Research on the medical emergencies that may occur in space include fatal and nonfatal arrhythmia, heart attacks, cardiac arrests, embolisms, massive hemorrhages, renal stone formations, fatal and non-fatal infections, and thrombotic complications. Of these conditions, only arrhythmia, renal colics, and infections have occurred in the history of spaceflight. The arrhythmia cases included occasional premature atrial contractions (PACs) and premature ventricular contractions (PVCs), which happened to 30% of astronauts at some point during periods of intense physical activity.