Ebook Implantable cardioverter - Defibrillators step by step: Part 2

(BQ) Part 2 book Implantable cardioverter - Defibrillators step by step includes: Cardiac tachyarrhythmias, ICD function with emphasis on stored electrograms, indications for ICD, ICD implantation and lead systems, cardiac resynchronization, complications of ICD therapy,...

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Implantable Cardioverter-Defibrillators Step by Step: An Illustrated Guide Roland X Stroobandt, S Serge Barold and Alfons F Sinnaeve

© 2009 R.X Stroobandt, S.S Barold and A.F Sinnaeve ISBN: 978-1-405-18638-4

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Implantable Cardioverter-Defibrillators Step by Step: An Illustrated Guide Roland X Stroobandt, S Serge Barold and Alfons F Sinnaeve

© 2009 R.X Stroobandt, S.S Barold and A.F Sinnaeve ISBN: 978-1-405-18638-4

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automaticity, afterdepolarizations and reentry.

1. Automaticity describes the spontaneous

devel-opment of a new site of depolarization (ectopicfocus) in non-nodal tissue

2. Afterdepolarizations consist of secondary

depolar-izations arising during repolarization (early depolarization: EAD) but also just after complete

after-repolarization of the cellular membrane: delayedafterdepolarization (DAD) Afterdepolarizationsmay reach the threshold level and generate anaction potential giving rise to tachycardia

3. Reentry occurs when an impulse depolarizes the

myocardium and then perpetuates activation in

an anatomic or functional loop producing itive beats or sustained tachycardias Reentrant circuits require three functional or structural conditions:

repet-(a) an impulse conducted through two pathwayswith different conduction velocities andrefractoriness;

(b) pathways joined proximally and distally toform a continuous loop;

(c) unidirectional block in one pathway to ate the process

initi-Slow conduction in the loop allows the previouslyblocked pathway time to recover excitability A crit-ically timed impulse conducts anterogradely throughthe only open pathway and returns retrogradely viathe other pathway responsible for unidirectionalblock If the anterograde pathway has recoveredfrom refractoriness, the returning impulse can thenreactivate the earlier site and start a self-perpetuatingreentrant tachycardia (Fig 1.02)

1.2 Supraventricular tachyarrhythmias

The maintenance of supraventricular mias (SVT) involves structures above the division of

tachyarrhyth-the His bundle Supraventricular tachyarrhythmiascomprise atrial tachycardia (AT), atrial flutter (AFL),atrial fibrillation (AF) and atrioventricular (AV)junctional tachycardias Atrioventricular junctionaltachycardias include AV nodal reentrant tachy-cardia (AVNRT) and AV reentrant (orthodromicreciprocating) tachycardia (AVRT/ORT) incorporat-ing one or more accessory AV pathways in a reentrycircuit

1.2.1 AV nodal reentrant tachycardia

Atrioventricular nodal reentrant tachycardia(AVNRT) involves an AV nodal reentry circuit thatconsists of two atrionodal connections or pathwaysand a component of atrial myocardium joining them

The common type of AVNRT (slow–fast) involvestwo functionally and anatomically separate atrionodalpathways with different refractory periods The fastconducting pathway possesses a long refractoryperiod and the slow pathway a short refractoryperiod During sinus rhythm, anterograde conduc-tion proceeds simultaneously over both AV nodalpathways but retrograde invasion of the impulsecoming from the fast pathway blocks anterogradeconduction over the slow pathway The tachycardiatypically starts with a premature atrial beat withcritical timing that finds the fast pathway refractorybut open conduction over the nonrefractory slowpathway (with a long PR-interval) Upon reachingthe lower common pathway of the reentry circuit,the wavefront returns retrogradely to the atria viathe fast pathway The process establishes a reentrantSVT using the slow pathway as the anterogradelimb and the fast pathway as the retrograde limb(Fig 1.03) Atrial and ventricular activation occurnearly simultaneously During the common slow–

fast AVNRT, retrograde P-waves hide within theQRS complex or emerge at the end of the QRS com-plex producing terminal “pseudo s waves” in theinferior leads or more frequently “pseudo r waves”

in lead V1 Rarely the P-wave precedes the ning of the QRS complex, resulting in “pseudo-qwaves” in the inferior leads

begin-The reverse form of AVNRT (fast–slow AVNRT)

is rare It engages the fast pathway as the grade limb and the slow pathway as the retrogradelimb of the reentry circuit

