Arrhythmias

Arrhythmias, also called cardiac dysrhythmias, are abnormalities in the rate, regularity, origin, or conduction of the electrical impulses that coordinate contraction of the heart. They can result from abnormal impulse generation, abnormal conduction through the cardiac conduction system, or a combination of these mechanisms.

Normal cardiac rhythm usually originates in the sinoatrial (SA) node, spreads through the atrial myocardium to the atrioventricular (AV) node, and then travels through the bundle of His, right and left bundle branches, Purkinje fibers, and ventricular myocardium. Disruption at any point in this electrical sequence can produce an arrhythmia.

Arrhythmias vary greatly in clinical importance. Some produce no symptoms and require little or no intervention, while others can reduce cardiac output, cause syncope, promote thromboembolism, or result in sudden cardiac arrest.

Normal Cardiac Rhythm

Normal sinus rhythm begins in the SA node, the dominant physiological pacemaker of the heart.

Electrical impulses generated by the SA node spread across both atria, producing atrial depolarization and contraction before reaching the AV node.

Normal Conduction Pathway

The normal electrical pathway can be represented as:

SA node → Atrial myocardium → AV node → Bundle of His → Right and left bundle branches → Purkinje fibers → Ventricular myocardium

Role of the SA Node

The SA node normally initiates each heartbeat because its pacemaker cells spontaneously depolarize more rapidly than other potential pacemaker tissues.

Autonomic influences modify the rate of SA nodal firing, allowing heart rate to change according to physiological requirements.

Role of the AV Node

The AV node conducts impulses from the atria toward the ventricles and introduces a physiological conduction delay.

This delay allows atrial contraction to contribute to ventricular filling before ventricular systole begins.

His-Purkinje System

The His-Purkinje system rapidly distributes electrical activation throughout the ventricles.

Rapid coordinated conduction helps produce an organized ventricular contraction capable of efficiently ejecting blood.

Classification of Arrhythmias

Arrhythmias can be classified according to rate, anatomical origin, mechanism, duration, or electrocardiographic characteristics.

Classification	Examples
Bradyarrhythmias	Sinus bradycardia, AV block, sinus node dysfunction
Supraventricular tachyarrhythmias	Atrial fibrillation, atrial flutter, AV nodal reentrant tachycardia
Ventricular arrhythmias	Premature ventricular complexes, ventricular tachycardia, ventricular fibrillation
Conduction disorders	AV blocks, bundle branch blocks

Mechanisms of Arrhythmias

Most arrhythmias arise through one or more fundamental electrophysiological mechanisms.

These include abnormal automaticity, triggered activity, reentry, and abnormalities of impulse conduction.

Abnormal Automaticity

Automaticity is the ability of cardiac cells to generate spontaneous electrical impulses.

Changes in pacemaker activity can cause the SA node to fire too rapidly or slowly, or allow other cardiac tissues to assume pacemaker activity.

Ectopic Pacemakers

An ectopic focus is a site outside the normal SA nodal pacemaker that generates cardiac impulses.

Ectopic activity can originate in atrial tissue, the AV junction, or ventricular myocardium.

Triggered Activity

Triggered activity results from abnormal depolarizations that occur during or after an action potential.

These abnormal electrical events can initiate premature beats or sustained tachyarrhythmias under certain conditions.

Reentry

Reentry occurs when an electrical impulse repeatedly circulates through a pathway and reactivates tissue that has recovered excitability.

It is an important mechanism underlying many tachyarrhythmias.

Requirements for Reentry

Reentrant arrhythmias generally require an appropriate conduction pathway, differences in conduction or refractory properties, and circumstances that permit an impulse to continue circulating rather than extinguishing.

Conduction Abnormalities

Electrical impulses can be delayed or blocked as they travel through the conduction system.

Depending on the site and severity of conduction impairment, this may produce bradycardia, altered activation sequences, or conditions favoring reentry.

