Low or high heart rateA heart rate under 50 BPM or over 120 BPM ,there will alert you take a notice or consultant with doctor.A heart rate can be low because of certain medications or if electrical signals are not properly conducted through the heart. Training to be an elite athlete can also lead to a low heart rate. Learn more about low heart rates from the American Heart Association. A high heart rate could be due to exercise, stress, nervousness, alcohol, dehydration, infection, AFib, or other arrhythmia. Learn more about high heart rates from the American Heart Association.