Discussion Question

Discussion Question

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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