An implantable cardioverter defibrillator (ICD) is a small electronic device that monitors and regulates heartbeat, and many have been implanted in patients — an estimated 114,000 in the U.S. in 2006. Part of the implantation process involves ventricular defibrillation testing, which temporarily disrupts brain activity by causing a drop in blood pressure and blood flow to the brain. In a study involving 52 patients having cognitive tests several days before ICD surgery and again six weeks and six and 12 months afterwards, more than a third of participants had significant cognitive problems six weeks and six and 12 months after ICD surgery. Attention, short-term memory of visual words and objects, and auditory (spoken) words were most commonly affected. Although most patients regained their normal abilities by 12 months after surgery, a few (10%) first developed difficulties at that point. The results were unrelated to measurements of anxiety, depression and quality of life.