Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins 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