Conduction System

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Learning Objectives

1) Describe the conduction system

2) Describe the basic function of the 3 pacemakers

3) Relate the pacemakers to the tracing on an ECG


Overview

The cardiac conduction system is a group of specialized cardiac muscle cells in the walls of the heart that send signals to the heart muscle causing it to contract. The main components of the cardiac conduction system are the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers. The SA node (anatomical pacemaker) starts the sequence by causing the atrial muscles to contract. From there, the signal travels to the AV node, through the bundle of His, down the bundle branches, and through the Purkinje fibers, causing the ventricles to contract. This signal creates an electrical current that can be seen on a graph called an Electrocardiogram (EKG or ECG). Doctors use an EKG to monitor the cardiac conduction system's electrical activity in the heart.



The Sino-Atrial Node



Figure 1

(SA) (Figure 1) is an oval shape structure that is located at the junction of the superior vena cava and the high lateral right atrium. It consists of specialized cells that are capable of initiating and generating electrical impulses. The normal sinus impulse originates in the SA node at a rate of between 60-100 beats per minute (bpm). It is the dominant pacemaker.



Figure 2


The first normal deflection in an ECG is the P wave (Figure 2). It signifies the electrical discharge from the SA node. Thus, when you refer to a Sinus Rhythm it simply means that the rhythm is initiated by the SA node. Hence also, all normal ventricular electrical activities (QRS) should be preceded by the P wave.





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