Cardiac Arrhythmias: Difference between revisions

The cardiac conduction system (figure 3) is the highway though which the electric activity of the heart spread from the atria to the ventricle. The characteristics of the different parts of the conduction system are a result of the different characteristics of the individual myocytes. On a larger level, function is controlled predominantly by the autonomic nerve system (both vagal and sympathetic nerve system). Especially the sinus node and atrioventricular node are responsive to the autonomic nerve system. The ganglionic plexi, a conglomeration of both vagal and sympathetic nerves, form the intrinsic cardiac nerve system and innervate through a network of nerve fibers the atria and ventricles. The vagal and sympathetic nerve system are both continually active in the heart, but vagal activity dominates the tonic background stimulation of the autonomic nerve system. Moreover the heart is more susceptible to vagal stimulation. Vagal stimulation provokes a rapid response and the effect dissipates swiftly in contrast to sympathetic stimulation which has a slow onset and offset. Vagal stimulation results in a reduction is in sinus node activation frequency and prolongs AV nodal conduction. These effects can occur simultaneously or independent of each other.  Sympathetic stimulation exerts reverse effects, accelerating the sinus node firing frequency and improving AV nodal conduction. The autonomic nerve system has a small effect on cardiomyocytes. Vagal stimulation tends to prolong the refractory period and decreasing the myocardial contractility. Sympathetic stimulation has opposite effects on the cardiac tissue. Whereas the physiological modulation of cardiac conduction is vital to adaptation of the heart to rest and exercise it  can attribute to the pathologic substrate in a diseased myocardium.

The sinus node is an densely innervated area located in the right atria which is supplied by the right (55%-60%) or circumflex (40%-45%) coronary artery. It is a small structure of 10-20mm long and 2-3mm wide and contains a diversity of cells. These include pacemaker cells which are discharged synchronously due to mutual entrainment. This results in an activation wave front activating the rest of the atria.

The impulse formed in the sinus node is conducted through the atrium to the AV-node. Evidence indicates three preferential conduction pathways. The pathways show preferential conduction due to their anatomical structure and rather than specialized conduction properties. The three pathways are; the anterior internodal pathway, the middle internodal tract the posterior internodal pathway. The anterior internodal pathway connect to the anterior interatrial band, also known as the Bachmann bundle. This bundle of muscular tissue conducts the sinus wave front from the right to the left atrium.  

From the bundle of His the right bundle branch continues to the right ventricular apex. The left bundle branch splits of and divides in two or more fascicular branches. Commonly the left bundle branch consist of an anterior fascicle, that activates the anterosuperior portion of the left ventricle. The thicker and more protected posterior fascicle activates the inferoposterior part of the left ventricle.