A pulse rate is considered dangerously low when it causes insufficient blood flow to the brain, with symptoms such as fatigue, dizziness, fainting or cardiac arrest. As the American Heart Association notes, the condition is defined as bradycardia.
According to Mayo Clinic, a healthy pulse rate during rest is accepted to be between 60 and 100 beats per minute. However, as WebMD explains, bradycardia--a pulse rate below 60 beats a minute--isn't always dangerous. The pulse or heart rate is simply an indicator of how hard a person's heart is working, and dangerously low pulse rates vary significantly among individuals based on their overall fitness, hormonal needs, medical conditions or medications.