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Understanding EKGs
A Practical Approach
For the Dental Hygienist
CHAPTER
4
The Electrical Conduction
System
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrical Conduction System
• Objectives
– Identify the location of the following
 SA node
 Internodal pathways
 Bachmann’s bundle
 AV node
 Bundle of His
 AV junction
 Bundle branches
 Purkinje network
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrical Conduction System
• Objectives (continued)
– Relate the normal path of an impulse traveling
through the electrical conduction system
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Cardiac Conduction System
Bachmann’s
Bundle
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The SA Node
• Sinoatrial (SA) node
– Located in the upper posterior portion of the
right atrial wall of the heart
 50% actual pacemaker cells, remaining cells
conduct electrical impulse
– “Primary pacemaker” of the heart
– Firing rate = 60-100 bpm
 Depolarization and myocardial contraction occur
after impulse leaves SA node
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Internodal Pathways
• Internodal pathways
– Receive the electrical impulse as it exits the
SA node
– Pathways Distribute impulse throughout atria
– Transmit the impulse from SA node to AV
node
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Bachmann’s Bundle
• Bachmann’s bundle
– Also known as Wenckebach’s bundle and
Thorel’s pathway
– A group of interatrial fibers contained in the
left atrium
– Subdivision of anterior internodal tract
– Conducts electrical activity from the SA node
to the left atrium
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The AV Node
• Atrioventricular (AV) node
– Located on floor of right atrium above
tricuspid valve
– Electrical activity delayed 0.05 seconds
– Allows for more complete filling of ventricles
– Only pathway of atrial electrical impulses
to ventricles
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The AV Junction
• AV junction
– SA node joins the bundle of His
– If the SA node fails or slows below the normal
rate, AV junctional tissues initiate electrical
activity and thus the role of secondary
pacemaker
 Accessory pathway: an irregular muscle
connection between atria and ventricles that
bypasses the AV node
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Bundle of His
• Bundle of His
– Conduction pathway that leads out of AV
node.
– Receives blood from left anterior and
posterior descending coronary arteries.
– Lies at top of interventricular septum
– Referred to as the common bundle
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Bundle of His
• Bundle of His
– Ability to self-initiate electrical activity at
intrinsic firing rate of 40-60 bpm
– Connection between upper and lower
chambers of the heart.
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Bundle Branches
• Bundle of His
– Divides into two main bundle branches
 Right bundle branch
 Left bundle branch
– Conducts electrical activity from bundle of
His to Purkinje’s network
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Purkinje’s Network
• Purkinje’s network
– Fibers that spread throughout the ventricles
– Carries impulses directly to ventricular muscle
cells
– Intrinsic firing rate of 20-40 bpm
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Review of Electrical Conduction
System of the Heart
Bartender
Bouncer
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Janitor
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Electrical Conduction System
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.