Third-degree block, also known as complete heart block, is a serious condition where the electrical signals between the heart's upper chambers (atria) and lower chambers (ventricles) are completely interrupted. This means that the atria and ventricles beat independently of each other, which can lead to a slow heart rate and reduced blood flow to the body. Symptoms of third-degree block may include fatigue, dizziness, and fainting. It can be caused by various factors, such as heart disease, certain medications, or damage to the heart from a heart attack. Treatment often involves the implantation of a pacemaker to help regulate the heart's rhythm.