Basic cardiac electrophysiology
The conduction system of the heart consists of specialized conduction tissue. It consists of the sinoatrial node (SA node), internodal pathways, atrioventricular node (AV node), bundle of His, bundle branches and Purkinje fibers.
Sinoatrial node is situated in the upper part of the right atrium near the opening of the superior vena cava. The SA node is a subepicardial structure. It is the natural pacemaker of the heart with highest automaticity among the specialized conduction tissue. The diastolic depolarization in the SA node which is responsible for the automaticity is mediated by the funny current(If). It has both sympathetic and parasympathetic innervation which modulates the heart rate. The heart rate after total sympathetic and parasympathetic blockade is known as intrinsic heart rate.
There are three internodal pathways:
- Posterior tract named after Thorel
- Middle tract named after Wenckebach
- Anterior tract gives off the Bachmann’s bundle, which carries impulses to the left atrium
Though these three tracts have been discussed, they are not clear cut anatomical bundles which can be easily localized like the bundle of His and its branches. They are microscopically identifiable preferentially oriented fibers. Many have doubted their presence and considered that internodal conduction is through the atrial myocardium.
AV node is situated in the lower part of the right atrium at the apex of Koch’s triangle formed by ostium of the coronary sinus, tendon of Todaro and the septal part of the tricuspid annulus. It is a subendocardial structure unlike the SA node. An important role of the AV node is to delay the signals coming from the SA node till the atrial contraction is over. This ensures that atrial and ventricular contractions are sequential rather than simultaneous. This ensures atrial booster action for ventricular filling during the terminal part of ventricular diastole. It can also act as a subsidiary pacemaker of the heart if the SA node were to fail, though its automaticity is less than that of the SA node. AV node has a slow pathway and a fast pathway (dual AV nodal physiology). The fast pathway is anterior, has a higher conduction velocity, but a longer refractory period. The slow pathway is posterior, has a slower conduction velocity, but a shorter refractory period.
Bundle of His
Bundle of His arises from the inferior border of the AV node and enters the upper part of the interventricular septum. Signals from the bundle of His can be recorded using intracardiac electrodes it is known as His bundle electrogram. The fibers destined for the bundle branches are segregated within the bundle of His even before its actual division.
The bundle of His divides into left and right branches destined for the corresponding ventricles. Part of the right bundle travels through the moderator band in the right ventricle. Left bundle branch further divides into a left anterior fascicle and a left posterior fascicle. Left anterior fascicle is thin while posterior fascicle is thicker and broader, with dual blood supply so that it is seldom involved in disease processes. Some authors describe an additional left septal fascicle.
Purkinje fibers are specialized conduction cells which ramify within the myocardium and conduct the impulses from the bundle branches through out the myocardium. It is finally the Purkinje fibers which carry the signals from the SA node to the myocardium for initiating excitation-contraction coupling.