There are medications that can selectively block the action of either sympathetic or parasympathetic influences on the heart. how could such medications be used to determine

Both sympathetic and parasympathetic systems may be stimulated or inhibited by medication. The sympathetic nervous system increase heart rate and blood pressure by allowing the body for a fight or flight response while the parasympathetic nervous system slow the heart rate and blood pressure.   Therefore, sympathetic systems increase the heart rate and decrease its inhibition. Meanwhile, the parasympathetic systems responds the other way.

Answerbasal cell carcinoma;