Bradycardia

Bradycardia refers to a slower-than-expected heart rate in the relevant physiological context.

It may be a normal adaptation, such as in well-trained individuals, or result from conduction disease, medications, metabolic abnormalities, autonomic influences, or other conditions.

Sinus Bradycardia

In sinus bradycardia, impulses continue to originate from the SA node but occur at a reduced rate.

The ECG retains the general sequence of sinus rhythm, with atrial activation preceding ventricular activation.

Tachycardia

Tachycardia refers to an abnormally rapid heart rate relative to the clinical setting.

Tachycardias may originate above the ventricles or within ventricular tissue.

Sinus Tachycardia

In sinus tachycardia, the SA node remains the pacemaker but fires more rapidly.

It commonly occurs as a physiological response to exercise, sympathetic activation, fever, reduced circulating volume, or other conditions that increase cardiac demand.

Premature Atrial Complexes

Premature atrial complexes are early depolarizations originating from atrial tissue outside the SA node.

They are common and may occur in otherwise healthy individuals, although frequent ectopic activity can also accompany cardiac or systemic conditions.

Premature Ventricular Complexes

Premature ventricular complexes (PVCs) originate prematurely within ventricular myocardium or the ventricular conduction system.

Because ventricular activation does not initially follow the normal His-Purkinje sequence, the QRS complex is typically broad and morphologically abnormal.

Supraventricular Tachycardia

Supraventricular tachycardia (SVT) broadly describes rapid rhythms arising above the ventricles.

Several distinct mechanisms can produce SVT, including reentrant circuits involving the AV node or accessory conduction pathways.

AV Nodal Reentrant Tachycardia

AV nodal reentrant tachycardia (AVNRT) is a common form of paroxysmal supraventricular tachycardia.

It involves a reentrant circuit associated with functionally distinct conduction pathways in or near the AV node.

AV Reentrant Tachycardia

AV reentrant tachycardia (AVRT) involves a reentrant circuit that uses the normal AV conduction system and an accessory atrioventricular pathway.

The accessory pathway provides an additional electrical connection between atrial and ventricular tissue.

Wolff-Parkinson-White Pattern

An accessory atrioventricular pathway can permit ventricular pre-excitation, producing the characteristic electrocardiographic pattern associated with Wolff-Parkinson-White syndrome when accompanied by clinically relevant tachyarrhythmias.

Accessory pathways can also participate in reentrant tachycardias.

Atrial Fibrillation

Atrial fibrillation (AF) is a supraventricular arrhythmia characterized by disorganized atrial electrical activation and ineffective coordinated atrial contraction.

AV nodal conduction of the irregular atrial activity produces a characteristically irregular ventricular rhythm when AV conduction is intact.

Electrical Activity in Atrial Fibrillation

During atrial fibrillation, organized sinus P waves are absent.

Instead, atrial electrical activity is rapid and disorganized, while ventricular activation occurs irregularly according to conduction through the AV node.

Loss of Atrial Contraction

Because atrial activation is disorganized, the normal coordinated atrial contribution to ventricular filling is reduced or absent.

This may be particularly important in individuals whose ventricular filling depends substantially on atrial contraction.

Thromboembolic Risk in Atrial Fibrillation

Atrial fibrillation can promote blood stasis, particularly within the left atrial appendage.

Thrombus formation in this region can lead to systemic embolization, including ischemic stroke.

Atrial Flutter

Atrial flutter is a macroreentrant atrial tachyarrhythmia characterized by rapid, organized atrial activity.

The AV node usually conducts only a proportion of atrial impulses to the ventricles.

Atrial Flutter Versus Atrial Fibrillation

Feature	Atrial Flutter	Atrial Fibrillation
Atrial activation	Rapid but organized	Rapid and disorganized
Mechanism	Usually macroreentry	Complex electrical mechanisms with multiple wavefronts and triggers
Ventricular rhythm	May be regular or variable depending on AV conduction	Typically irregularly irregular

Ventricular Tachycardia

Ventricular tachycardia (VT) is a rapid rhythm originating within the ventricles.

