Roger A. Freedman, M.D.
• Anatomy of the cardiac conduction
system
• Relationship between intracardiac
events and ECG
• Bradyarrhythmias
• Pacemakers
THE CONDUCTION SYSTEM
Heart Beat Anatomy
SINUS NODE
Sinus Node
(SA Node)
• The Heart’s ‘Natural Pacemaker’
- 60-100 BPM at rest
Heart Beat Anatomy
AV NODE
Sinus Node
(SA Node)
Atrioventricular
Node (AV Node)
• Receives impulse from
SA Node
• Delivers impulse to the HisPurkinje System
• 40-60 BPM if SA Node fails to
deliver an impulse
Heart Beat Anatomy
BUNDLE OF HIS
Sinus Node
(SA Node)
Atrioventricular
Node (AV Node)
Bundle of His
• Begins conduction to
the Ventricles
• AV Junctional Tissue:
40-60 BPM
Heart Beat Anatomy
THE PURKINJE NETWORK
Sinus Node
(SA Node)
Atrioventricular
Node (AV Node)
Bundle of His
Bundle Branches
Purkinje Fibers
• Bundle Branches
• Purkinje Fibers
• Moves the impulse through
the ventricles for contraction
• Provides ‘Escape Rhythm’:
20-40 BPM
Impulse Formation In SA Node
Atrial Depolarization
Delay At AV Node
Conduction Through Bundle Branches
Conduction Through Purkinje Fibers
Ventricular Depolarization
Plateau Phase of Repolarization
Final Rapid (Phase 3) Repolarization
Normal EKG Activation
Reading EKGs
Intervals and Timing
Normal Ranges
in Milliseconds:
• PR Interval 120 – 200 ms
• QRS Complex 60 – 100 ms
• QT Interval 360 – 440 ms
Reading EKG Squares
Intervals and Timing
• Each square = 40 ms
• Each interval = 200 ms
Ca2+ in
Calcium channel
blockers
(verapamil,
diltiazem) slow
sinus rate and
slow or block
conduction
through AV node
K+ out
Na+ in
Sodium channel
blockers
(flecainide,
propafenone,
amiodarone)
widen the QRS
K+ out
Potassium channel
blockers
(dofetilide,
sotalol,
amiodarone)
prolong the QT
interval
Prolonged QT interval
RHYTHM DISORDERS
Bradyarrhythmias
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Bradycardias
Impulse Conduction
Disorders
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Bradycardia
Sinus Bradycardia
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Bradycardia
• Sinus Arrest
Sinus Arrest
*
Animation
• Failure of sinus node discharge
• Absence of atrial depolarization
• Periods of asystole
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
• Sinus Bradycardia
• Sinus Arrest
• Brady/Tachy Syndrome
Impulse Conduction
Disorders
Brady/Tachy Syndrome
• Intermittent episodes of slow and
fast rates from the SA node or atria
• Brady <60 BPM
• Tachy >100 BPM
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Arrest
• Sinus Bradycardia
• Brady/Tachy Syndrome
• Sino-atrial Exit Block
Sino-atrial Exit Block
• Transient block of impulses from the SA node
• Identified by P-P interval relationship
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Arrest
• Sinus Bradycardia
• Brady/Tachy Syndrome
• Sino-atrial Exit Block
• AV block
First-Degree AV Block
• PR interval > 200 ms
• Delayed conduction through the AV Node
- Example shows PR Interval = 320 ms
Second degree AV block
Second-Degree AV Block - Mobitz I
*
Animation
Known as Wenckebach Block
• Progressive prolongation of the PR interval until
there is failure to conduct and a ventricular beat
is dropped
Third-Degree AV Block
*
Animation
• No impulse conduction from the atria to the
ventricles
– Ventricular rate = 37 BPM
– Atrial rate = 130 BPM
– PR interval = variable
Third degree AV block
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Arrest
• Sinus Bradycardia
• Brady/Tachy Syndrome
• Exit Block
• 1st Degree AV Block
• 2nd Degree AV Block
• 3rd Degree AV Block
Bradyarrhythmia Classifications
Classification Based on Disorder
Impulse Formation
Disorders
Impulse Conduction
Disorders
• Sinus Arrest
• Sinus Bradycardia
• Brady/Tachy Syndrome
• Exit Block
• 1st Degree AV Block
• 2nd Degree AV Block
• 3rd Degree AV Block
• Bundle Branch Block
Bundle Branch Block
Left bundle
branch block
Left bundle branch block
Right bundle branch block
Causes of Bradyarrhythmias
Congenital Heart Disease
• Present at birth due to genetics,
environment
Acquired Heart Disease
• Acute Myocardial Infarction, Ischemic
Heart Disease, Dilated or Hypertrophic
Cardiomyopathy, Hypertension, Valvular
Heart Diease, Post-operative
Drug-induced
Neurocardiogenic
• Hypersensitive Carotid Sinus Syndrome
• Vasovagal Syncope
Biventricular pacing
• Also known as Cardiac Resynchronization
Therapy (CRT)
• Therapy for patients with bundle branch
block, depressed left ventricular ejection
fraction, and heart failure
• Goal is to pace RV and LV simultaneously to
resynchronize contraction of the heart
(both interventricular resynchyronization
and intraventricular LV resynchronization)
CRT-pacemaker
Summary
• Anatomy of the cardiac conduction
system
• Relationship between intracardiac
events and ECG
• Bradyarrhythmias
• Pacemakers