Hearts with atrial fibrillation are notoriously inefficient. Imagine for a moment a band where the fiddle plays in one key but the guitar and bass are in different keys. The result is not good! Such is the case with atrial fibrillation.
Hearts in atrial fibrillation are unable to pump enough blood to adequately supply the body with enough blood to stay healthy and well oxygenated. The inefficiency is the result of an irregular -- and sometimes too-rapid -- heart beat caused when the upper chambers of the heart, or atria, are out of sync with the ventricles, or lower chambers of the heart.
Symptoms
Atrial fibrillation is not a heart attack or heart failure and it’s generally not life threatening. However, the symptoms can be bothersome and may include conditions such as shortness of breath, general weakness, heart palpitations, low blood pressure, lightheadedness, chest pain, and confusion.
https://www.empowher.com/atrial-fibrillation/content/atrial-fibrillation-overview