It often produces broad QRS complexes because ventricular activation occurs outside the normal rapid conduction sequence.

Monomorphic Ventricular Tachycardia

In monomorphic ventricular tachycardia, successive QRS complexes have a relatively consistent morphology.

It can occur when a stable reentrant circuit develops within diseased or scarred ventricular myocardium.

Polymorphic Ventricular Tachycardia

Polymorphic ventricular tachycardia is characterized by changing QRS morphology during the tachycardia.

It can occur in several electrophysiological settings, including conditions associated with abnormal ventricular repolarization.

Torsades de Pointes

Torsades de pointes is a form of polymorphic ventricular tachycardia associated with prolongation of the QT interval.

The ventricular complexes appear to change amplitude and electrical axis around the ECG baseline.

Ventricular Fibrillation

Ventricular fibrillation (VF) consists of chaotic ventricular electrical activity without coordinated ventricular contraction.

Effective cardiac output ceases, making ventricular fibrillation a cause of cardiac arrest requiring immediate resuscitative treatment.

Asystole

Asystole represents absence of effective ventricular electrical activity.

It produces no effective cardiac output and is a cardiac arrest rhythm.

Sinus Node Dysfunction

Sinus node dysfunction describes abnormalities of impulse formation and propagation associated with the SA node and surrounding atrial tissue.

Manifestations can include inappropriate sinus bradycardia, sinus pauses, sinoatrial exit block, and alternating slow and rapid atrial rhythms.

Atrioventricular Block

AV block occurs when conduction of impulses from the atria to the ventricles is delayed or interrupted.

It is traditionally classified into first-degree, second-degree, and third-degree AV block.

First-Degree AV Block

In first-degree AV block, atrial impulses are conducted to the ventricles but AV conduction is prolonged.

On the ECG, this is represented by prolongation of the PR interval.

Second-Degree AV Block

In second-degree AV block, some atrial impulses fail to conduct to the ventricles.

Different conduction patterns occur depending on the underlying type and anatomical level of block.

Mobitz Type I

Mobitz type I, or Wenckebach block, typically demonstrates progressive prolongation of AV conduction until an atrial impulse fails to produce a ventricular response.

Mobitz Type II

In Mobitz type II block, conduction can fail suddenly without the characteristic progressive PR prolongation of Mobitz type I.

This pattern often indicates disease within the His-Purkinje conduction system and can progress to more advanced block.

Third-Degree AV Block

Third-degree AV block, or complete heart block, occurs when atrial impulses do not conduct to the ventricles.

The atria and ventricles then depolarize independently, with ventricular activity maintained by a subsidiary escape pacemaker when present.

Bundle Branch Block

A bundle branch block delays or prevents conduction through one of the major ventricular bundle branches.

The affected ventricle is activated through altered pathways, producing characteristic changes in QRS morphology and duration.

Right Bundle Branch Block

In right bundle branch block, conduction through the right bundle branch is delayed or interrupted.

The right ventricle is activated later through electrical spread from the left ventricular conduction system.

Left Bundle Branch Block

In left bundle branch block, normal rapid conduction through the left bundle system is impaired.

This substantially changes the sequence of ventricular activation and produces characteristic broad QRS abnormalities.

Escape Rhythms

When the dominant pacemaker fails or impulses are blocked, subsidiary pacemaker tissues may generate an escape rhythm.

Escape activity can arise from the AV junction or ventricular conduction system and may preserve a degree of cardiac output.

Electrocardiography

The electrocardiogram (ECG) records electrical activity generated by the heart and is central to identifying and classifying arrhythmias.

Analysis includes heart rate, rhythm regularity, P waves, PR interval, QRS duration and morphology, QT interval, and relationships between atrial and ventricular activity.

