So what is heart rate variability (HRV) and what difference does it make whether or not certain games affect it? HRV is a measure of the normal cardiac fluctuations that occur during respiration. Your heart beats a little faster when you inhale and a little slower when you exhale. HRV is also used to monitor vagal activity (the vagus nerve runs from the brainstem throughout the abdomen, and controls the heart among other things). Excessive activation of the vagal nerve due to stress (for example, shooting a bunch of aliens) can cause a vasovagal syncope (a seizure-esque episode) and, in extreme cases, loss of bladder control. That's right, too much stimulation from a violent video game can, theoretically, make you pee your pants.