id explain myself but i feel this article and research explains it better
Unfortunately, the action that Viagra has on nerve activity that causes relaxation of the smooth muscles in the penis, also seems to increase sympathetic nerve activity (the nerve activity that causes blood vessels to constrict). There have been several reports of heart attack, heart arrhythmia and even deaths that have been related to victims who were using Viagra.
A study published in the Dec. 19 issue of the professional journal Circulation looked at this nerve activity, focusing on its effect on heart function especially during physical and mental stress - exactly the kind of stress a man could encounter during sexual activity.
This study comes from the University of Iowa Cardiovascular Research Center. The researchers looked at the effect of Viagra both at rest and during stressful situations. The study participants were 14 healthy men, with no documented heart disease, between the ages of 25 and 39. Baseline measurements of blood pressure, heart rate, noradrenaline levels and sympathetic nerve activity were measured. During the study, the research subjects were given 100 mg tablets of Viagra or a placebo and then have a 60-minute rest period. The measurements were repeated at 30 and 60 min. After the rest period, each participant's cardiovascular response was measured during stressful situations which included exercise, mental stress and cold exposure.
The researchers found that study participants who took Viagra had more than double the sympathetic nerve activity and a 30 percent increase in blood levels of noraderenaline. Sympathetic nerve activity increased even more dramatically during stressful conditions.
Patients had increased sympathetic nerve activity. The high and sustained sympathetic activity caused by Viagra during rest and stress is damaging to the heart and heart function over a long period.
hope this helped
also should mention people with pre exisiting heart conditions, or cardio vascular complications are not reccomened to take viagra due to its effects on the heart and blood pressure, especially those with cardio vascular disease