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Cardiac Anatomy - The Conduction System

Conduction System

          The conduction system of the heart is a complex system of preferential conductive zones that connects the myocardium of the various cardiac chambers. To adequately describe a system of this complexity would require a great deal of time, text and images. In this area, I will try to provide a brief review of this immensely complex subject.

From the Sinus Node to the Ventricles - The path of normal conduction

Cardiac Conduction

           The cardiac conduction system is the foundation of all heart rhythms, both normal and abnormal. Understanding how the normal conduction occurs allows us to understand abnormal conduction patterns and therefore, provides us with the capability of unraveling the mechanisms that contributed to arrhythmias of all types. This section contains information on the primary structures that are involved with initiation and transmission of a normal heart beat.

• Sinus Node

          The sinus node is located in the right atrium at the junction of the right atrium and the superior vena cava along the posterior aspect of the lateral wall of the atrium. The node lies medial to the crista terminalis with the cells of the node surrounding the sinus node artery.(1) The sinus node activates as a focal event with the initial depolarization occurring due to a difference between the voltage inside the cell compared to the voltage outside the cell. Most cardiac cells have a stable resting potential. The cells responsible for initiating depolarization will have a current leak across the cellular membrane that slowly shifts the transmembrane potential. When the difference between the intracellular and extra cellular voltage reaches a critical threshold, the Na- channels open and depolarization occurs.

• Intra-Atrial Tracks / Bachman’s Bundle to LA

          Once an electrical signal propagates into the myocardium, it traverses the atrial tissue of both the right and left atrium. The transition through the right atrium occurs through 3 preferential pathways in the right atrium. These pathways are preferential due to their fiber alignment.(2) When the phrase preferential is used to describe cardiac conduction, think in terms of driving your car. If you need to get downtown, do you want to use the freeway or go on the side streets. Assuming that there are no accidents and that the roads are in good condition, the freeway is probably the preferred choice. You can travel faster and you will get there sooner. A preferential conduction pathway is very similar. It is easier for the electrical signal to travel down tracks where the fibers (think in terms of the number of lanes on the freeway) are all in good condition. Travel is smooth and quick. Once the electrical conduction wave front gets off the "freeway", it slows down, gets frustrated, turns on the radio and starts mumbling under its breath at other cardiac wavelets!!

• AV Node

          The AV Node acts as the primary electrical connection between the atria at the top of the heart and the ventricles in the bottom of the heart. After the depolarization wave front passes through the atria, it enters that AV node which is located at the apex of the triangle of Koch. (Triangle of Koch - Right Atrial Anatomy) * The AV node plays a very important role in cardiac conduction. As the only connection between the atria and the ventricles (in normal hearts), the AV node controls how fast signals from the top of the heart get to the bottom of the heart. This becomes an important safety factor when tachycardias occur. The faster the cardiac muscle contracts the less blood it pumps with each contraction. (For more information on this, read up on the association between heart rate and cardiac output). When an atrial tachycardia occurs, it is the AV Node that acts as the regulator which prevents the ventricles from contracting so fast that the cardiac output drops too low.

          There are two functions that are key to how the AV node regulates conduction between the top and bottom halves of the heart. The first of these involves the conduction velocity of the tissue within the node. When the electrical wave front from the atrium reaches the AV node, it slows down due to the slower conduction velocity inside the node. This causes a small delay between the time that the atria finish contracting and the ventricles start to contract. This delay allows for maximum filling time of the ventricles and thus optimizes the cardiac output.

          The second delaying property of the AV node is referred to as decremental conduction. This term is used to describe tissue that conducts slower when it receives signals faster. When the atria are affected by a tachycardia that is very rapid, the AV node may begin to block signals due to decremental conduction. This event is most clearly seen when a patient has atrial fibrillation. If the AV node conducted every atrial depolarization through to the ventricle, atrial fibrillation would be a lethal rhythm. But the decremental properties of conduction through the AV node prevent every signal from passing to the ventricles and thus prevents atrial fibrillation from being instantly fatal.

• His Bundle

          The HIS Bundle was first recorded by Dr. Ben Scherlag in 1965. This event represented the start of the modern era of cardiac electrophysiology as this was the first time a signal was recorded using intracardiac catheters that represented a physiologic phenomena that could not be visualized on the surface electrocardiogram. This signal represented activation of the Bundle of HIS below the AV node. Being able to visualize activation of the HIS Bundle became key to understanding AV nodal reentry tachycardia as well as providing a differentiating tool for determining the difference between physiologic and non-physiologic block below the node.

          Physiologic block below the node is due to events such as increased vagal tone and represents a situation where the patient generally will not require a pacemaker implant. Non-physiologic block is visualized by a block that occurs after a HIS signal is seen on the recording. This indicates damaged conduction pathways and usually is an indication for pacemaker implant.

• Right Bundle, Left Anterior Fascicle, Left Posterior Fascicle

          After the electrical wave front passes through the AV node, it enters the left and right bundles within the ventricles. The bundles are protected conduction pathways that allow the depolarization of the ventricles to start at the apex and contract to the base providing optimal muscle contraction to push blood from the ventricles out to the pulmonary artery and the aorta. If the ventricles contracted from the base first, the cardiac output would be reduced as some blood would be trapped down in the apex.

          The bundles have three branches, one on the right and two on the left. The left bundles separate into anterior and posterior fascicles. The signals from the bundles pass into the purkinje network and disperse into the ventricular tissue.

• Purkinje Network

          The transition from the bundles into the ventricular myocardium occurs in the purkinje fibers. These fibers disperse the electrical wave front from the concentrated zone within the protected conduction pathway of the bundles throughout the ventricles in a manner that helps to ensure a smooth even contraction of the ventricular myocardium. This transition zone is referred to as the purkinje network. While the entire function of the purkinje fibers is not yet fully understood, there is evidence that the back up ventricular pacemaker can be found within these fibers.(1)

(1) Information found in: Cardiac Electrophysiology - From Cell to Bedside 3rd Edition: Douglas P. Zipes, MD & Jose Jalife, MD
(2) Information found in: Clinical Cardiac Electrophysiology - Techniques and Interpretations: Mark E. Josephson, MD
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