Conduction system basics and sinus rhythm
To learn some basics on the conduction system it is necessary to know some basic anatomical structures of the heart. In the following post I use a schematic drawing of the heart which includes the following anatomical structures:
SVC = superior vena cava; IVC = inferior vena cava; RSPV = right superior pulmonary vein; RIPV = right inferior pulmonary vein; LSPV = left superior pulmonary vein; LIPV = left inferior pulmonary vein; TV = tricuspid valve; MV = mitral valve; CS = coronary sinus.
The cardiac conduction system consists of specialized cardiac tissue / cardiomyocytes that are able to depolarize and conduct the electrical impulse. The intrinsic pacemaker of the heart is the crescent shaped sinus node (SAN = sinuatrial node). Sinus nodal tissue spontaneously depolarizes at different heart rates, influenced by sympathetic / parasympathetic input.
The electrical impulse then propagates via intraatrial tracts throughout the right atrium, and passes over to the left atrium via three passages: the main passage being the Bachmann bundle, located septal at the roof of the right atrium. The other two passage points where the electrical impulse can enter the left atrium are the fossa ovalis and the coronary sinus. The latter two playing an inferior role for transmission.
The level of the atrioventricular valves (TV & MV) works as an insulation within the heart, separating atria and ventricles. Usually, there is only one point where the electrical impulse can travel from the atria to the ventricles - the bundle of His.
Before reaching the bundle of His, the impulse travels through the atrioventricular node (AVN), which functions as a "gatekeeper". The AVN slows conduction to the ventricles and protects them from very high atrial rates (e.g. atrial fibrillation).
Once passed through the AVN and bundle of His, the electrical impulse travels via the right bundle branch, left anterior and left posterior bundle branches which then split into Purkinje fiber. The latter are interwoven with the myocardial tissue and finally conduct the impulse to the working myocardium. From the apex, the electrical current continues along the lateral walls of the ventricles until it reaches the AV valve level again. This where depolarization usually ends.
SAN = sinusatrial node; AVN = atrioventricular node; HIS = bundle of His; RBB = right bundle branch; LPBB = left posterior bundle branch; LABB = left anterior bundle branch; PF = Purkinje fibers.