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Implantable Cardioverter-Defibrillators Step by Step: An Illustrated Guide Roland X Stroobandt, S Serge Barold and Alfons F Sinnaeve

© 2009 R.X Stroobandt, S.S Barold and A.F Sinnaeve ISBN: 978-1-405-18638-4

1.2.2 Orthodromic reciprocating tachycardia

Orthodromic reciprocating tachycardia (ORT) oftenoccurs in patients with an obvious accessory path-way (atrium to ventricle as in the Wolff–Parkinson–

White syndrome) However, some patients show

no evidence of preexcitation during sinus rhythmbecause the accessory pathway can only conduct

in the retrograde direction from ventricle to atrium

In this situation, an ORT utilizes the accessory pathway as the retrograde limb and the AV node asthe anterograde limb of a reentry circuit (Fig 1.04)

Orthodromic reciprocating tachycardia usually startswith either an atrial or ventricular premature beat

When the accessory pathway conducts rapidly, theretrograde P-wave is inscribed at approximately

140 ms (range 80–160 ms) after the QRS complex with

an RP interval shorter than the PR interval The QRScomplex is identical to that during sinus rhythm unlessrate-related bundle branch aberrancy supervenes

1.2.3 Atrial tachycardia

Atrial tachycardia can originate from anywhere inthe atria The mechanisms are unclear but may berelated either to localized atrial reentry or a focus ofenhanced automaticity The atrial rate generallyruns between 150 and 200 b.p.m but occasionally asfast as 300 b.p.m During atrial tachycardia the P-waveprecedes the QRS complex The morphology of theP-wave depends on the site of origin and differsfrom the sinus P-wave The ventricular rate depends

on AV nodal transmission The AV relationship can

be 1:1, or governed by second-degree AV block,which may itself also be variable (Fig 1.05) The QRScomplex remains the same as during sinus rhythm

of conduction During the less common “reversetypical” type of right AFL the macroreentry circuitrotates in a clockwise (CW) direction, down theinteratrial septum and up the right atrial free wall(Fig 1.06)

In typical CCW AFL, atrial activity (F-waves)inscribes a characteristic “saw-tooth” configuration

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with a dominant negative deflection in the inferiorleads In the uncommon CW AFL the inferior leadsregister positive flutter waves The atrial rateapproximates 300 b.p.m., and the AV ratio is usually2:1 or greater depending on the status of AV nodalconduction The AV ratio may change irregularly.Atrial flutter can also occur in the left atrium, espe-cially after pulmonary vein isolation (ablation) forthe treatment of atrial fibrillation

A 1:1 AV ratio is exceptional It may result fromthe proarrhythmic effect of class IC antiarrhythmicdrugs that reduce atrial conduction velocity Themarked slowing of the atrial rate (190–240 b.p.m.)(Fig 1.07) permits the development of 1:1 “slow” AFLwith rate-related bundle branch block – a situationoften misdiagnosed as ventricular tachycardia (VT)

During atrial fibrillation the fibrillatory waves (f-waves) exhibit a rate of 350–500/min Only part

of the many atrial impulses actually travel at gular intervals through the AV nodal filter to the ventricles The electrocardiogram (ECG) shows anirregular ventricular response The ventricular ratebecomes regular and not fast, only in combinationwith complete AV block In the presence of a fastventricular response (> 170 b.p.m.), the ventricularrate tends to regularize, and an implantable car-dioverter defibrillator (ICD) may therefore facedifficulty in discriminating atrial fibrillation fromventricular tachycardia

irre-1.3 Ventricular arrhythmias

Ventricular tachycardia (VT) and ventricular lation (VF) are the major causes of sudden cardiacdeath in patients with structural heart disease.Ventricular fibrillation (VF) and/or ventriculartachycardia (VT) can also occur far less commonly inpatients with structurally normal hearts Suddencardiac death is defined as death from an unsus-pected circulatory arrest, usually due to an arrhy-thmia occurring within an hour of the onset ofsymptoms and when medical intervention such asdefibrillation reverses the event

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