Mechanism of ArrhythmiasMechanism of Arrhythmias

Dr Fuad FarooqDr Fuad Farooq

Implies normal sequence of conduction, originating in the sinus node and proceeding to the ventricles via the AV node and His-Purkinje system

EKG Characteristics: Regular narrow-complex rhythm

Rate 60-100 bpm

Each QRS complex is proceeded by a P wave

P wave is upright in lead II & downgoing in lead aVR

Normal Sinus RhythmNormal Sinus Rhythm

The mechanisms responsible for cardiac The mechanisms responsible for cardiac arrhythmias are generally divided intoarrhythmias are generally divided into

– Disorders of impulse formationDisorders of impulse formation– Disorders of impulse conductionDisorders of impulse conduction– Combinations of both Combinations of both

Can beCan be– Disorder in Automaticity Disorder in Automaticity – Trigger activityTrigger activity

Disorders of Impulse FormationDisorders of Impulse Formation

Automaticity is the property of a fiber to initiate an Automaticity is the property of a fiber to initiate an impulse spontaneously, without need for prior stimulationimpulse spontaneously, without need for prior stimulation

Characterized by Characterized by – Inappropriate discharge rate of the normal pacemaker, the Inappropriate discharge rate of the normal pacemaker, the

SA node (e.g., sinus rates too fast or too slow for the SA node (e.g., sinus rates too fast or too slow for the physiologic needs of the patient)physiologic needs of the patient)

– Discharge of an ectopic pacemaker that controls atrial or Discharge of an ectopic pacemaker that controls atrial or ventricular rhythmventricular rhythm

Ectopic pacemakers are often called latent or subsidiary Ectopic pacemakers are often called latent or subsidiary pacemakerspacemakers

AutomaticityAutomaticity

Ectopic pacemakers are usually suppressed by Ectopic pacemakers are usually suppressed by overdrive suppression by the more rapidly firing SA node overdrive suppression by the more rapidly firing SA node or by concealed conduction from neighboring fibersor by concealed conduction from neighboring fibers

– Manifested when the SA discharge rate slows or block Manifested when the SA discharge rate slows or block occurs at some level between the SA node and the ectopic occurs at some level between the SA node and the ectopic pacemaker sitepacemaker site

– The discharge rate of the latent pacemaker can speed The discharge rate of the latent pacemaker can speed inappropriately and usurp control of cardiac rhythm from inappropriately and usurp control of cardiac rhythm from the SA nodethe SA node

AutomaticityAutomaticity

Recognizing Altered Automaticity on EKGRecognizing Altered Automaticity on EKG

Gradual onset (warming) and termination of the Gradual onset (warming) and termination of the arrhythmiaarrhythmia

The P wave of the first beat of the arrhythmia is The P wave of the first beat of the arrhythmia is typically the same as the remaining beats of the typically the same as the remaining beats of the arrhythmia (if a P wave is present at all)arrhythmia (if a P wave is present at all)

Sinus Bradycardia

Decreased AutomaticityDecreased Automaticity

Sinus tachycardia

Junctional tachycardia

Ectopic atrial tachycardia

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Increased/Abnormal AutomaticityIncreased/Abnormal Automaticity

• Triggered activity is initiated by after-depolarizations, which Triggered activity is initiated by after-depolarizations, which are depolarizing oscillations in membrane voltage induced by are depolarizing oscillations in membrane voltage induced by one or more preceding action potentialsone or more preceding action potentials

• Thus, triggered activity is pacemaker activity that results Thus, triggered activity is pacemaker activity that results consequent to a preceding impulse or series of impulses, consequent to a preceding impulse or series of impulses, without which electrical quiescence will occurswithout which electrical quiescence will occurs

TriggeredTriggered ActivityActivity

Phase 4: Resting phase•Associated with diastole portion of heart cycle•Potassium outward current through open, inwardly rectifying K+ channels ( Ikl )

Phase 0 : Rapid depolarization •Opening of fast Na channels

Phase 1: Initial rapid repolarization

•Closure of the fast Na+ channels•Phase 0 and 1 together correspond to the R and S waves of the ECG

Phase 2 Plateau phase

•Balance between the inward movement of Ca+ and outward movement of K + •Corresponds to ST segment of the ECG

Phase 3 Repolarization

•K+ channels remain open •Allow K+ to build up outside the cell, causing the cell to repolarize•Corresponds to T wave on the ECG

Can be: Can be: • Early after-depolarizations (EADs), arise from a reduced level of Early after-depolarizations (EADs), arise from a reduced level of

membrane potential during phases 2 (type 1 – augmented membrane potential during phases 2 (type 1 – augmented opening of Ca channels) and 3 (type 2 – augmented opening of opening of Ca channels) and 3 (type 2 – augmented opening of Na channels) of the cardiac action potentialNa channels) of the cardiac action potential

• Late or delayed after-depolarizations (DADs), occur after Late or delayed after-depolarizations (DADs), occur after completion of repolarization (phase 4 – secondary to increase completion of repolarization (phase 4 – secondary to increase cytosolic Cacytosolic Ca++++ levels), generally at a more negative membrane levels), generally at a more negative membrane potential than that from which EADs arisepotential than that from which EADs arise

• Causes of EAD include:Causes of EAD include:1.1. Low potassium blood levelLow potassium blood level

2.2. Slow heart rateSlow heart rate

3.3. Drug toxicity (i.e. quinidine causing torsades de Drug toxicity (i.e. quinidine causing torsades de pointes)pointes)

• DAD may cause triggered activity due to:DAD may cause triggered activity due to:1.1. Premature beatsPremature beats

2.2. Increased serum CaIncreased serum Ca++++ level level

3.3. Increased adrenaline levels Increased adrenaline levels

4.4. Digitalis toxicityDigitalis toxicity

Can be:Can be:– Conduction blockConduction block– Re-entryRe-entry

Disorder of Impulse ConductionDisorder of Impulse Conduction

Mechanism of ReentryMechanism of Reentry

Mechanism of ReentryMechanism of Reentry

AV nodal reentrant tachycardia (AVNRT)AV nodal reentrant tachycardia (AVNRT)

AV reentrant tachycardia (AVRT)AV reentrant tachycardia (AVRT)– OrthodromicOrthodromic– AntidromicAntidromic

Atrial flutterAtrial flutter

Atrial fibrillationAtrial fibrillation

Ventricular tachycardiaVentricular tachycardia

Reentrent RhythmsReentrent Rhythms

Abrupt onset and termination of the arrhythmiaAbrupt onset and termination of the arrhythmia

The P wave of the first beat of the arrhythmia is The P wave of the first beat of the arrhythmia is different as the remaining beats of the different as the remaining beats of the arrhythmia (if a P wave is present at all)arrhythmia (if a P wave is present at all)

Recognizing Reentry on EKG

Example of AVNRTExample of AVNRT

Most cases of atrial flutter are caused by a large reentrant circuit in the wall of the right atrium

Atrial FlutterAtrial Flutter

Atrial fibrillation is caused by numerous wavelets of depolarization spreading throughout the atria simultaneously, leading to an absence of coordinated

atrial contraction

Atrial Fibrillation

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