I've only recently heard of the device called the Stress Eraser (StressEraser), which claims to help alter a person's breathing in order to reduce stress. The Stress Eraser Web site says that it can reverse the effects of "ergotropic tuning," which it says is "a biological process that changes the way the nerves in your body respond to stress." The idea is that their device can give a person feedback on how to adjust his or her breaking to reduce ergotropic tuning, and thus stress.
Although I'm not familiar with this device, it reminds me of another device that claims to help lower blood pressure by helping the user adjust breathing rate. That device, called the RESPeRATE®, is a small, portable electronic device that guides the user to lowering the number of breaths they take per minute, which is said to lower blood pressure naturally. The RESPeRATE Web site has positive comments from the Mayo Clinic and from Dr. Andrew Weil. According to their Web site, the RESPeRATE breathing exercises work by "relaxing the constricted muscles surrounding the small blood vessels, allowing the blood to flow more freely and yielding a significant and lasting reduction of blood pressure." RESPeRATE has been approved by the FDA.