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Cardiac conduction system

The cardiac conduction system is a specialized network of cells within the heart that generates electrical impulses and coordinates the rhythmic contractions of the atria and ventricles to ensure efficient pumping of blood.^[1] This system comprises key components including the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers, which together form an intrinsic pacemaker and conduction pathway independent of external neural input, though modulated by the autonomic nervous system.^[2]^[3] The SA node, located in the upper right atrium near the superior vena cava, serves as the primary pacemaker by spontaneously depolarizing at a rate of 60-100 times per minute,^[4] initiating atrial contraction and propagating the impulse across both atria via internodal pathways.^[2] The electrical signal then reaches the AV node, situated at the base of the right atrium within the triangle of Koch near the tricuspid valve, where it is delayed for approximately 0.1 seconds to allow complete atrial emptying before ventricular filling.^[3]^[5] From the AV node, the impulse travels through the bundle of His, which penetrates the fibrous septum between the atria and ventricles, and divides into left and right bundle branches that course along the interventricular septum.^[2] The bundle branches terminate in an extensive Purkinje fiber network that ramifies through the ventricular myocardium, rapidly distributing the depolarization wave to synchronize ventricular contraction from apex to base, achieving a conduction velocity up to 4 meters per second—far faster than in ordinary myocardial cells.^[2]^[6] This coordinated sequence ensures atrial systole precedes ventricular systole, optimizing cardiac output, and the system's activity can be monitored noninvasively via electrocardiography (ECG) to detect rhythm disorders such as bradycardia or arrhythmias.^[1] Dysfunctions in this system, often due to aging, ischemia, or fibrosis, can lead to conduction blocks or ectopic pacemakers, underscoring its critical role in maintaining cardiovascular homeostasis.^[3]

Anatomy and development

Structural components

The sinoatrial (SA) node is a crescent-shaped structure located subepicardially in the right atrium at the junction with the superior vena cava, with its tail extending 10-20 mm along the crista terminalis.^[7] Its intramural architecture features a central body approximately 15 mm long and 4 mm wide, with radiating projections toward the terminal crest, pectinate muscles, and interatrial septum, enclosed partly by myocardium and epicardial fat.^[8] The node consists of specialized nodal cells organized into central and peripheral zones, including pacemaker (P) cells that are small, ovoid, and pale-staining with sparse myofibrils, and transitional (T) cells that bridge to atrial myocardium.^[9] These P cells exhibit fewer myofibrils and fewer gap junctions compared to surrounding contractile myocytes, supporting automaticity while allowing controlled impulse transmission to the atria.^[10] The atrioventricular (AV) node resides at the apex of the triangle of Koch in the right atrium, positioned posterior to the septal leaflet of the tricuspid valve and adjacent to the coronary sinus ostium.^[11] It comprises a compact nodal portion (N region) of small, pale cells with low connexin expression, transitioning to the penetrating portion that extends anteriorly through the central fibrous body, and the nodo-His region with mixed connexin profiles.^[7] Histologically, AV nodal cells are smaller than contractile myocytes, with reduced myofibrils and low connexin expression, contributing to slow conduction and electrical isolation from adjacent atrial tissue.^[7] The bundle of His emerges as the continuation of the AV node's penetrating portion, traversing the fibrous septum to bifurcate into the right and left bundle branches on the interventricular septum.^[8] The right bundle branch forms a narrow, ribbon-like tract descending along a septal trabeculation, while the left bundle branch spreads as a broad, trifascicular sheet (anterior, posterior, and septal fascicles) across the left septal endocardium.^[8] These branches connect to an extensive Purkinje fiber network, which ramifies as fine, low-density strands on the ventricular endocardial surfaces, denser on the left side in a cone-like distribution toward the apex and papillary muscles.^[8] Purkinje fibers consist of elongated cells with fewer myofibrils and abundant gap junctions relative to contractile ventricular myocytes, optimizing conduction velocity.^[10] Interatrial synchronization is supported by specialized tracts, including internodal pathways from the SA node along the crista terminalis and interatrial septum, and Bachmann's bundle, a prominent muscular band of parallel myocardial strands crossing the anterior interatrial groove from right to left atrium.^[12] These structures feature aligned cardiomyocytes with enhanced gap junction density compared to ordinary atrial myocytes, aiding efficient spread of impulses.^[12]

Embryological origins

The cardiac conduction system originates during early embryogenesis from the cardiac crescent, a mesodermal structure that forms around the third week of gestation (approximately day 21-23 in humans), which gives rise to the primary heart tube and initiates rhythmic contractions through peristaltic waves.^[13] This primitive conduction begins with the primordial atrium serving as the initial pacemaker, transitioning to the sinus venosus as the heart tube elongates and loops between weeks 3 and 4.^[13] By the end of week 3, precursors of the conduction system are specified within the myocardial wall of the heart tube, setting the stage for regional differentiation into pacemaker and conductive tissues.^[14] The sinoatrial (SA) node develops from tissues of the right common cardinal vein and sinus venosus, which incorporate into the right atrium during heart looping around week 5.^[13] These venous pole structures express early pacemaker markers like Hcn4 and Tbx3, forming the mature SA node that becomes the primary pacemaker in the adult heart.^[14] In contrast, the atrioventricular (AV) node and His bundle derive from the AV canal myocardium and associated endocardial cushions, which undergo epithelial-to-mesenchymal transformation to contribute to the insulating annulus fibrosus and conductive axis by weeks 5-7.^[13] The AV canal's slow-conducting properties persist in the AV node, ensuring timed ventricular activation.^[14] Purkinje fibers emerge later, during mid-to-late fetal stages (around weeks 8-10), through differentiation and migration from the subendocardial ventricular myocardium, guided by epicardial-mesenchymal interactions involving epicardium-derived cells.^[13] These cells secrete factors that promote Purkinje network expansion across the ventricles, forming a rapid-conducting subendocardial meshwork.^[14] Specification of the conduction system is tightly regulated by transcription factors such as Tbx5 and Nkx2.5, which promote AV bundle and Purkinje fiber development while repressing chamber-like gene expression in conductive tissues.^[14] Signaling pathways, including Wnt (which inhibits Nkx2.5 to maintain SA node identity) and Notch (which drives Purkinje differentiation via Bmp10 upregulation), further refine these lineages during heart tube maturation.^[14] Disruptions in these processes can lead to congenital anomalies, such as Wolff-Parkinson-White syndrome from genetic mutations (e.g., PRKAG2 in humans) causing accessory pathways, or persistent left superior vena cava altering SA node positioning and function.^[14]^[13]