Major ECG Components

ECG Component	Electrical Event
P wave	Atrial depolarization
PR interval	Conduction from atria through AV conduction system toward ventricles
QRS complex	Ventricular depolarization
ST segment	Period after ventricular depolarization before major repolarization is complete
T wave	Ventricular repolarization
QT interval	Total duration of ventricular depolarization and repolarization

Ambulatory Rhythm Monitoring

Intermittent arrhythmias may not occur during a standard short ECG recording.

Longer monitoring methods can record cardiac rhythm during daily activity and help correlate symptoms with electrical events.

Electrophysiology Studies

An invasive electrophysiology study uses intracardiac electrodes to record electrical activity and stimulate the heart.

It can help define conduction pathways, identify arrhythmia mechanisms, and guide catheter ablation.

Effects on Cardiac Output

Arrhythmias can reduce cardiac output by changing heart rate, ventricular filling time, atrioventricular coordination, or the synchrony of ventricular contraction.

The hemodynamic effect depends on the type and duration of the arrhythmia and the underlying condition of the heart.

Very Rapid Heart Rates

Marked tachycardia shortens diastolic filling time.

If ventricular filling becomes inadequate, stroke volume and cardiac output may fall despite the increased heart rate.

Very Slow Heart Rates

Severe bradycardia can reduce cardiac output when stroke volume cannot increase sufficiently to compensate for the low rate.

Loss of AV Synchrony

Some arrhythmias disrupt the normal timing between atrial and ventricular contraction.

Loss of coordinated atrial contribution to ventricular filling can reduce hemodynamic efficiency.

Common Symptoms

Arrhythmias may be asymptomatic or produce symptoms related to abnormal cardiac activity and altered tissue perfusion.

• Palpitations
• Dizziness or lightheadedness
• Syncope or near-syncope
• Chest discomfort
• Shortness of breath
• Fatigue
• Reduced exercise tolerance

Causes and Predisposing Factors

Arrhythmias can occur in structurally normal hearts or in association with cardiovascular and systemic disorders.

Potential contributors include myocardial ischemia, myocardial scar, structural heart disease, electrolyte abnormalities, medications, autonomic influences, endocrine disturbances, inherited ion-channel abnormalities, and degeneration of the conduction system.

Electrolyte Abnormalities

Changes in extracellular potassium, magnesium, calcium, and other electrolytes can alter cardiac membrane potentials, conduction, and repolarization.

Severe abnormalities can increase susceptibility to clinically important arrhythmias.

Myocardial Ischemia

Reduced coronary blood flow can alter the electrical properties of myocardial cells.

Acute ischemia can produce conduction slowing, abnormal automaticity, and heterogeneous refractoriness that may facilitate arrhythmias.

Myocardial Scar

Fibrosis or scar tissue can create regions of abnormal electrical conduction.

Slow and nonuniform conduction around scarred myocardium can provide a substrate for reentrant ventricular or atrial arrhythmias.

Autonomic Nervous System

Sympathetic and parasympathetic activity influence pacemaker rate, AV nodal conduction, refractoriness, and myocardial excitability.

Changes in autonomic balance can therefore trigger or modify certain arrhythmias.

Clinical Significance

Syncope

Arrhythmias can cause transient loss of consciousness when cardiac output falls enough to reduce cerebral perfusion.

Both severe bradyarrhythmias and rapid tachyarrhythmias can produce arrhythmic syncope.

Stroke

Atrial fibrillation is an important cause of cardioembolic ischemic stroke.

Loss of coordinated atrial contraction and altered atrial flow can promote thrombus formation, particularly within the left atrial appendage.

Heart Failure

Persistent rapid arrhythmias can impair ventricular function, while pre-existing heart failure can increase susceptibility to arrhythmias.

Loss of AV synchrony and irregular ventricular activation can further compromise cardiac performance in susceptible individuals.

Sudden Cardiac Arrest

Ventricular fibrillation and certain ventricular tachyarrhythmias can abruptly eliminate effective cardiac output.

