heart and blood vessel disorders
Overview of Heart Block
Normal electrical pathway
Contraction of the muscle fibers in the heart is controlled by electricity that flows through the heart in a precise manner along distinct pathways at a controlled speed. The electrical current that begins each heartbeat originates in the heart’s pacemaker (called the sinus node or sinoatrial node), located in the top of the upper right heart chamber (right atrium). The rate at which the pacemaker discharges the electrical current determines the heart rate. This rate is influenced by nerve impulses and by levels of certain hormones in the bloodstream. The electrical current from the sinoatrial node flows first through the right atrium and then through the left atrium, causing the muscles of these chambers to contract and blood to be pumped from the atria into the lower heart chambers (ventricles). The electrical current then reaches the atrioventricular node, located in the lower part of the wall between the atria near the ventricles. The atrioventricular node provides the only electrical connection between the atria and ventricles. Otherwise, the atria are insulated from the ventricles by tissue that does not conduct electricity. The atrioventricular node delays transmission of the electrical current so that the atria can contract completely and the ventricles can fill with as much blood as possible before the ventricles are electrically signaled to contract. The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract. After passing through the atrioventricular node, the electrical current travels down the bundle of His, a group of fibers that divide into a left bundle branch for the left ventricle and a right bundle branch for the right ventricle. The electrical current then spreads in a regulated manner over the inner surface of the ventricles, from the bottom up, initiating contraction of the ventricles, which eject blood from the heart. 