This question depends on the person. Usually by around 200 to 210, your heart begins to go into an abnormal rhythm or disrythmia, called VT or PVT. The respectively, stand for Ventricular Tachycardia and Pulseless Ventricular Tachycardia (PVT is the more dangerous variety). At around this stage a patient runs the risk of cardiac arrest, which is dangerous because while the heart continues to beat, the contractions are so fast, blood isn't being let into the chambers of the heart. Without blood actually being pumped, the person is unable to perfuse, and goes into shock, seizure, coma, and then death.
