Disorders of Cardiac Conduction and Rhythm
Chapter 28: Disorders of Cardiac Conduction and Rhythm
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Cardiac Conduction System
Controls the rate and direction of electrical impulse conduction in the heart
Impulses are generated in the SA node, which has the fastest rate of firing, and travel to the Purkinje system in the ventricles.
In certain areas of the heart, the myocardial cells have been modified to form the specialized cells of the conduction system.
The conduction system maintains the pumping efficiency of the heart.
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Phases of Cardiac Potentials
Phase 0: rapid upstroke of the action potential
Phase 1: early repolarization
Phase 2: plateau
Phase 3: final repolarization period
Phase 4: diastolic repolarization period
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Refractory Periods #1
Absolute Refractory Period
No stimuli can generate another action potential.
Includes phases 0, 1, 2, and part of phase 3.
The cell cannot depolarize again.
Relative Refractory Period
Greater than normal stimulus response
Repolarization returns the membrane potential to below the threshold, although not yet at the resting membrane potential.
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Refractory Periods #2
Begins when the transmembrane potential in phase 3 reaches the threshold potential level
Ends just before the terminal portion of phase 3
Supernormal Excitatory Period
A weak stimulus can evoke a response.
Extends from the terminal portion of phase 3 until the beginning of phase 4
Cardiac arrhythmias develop.
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Electrocardiography
Twelve leads
Diagnostic ECG
Each providing a unique view of the electrical forces of the heart
Diagnostic criteria are lead specific.
Improper lead placement can significantly change the QRS morphology.
Misdiagnosis of cardiac arrhythmias or the presence of conduction defects can be missed.
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Goals of Continuous Bedside Cardiac Monitoring
Shifted from simple heart rate and arrhythmia monitoring to
Identification of ST segment changes
Advanced arrhythmia identification
Diagnose
Provide treatment
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Question #1
Is the following statement true or false?
The electrocardiogram is a reflection of cardiac muscle contraction. You can directly diagnose specific defects in muscle activity.
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Answer to Question #1
False
Rationale: The electrocardiogram is a reflection of cardiac muscle contraction. You can directly diagnose specific defects in muscle activity.
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Types and Causes of Disorders of the Cardiac Conduction System
Types
Disorders of rhythm
Disorders of impulse conduction
Causes
Congenital defects or degenerative changes in the conduction system
Myocardial ischemia and infarction
Fluid and electrolyte imbalances
Effects of drug ingestion
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Types of Arrhythmias #1
Sinus Node Arrhythmias
Sinus bradycardia
Sinus tachycardia
Sinus arrest
Arrhythmias of Atrial Origin
Paroxysmal supraventricular tachycardia
Atrial flutter
Atrial fibrillation
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Types of Arrhythmias #2
Junctional Arrhythmias
Disorders of Ventricular Conduction and Rhythm
Long QT Syndrome and Torsades de Pointes
Ventricular Arrhythmias
Premature ventricular contractions
Ventricular tachycardia
Ventricular flutter and fibrillation
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Types of Arrhythmias #3
Disorders of Atrioventricular Conduction
First-degree AV block
Second-degree AV block
Third-degree AV block
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Diagnostic Methods
Signal-averaged electrocardiogram
Holter monitoring
Exercise stress testing
Electrophysiologic studies
QT dispersion
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Pharmacologic Treatment of Arrhythmias
Class I drugs: act by blocking the fast sodium channels
Class II agents: β-adrenergic–blocking drugs that act by blunting the effect of sympathetic nervous system stimulation on the heart
Class III drugs: act by extending the action potential and refractoriness
Class IV drugs: act by blocking the slow calcium channels, thereby depressing phase 4 and lengthening phases 1 and 2
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Correction of Conduction Defects, Bradycardias and Tachycardias
Electronic pacemaker
Temporary
Permanent
Cardioversion
Defibrillation
Synchronized
Ablation
Surgical interventions
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Question #2
Sinus arrhythmias will have a greater affect on the total heart than atrial arrhythmias because________.
they will not be different, and each is equally harmful
the sinus node will directly cause a fibrillation
the sinus node will stimulate the rest of the heart directly into a new rhythm
the sinus node will not activate the atrioventricular node
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Answer to Question #2
C. the sinus node will stimulate the rest of the heart directly into a new rhythm
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