Download LWW PPT Slide Template Master

Chapter 25
Disorders of Cardiac Conduction
and Rhythm
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Cardiac Conduction System
• Controls 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.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Refractory Periods
• 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
threshold, although not yet at the resting membrane
potential.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Refractory Periods (cont.)
– 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
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
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.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
• False: The ECG is the electrical activity of the heart and
is used to infer changes in mechanical activity.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Electrocardiography
• 12 leads
• Diagnostic ECG
– Each provides a unique view of the electrical forces
of the heart
• Diagnostic criteria are lead specific.
• Improper lead placement can significantly change QRS
morphology.
– Misdiagnosis of cardiac arrhythmias or the presence
of conduction defects can occur.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Goals of Continuous Bedside Cardiac
Monitoring
• Shifted from simple heart rate and arrhythmia monitoring
to
– Identification of ST segment changes
– Advanced arrhythmia identification
– Diagnosis
– Treatment
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Arrhythmias
• Sinus node arrhythmias
– Sinus bradycardia
– Sinus tachycardia
– Sinus arrest
• Arrhythmias of atrial origin
– Paroxysmal supraventricular tachycardia
– Atrial flutter
– Atrial fibrillation
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Arrhythmias (cont.)
• 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
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Types of Arrhythmias (cont.)
• Disorders of atrioventricular conduction
– First-degree AV block
– Second–degree AV block
– Third–degree AV block
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Diagnostic Methods
• Signal-averaged electrocardiogram
• Holter monitoring
• Exercise stress testing
• Electrophysiologic studies
• QT dispersion
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Correction of Conduction Defects,
Bradycardias, and Tachycardias
• Electronic pacemaker
– Temporary
– Permanent
• Cardioversion
– Defibrillation
– Synchronized
• Ablation
• Surgical interventions
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
• Sinus arrhythmias will have a greater effect on the total
heart than atrial arrhythmias because________.
a. They will not be different; each is equally harmful.
b. The sinus node will directly cause a fibrillation.
c. The sinus node will stimulate the rest of the heart
directly into a new rhythm.
d. The sinus node will not activate the atrioventricular
node.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
a. They will not be different, each is equally harmful
b. The sinus node will directly cause a fibrillation
c. The sinus node will stimulate the rest of the heart
directly into a new rhythm.
d. The sinus node will not activate the atrioventricular node
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins