental stress, bolstering recommendations that men with severe cardiovascular disease use caution when taking the drug.
This finding comes from researchers at University of Iowa Cardiovascular Research Center who studied the effect of Viagra on the cardiovascular system at rest and during stressful conditions. The results are published in the Dec. 19 issue of the journal Circulation.
"Little is known about Viagra's effect on the cardiovascular system, particularly during situations when the cardiovascular system is under stress, as it is during sexual activity," said Bradley G. Phillips, Pharm.D., assistant professor in the UI College of Pharmacy. "Recent concern and reports of heart attacks, arrhythmias and even deaths temporally related to Viagra use in patients with heart failure initially raised questions about the drug's effect on the cardiovascular system."
Phillips and other investigators, including Virend Somers, M.D., Ph.D., and Catherine Pesek, D.O., from the UI Department of Internal Medicine, conducted the study. Somers is now a researcher at the Mayo Clinic in Rochester, Minn.
The researchers studied 14 healthy men, ages 25 to 39, who were given a 100-milligram dose of Viagra or placebo on two separate study days. Men who received Viagra on the first study visit were given placebo on their second visit and vice-versa. Neither the participants nor the researchers knew which drugs were being administered on either of the study days.
The researchers took baseline measurements of the subjects' blood pressure rates, heart rates, noradrenaline levels and sympathetic nerve activity (nerve activity that causes blood vessels to constrict). The investigators took similar measurements 30 and 60 minutes after administering Viagra and placebo while study subjects rested.
Immediately following the 60-minute "rest" period, each participant's cardiovascular response was evaluated during stressful conditions, including exercise, mental stress and cold exposure.
Compared to placebo, the researchers found that Viagra resulted in more than a doubling of sympathetic nerve activity and a 30 percent increase in blood levels of noradrenaline. Moreover, they found that sympathetic nerve activity after Viagra increased even more dramatically during stressful conditions.
"It is well recognized that sympathetic nerve activity is already increased in patients with cardiovascular diseases like heart failure and that this high sympathetic activity is detrimental over the long term," Phillips said. "Our study showed that Viagra increases this type of nerve activity at rest and even further during stressful situations. For people with unstable cardiovascular disease, this could be a problem."
The U.S. Food and Drug Adminstration approved Viagra in March 1998 as the first oral pill to treat erectile dysfunction, a condition that affects millions of men in the United States and as many as 100 million men worldwide. Phillips noted that the study findings reinforce recommendations from the American Heart Association and the American College of Cardiology that people with cardiovascular disease should avoid using Viagra. Viagra also should not be combined with other drugs, such as nitrates, Phillips added.
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