Atrial fibrillation (AF) often starts with short episodes of rapid irregular heartbeats that are only detected by implanted pacemakers or defibrillators. Usually people don't know that they have these episodes. Over time, these episodes can happen more often and last for longer periods. In some people, the heart eventually remains permanently in a fast irregular rhythm, known as atrial fibrillation. This condition can lead to strokes and blood clots. If physicians could detect atrial fibrillation at a very early stage they could treat people early and possibly prevent the condition from becoming permanent. People with implanted devices allow a unique opportunity to constantly monitor the heart rhythm so investigators can detect any irregularities immediately. Investigators don't know which people are at risk of developing short episodes of fast irregular heartbeats that can lead to atrial fibrillation. The purpose of this study is to find out if there are proteins or chemical changes in the blood that can predict the risk of developing atrial fibrillation. These chemical changes, also known as biomarkers, may also be able to give investigators other clues about atrial fibrillation.