A dangerous heart rate refers to an abnormal or irregular heart rhythm that poses a serious risk to your health. Typically, the heart beats at a rate of 60 to 100 beats per minute (bpm) when at rest. A heart rate that is consistently too high (tachycardia) or too low (bradycardia) can be life-threatening. Tachycardia occurs when the heart rate exceeds 100 bpm, increasing the risk of heart attack, stroke, and sudden cardiac arrest. Bradycardia, a heart rate below 60 bpm, can cause fatigue, dizziness, and fainting due to insufficient blood flow.