1. Anatomy of the cardiac conducting system
Under normal conditions, the pacemaker function of the heart is located in the sinoatrial (SA) node, which lies at the junction of the right atrium and superior vena cava. Cells of sinus node are modified cardiac myocytes but they possess some contractile filaments, and do not contract. The SA node is approximately 1.5 cm long and 2 to 3 mm wide and is supplied by the sinus node artery, which arises from either the right coronary artery (60%) or the left circumflex coronary artery (40%).
The impulse exits the sinus node and perinodal tissue; it traverses the atrium until it reaches the atrioventricular (AV) node. The blood supply of the AV node is derived from the posterior descending coronary artery (90%). The AV node lies at the base of the interatrial septum just above the tricuspid annulus and anterior to the coronary sinus. The electrophysiologic properties of the AV node result in slow conduction, which is responsible for the normal delay in AV conduction, or in the PR interval.
The bundle of His emerges from the AV node, enters the fibrous skeleton of the heart, and courses anteriorly across the membranous interventricular septum. It has a dual blood supply from the AV nodal artery and a branch of the anterior descending coronary artery. The branching (distal) portion of the bundle of His gives rise to a broad sheet of fibers that course over the left side of the interventricular septum to form the left bundle branch and a narrow cable-like structure on the right side that forms the right bundle branch. The arborization of both the right and left bundle branches gives rise to the distal His-Purkinje system, which ultimately extends throughout the endocardium of the right and left ventricles.
The normal sequence of electrical activation of the chambers of the heart is called sinus rhythm. It occurs each time the heart beats, usually about 60 to 80 times every minute. In a normal heartbeat, the atria contract simultaneously while the ventricles relax.
Then, the ventricles relax and the atria contract. The term systole refers to contraction and the term diastole refers to relaxation. A heartbeat consists of the systole and diastole of the atria and the systole and diastole of the ventricles.
Despite the heart’s ability to maintain its self generated beat, there are times when the heart needs to adjust in response to the varying demands of the physical human body. This is controlled by intervention of the autonomic nervous system.
Both the atrioventricular node and the sinoatrial node are innervated with sympathetic and parasympathetic innervations. When the heart responds to the increased needs of the body, as well as its own increased need, this is known as sympathetic stimulation. It is marked by the acceleration of the heart rate and dilation of the coronary arteries. Parasympathetic stimulation refers to the heart’s ability to slow down in response the decreased needs of both the body and the heart. Fibers from the cervical as well as the upper thoracic ganglia are responsible for the sympathetic innervation. Branches of the vagus nerves stimulate the parasympathetic innervation. The sinoatrial node is innervated by the branches coming from the right vagus nerves while the branches from the left vagus nerves are responsible for the innervation of the atrioventricular node.