• Electrophysiology of the Heart
• Arrhythmia
• Mechanisms of Cardiac Arrhythmias
• 1. Automaticity
• Altered Normal Automaticity
• Abnormal Automaticity
• 2. Triggered Activity
• Early After-Depolarization
• Delayed After-Depolarization
• 3. Reentry
• Reentry Mechanism
• Reentry Formation
• Classification of Reentry
• Anatomical Reentry
• Functional Reentry
• Leading Circle Reentry
• Anisotropic Reentry
• Figure of 8 Reentry
• Reflection Reentry
• Spiral Wave (Rotor) Reentry
• Phase 2 Reentry

Electrophysiology of the Heart

• Cardiomyocytes have the ability of automaticity:
• Spontaneously generate impulses (action potential)
• Impulse propagation:
• Rapidly through the conduction system
• Slowly through the working myocardium
• Overdrive suppression
• Is a fundamental electrophysiological principle of arrhythmias
• The focus with the highest frequency (SA node)
• deactivates foci with lower frequency
• Refractory period
• Is the period immediately after depolarization of the cardiomyocyte by an impulse
• During this period, the cardiomyocyte is non-excitable
• Does not respond to another impulse

Mechanisms of Cardiac Arrhythmias

• Arrhythmias occur through 3 mechanisms:
  
  
1. Automaticity
• Change in normal automaticity
• Abnormal automaticity
2. Triggering Activity
• Early afterdepolarization
• Late afterdepolarization
3. Reentry
• Anatomical reentry
• Functional reentry

1. Automaticity

• Each cardiomyocyte has the ability for automaticity (spontaneously generate impulses)
• Impulses are generated with the highest frequency in the SA node (primary pacemaker)
• The SA node then deactivates other secondary pacemakers through overdrive suppression:
• AV node, Purkinje fibers in the ventricles, atrial myocardium
• Automaticity can induce arrhythmia through 2 mechanisms:
• Change in normal automaticity
• Abnormal automaticity

Altered Normal Automaticity

Altered normal automaticity

• It is a change in normal automaticity in cardiac pacemakers:
• SA node (primary pacemaker)
• AV junction (secondary pacemaker)
• Purkinje fibers in the ventricles (tertiary pacemaker)
  
  
• Arrhythmias due to altered normal automaticity
• Sinus tachycardia
• Sinus bradycardia
• Inappropriate sinus tachycardia
• Accelerated ventricular rhythm
• Junctional tachycardia

2. Triggered Activity

• ARP (Absolute refractory period)
• During ARP, cardiomyocytes do not respond to another impulse
• RRP (Relative refractory period)
• During RRP, cardiomyocytes only respond to a suprathreshold impulse
• Triggered activity means
• that an impulse occurs in a cardiomyocyte
• Shortly after depolarization (during ARP)
• A premature action potential is generated, which triggers arrhythmia
• Triggered activity can induce arrhythmia through 2 mechanisms:
• Early after-depolarization (occurs during phases 2 and 3)
• Delayed after-depolarization (occurs during phase 4)

Early After-Depolarization and Torsades de Pointes

• With a prolonged QT interval
• The action potential is prolonged - mainly repolarization (green curve)
• Prolonged repolarization is seen on the ECG during sinus rhythm as a prolonged QT interval
• Early After-Depolarization (EAD)
• Triggers a ventricular extrasystole (R-on-T phenomenon)
• and initiates Torsades de Pointes
• TdP is sustained by a reentry mechanism due to transmural dispersion of repolarization

Delayed After-Depolarization

Delayed Afterdepolarization Induced Triggered Activity

• Depolarization occurs prematurely
• During the 4th phase of the action potential
• Risk factors for delayed afterdepolarization include:
• Digoxin toxicity
• Catecholamines (high levels)
• Hypercalcemia
• Occurs during tachycardia
  
  
• Arrhythmias associated with delayed afterdepolarization
• Bidirectional Ventricular Tachycardia
• During digoxin toxicity
• During extreme physical exertion (catecholamines)
• Accelerated Ventricular Rhythm during myocardial infarction

3. Reentry

• It is the most common mechanism of arrhythmias
• The impulse (action potential) circulates (most commonly around an anatomical obstacle)
• From the reentry (loop), impulses emerge and activate the myocardium
• Impulses emerge from the reentry exactly like from the SA node in sinus rhythm
  