These rhythms are major electrical mechanisms of sudden cardiac arrest.

General Treatment Approaches

Management depends on the specific rhythm, symptoms, hemodynamic effects, underlying disease, and risk of complications.

Approaches may include treatment of reversible causes, medications, electrical cardioversion or defibrillation, pacing, catheter ablation, and implantable cardiac devices.

Antiarrhythmic Medications

Antiarrhythmic drugs modify cardiac ion channels, autonomic influences, conduction, or refractoriness.

The choice of medication depends on the specific arrhythmia and clinical context because these drugs can have important adverse effects and may themselves promote arrhythmias.

Electrical Cardioversion

Synchronized cardioversion delivers an electrical shock timed to cardiac electrical activity.

It can terminate selected tachyarrhythmias by depolarizing a critical mass of myocardium and interrupting abnormal electrical circuits.

Defibrillation

Defibrillation delivers an unsynchronized electrical shock and is used for shockable cardiac arrest rhythms such as ventricular fibrillation and pulseless ventricular tachycardia.

Pacemakers

An implanted pacemaker can provide electrical stimulation when intrinsic impulse formation or conduction is inadequate.

Pacemakers are commonly used for selected clinically significant bradyarrhythmias and conduction disorders.

Implantable Cardioverter-Defibrillator

An implantable cardioverter-defibrillator (ICD) monitors cardiac rhythm and can deliver therapy for certain dangerous ventricular tachyarrhythmias.

Depending on the device and rhythm, therapy may include antitachycardia pacing or electrical shock.

Catheter Ablation

Catheter ablation targets cardiac tissue responsible for initiating or sustaining an arrhythmia.

Energy is delivered through an intracardiac catheter to modify or eliminate the abnormal electrical substrate.

Arrhythmia Overview

Arrhythmia	Primary Site or Mechanism	Typical Electrical Feature
Sinus bradycardia	SA node	Slow sinus rhythm
Sinus tachycardia	SA node	Rapid sinus rhythm
Atrial fibrillation	Atria	Disorganized atrial activity with irregular ventricular response
Atrial flutter	Atrial macroreentry	Rapid organized atrial activity
AVNRT	AV nodal region	Reentrant supraventricular tachycardia
Ventricular tachycardia	Ventricles	Rapid ventricular rhythm, commonly broad QRS
Ventricular fibrillation	Ventricles	Chaotic ventricular electrical activity
AV block	AV conduction pathway	Delayed or interrupted atrioventricular conduction

Key Features of Arrhythmias

Feature	Key Point
Definition	Abnormality of cardiac rate, rhythm, impulse origin, or conduction
Normal pacemaker	Sinoatrial node
Major mechanisms	Abnormal automaticity, triggered activity, reentry, and conduction abnormalities
Primary diagnostic tool	Electrocardiogram
Common sustained arrhythmia	Atrial fibrillation
Major AF complication	Systemic thromboembolism, including stroke
Dangerous ventricular rhythms	Ventricular tachycardia and ventricular fibrillation
Potential hemodynamic effect	Reduced cardiac output and tissue perfusion

Anatomical and Physiological Importance

Arrhythmias illustrate the dependence of effective cardiac pumping on the precise organization of the cardiac conduction system. Normal impulse generation in the SA node, physiological delay through the AV node, and rapid conduction through the His-Purkinje network allow atrial and ventricular contractions to occur in a coordinated sequence.

Abnormal impulse formation or conduction can disrupt this sequence at many levels. Atrial arrhythmias may impair coordinated atrial contraction, conduction blocks can separate atrial and ventricular electrical activity, and ventricular arrhythmias can severely compromise the organized contraction required to maintain cardiac output.

Understanding the anatomical pathways of cardiac conduction together with the electrophysiological mechanisms of automaticity, refractoriness, and reentry provides the foundation for interpreting arrhythmias and their effects on cardiovascular function.