  
• In the past, the reentry mechanism for SVT was referred to by terms:
• Reciprocating tachycardia, Circus movement tachycardia
• These terms are no longer used

Action Potential

• Impulse in the myocardium of the ventricles is seen on EKG as the QT interval
• Begins with depolarization (phase 0)
• Ends with repolarization (phase 4)
• After depolarization follows ARP and RRP
• During ARP (Absolute Refractory Period)
• Cardiomyocytes do not respond to a subsequent impulse
• During RRP (Relative Refractory Period)
• Cardiomyocytes respond only to a suprathreshold impulse

Reentry and Excitation Gap

• The impulse circles most often around an anatomical obstacle
• The tip must not encounter the absolute refractory period
• Reentry would be interrupted
• Excitation gap
• It is the segment between the tip and the end of ARP
• It is an excitability zone within reentry
• It is important for electrophysiological studies
• Used in the diagnosis and treatment of reentry

Ventricular Activation via Purkinje Fibers

• The ventricular myocardium is activated by an impulse
• from the conducting system through Purkinje fibers
• There are 2 parallel pathways
• The fibers are separated by an anatomical obstacle
• It is a physiological obstacle (not a scar from an infarction)
• Behind the obstacle, impulses dissipate and disappear
• The remaining impulses continue to the myocardium

Start of Reentry

• Reentry begins with a atrial or ventricular extrasystole
• The extrasystole enters the reentry circuit during the excitation gap
• The impulse moves through
• Anterogradely only through 1 Purkinje fiber
• Retrogradely through the slow ischemic portion
• At the end of the slow pathway
• There is no longer anterograde impulse in the ARP
• (ARP - Absolute Refractory Period)
• Retrograde impulse from the slow pathway:
• Begins to circulate in reentry

Classification of Reentry

• Anatomical reentry (most common)
• Functional reentry
• Leading circle reentry
• Anisotropic reentry
• Figure of 8 reentry
• Reflection reentry
• Spiral wave (rotor) reentry
• Phase 2 reentry

Functional Reentry

• A functional barrier develops
• It involves an electrophysiological change in the myocardium due to:
• Ischemia
• Mineral imbalance
• Administration of certain antiarrhythmic drugs
• Functional reentry can involve several mechanisms:
• Leading circle reentry
• Anisotropic reentry
• Figure of 8 reentry
• Reflection reentry
• Spiral wave (rotor) reentry
• Phase 2 reentry

Anisotropic Reentry

• Isotropic conduction
• Has the same conduction properties in all directions
• Anisotropic conduction
• Does not have the same properties in all directions
• Conduction of impulses through myocardium is anisotropic
• Impulse propagation
• Fast - longitudinal direction (gap junction)
• Slow - transversal direction
• In anisotropic reentry
• A refractory line forms in the myocardium
• around which reentry circulates
• The speed of impulse is shown in ms
  
  
• Arrhythmias associated with Anisotropic Reentry:
• Ventricular tachycardia

Reflection Reentry

• It is an atypical reentry mechanism
• Because the impulse does not circulate in a loop
• The impulse oscillates along a linear axis
• Linear section of the myocardium
• Has electrophysiologically altered area
• At the boundary of the altered myocardium, there is a zone
• That reverses the direction of the impulse (action potential)
• The impulse thus oscillates "up and down"
• Along the linear section of the myocardium
• Can occur on an area of 1-2mm²
• Difficult to differentiate from ectopic automaticity

Phase 2 Reentry

• Action Potential (AP) in the myocardial wall
• Differs partially in
• The epicardium
• The mid-myocardium (M-cells)
• The endocardium
• AP also partially differs in
• The right and left ventricles
• Transmural Repolarization Dispersion
• Is the difference in the duration of action potentials (primarily repolarization)

• Phase 2 reentry
• It is a functional reentry that occurs during a disturbance of phase 2 of the action potential
• Reentry is most commonly triggered by ventricular extrasystole (VES)
• The reentry circuit circulates between the endocardium and epicardium
• The impulse does not encounter its own absolute refractory period
• The impulse essentially oscillates between the endocardium and epicardium
• This is not a typical circular reentry