Physiology

Action potential mechanisms

The action potential in cells of the cardiac conduction system, particularly pacemaker cells, differs from that in contractile myocardial cells, enabling spontaneous electrical activity essential for initiating heartbeats. In pacemaker cells such as those in the sinoatrial (SA) node and atrioventricular (AV) node, the action potential consists of three main phases, lacking the distinct early repolarization (phase 1) and plateau (phase 2) seen in ventricular myocytes. Phase 0 involves rapid depolarization, but unlike the fast sodium-driven upstroke in working myocardium, it relies primarily on calcium influx through L-type calcium channels, resulting in a slower upstroke velocity of approximately 10-20 V/s compared to 200-500 V/s in ventricular cells.^[15] Phase 3 is repolarization driven by potassium efflux, while phase 4 is characterized by spontaneous diastolic depolarization, which progressively brings the membrane potential to the threshold for the next action potential, conferring automaticity to these cells.^[16] This phase 4 depolarization is mediated by the "funny" current (I_f), a hyperpolarization-activated mixed sodium-potassium inward current, alongside contributions from calcium clock mechanisms involving sarcoplasmic reticulum calcium release.^[17] Key differences between pacemaker and working myocardial action potentials arise from distinct ion channel expression and function. Pacemaker cells exhibit low expression of the inward rectifier potassium channel Kir2.1, which underlies the stable resting potential (I_{K1}) in ventricular myocytes, preventing a true resting phase and allowing phase 4 drift.^[18] In SA and AV nodal cells, phase 0 depolarization depends on L-type calcium channels (Cav1.2) rather than voltage-gated sodium channels (Nav1.5), which are sparsely expressed or inactivated at nodal diastolic potentials, leading to slower conduction and reduced excitability compared to the rapid, sodium-mediated propagation in atrial or ventricular tissue.^[15] The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, particularly HCN4 in the SA node, conduct I_f, activating upon hyperpolarization at the end of repolarization to initiate phase 4 with slow kinetics that contribute to the timing of subsequent beats.^[19] These mechanisms establish a hierarchy of automaticity among conduction system components, ensuring the SA node dominates rhythm generation under normal conditions. The SA node exhibits the fastest intrinsic firing rate of 60-100 beats per minute, followed by the AV node at 40-60 beats per minute, and Purkinje fibers at 20-40 beats per minute, with dominance determined by the steepest phase 4 slope reaching threshold first.^[20] This hierarchy relies on graded differences in I_f density and activation kinetics, with higher HCN expression and faster diastolic depolarization in the SA node.^[21]

Propagation and synchronization

The electrical impulse in the heart originates in the sinoatrial (SA) node and propagates through a specialized conduction pathway to ensure coordinated contraction. From the SA node, the impulse travels rapidly across the atria via internodal pathways, depolarizing the atrial myocardium and leading to atrial systole. The impulse then reaches the atrioventricular (AV) node, where it is briefly delayed to allow complete atrial emptying, before proceeding through the His bundle, which divides into left and right bundle branches. These branches further ramify into the Purkinje fiber network, distributing the impulse swiftly to the ventricular myocardium for synchronized ventricular contraction.^[22] Efficient propagation along this pathway relies on low-resistance electrical coupling between cardiomyocytes, primarily mediated by gap junctions composed of connexin proteins. In the atria and conduction system, connexin 40 (Cx40) predominates, forming channels that support rapid intercellular current flow, while connexin 43 (Cx43) is more abundant in the ventricular working myocardium, ensuring reliable signal transmission. These gap junctions create a functional syncytium, minimizing conduction delays and maintaining the integrity of the impulse as it traverses from specialized tissues to contractile cells.^[23]^[24] Synchronization of atrial and ventricular contractions is critical for optimal cardiac output, with atrial systole preceding ventricular systole to enhance ventricular filling during late diastole. This "atrial kick" contributes approximately 20-30% of ventricular end-diastolic volume, stretching myocardial fibers and invoking the Frank-Starling mechanism, whereby increased preload augments contractile force. Disruptions in this timing could impair filling and reduce stroke volume, underscoring the conduction system's role in temporal coordination. Conduction velocities vary along the pathway to achieve this: atrial tissue conducts at about 0.4 m/s, sufficient for orderly atrial activation, while the Purkinje system achieves 2-4 m/s, enabling near-simultaneous ventricular depolarization from apex to base.^[25]^[26]^[27] The conduction system also maintains dominance of the primary pacemaker through overdrive suppression, where faster rhythmic activity from the SA node inhibits subsidiary pacemakers. This occurs via hyperpolarization of the membrane potential in slower foci during accelerated firing, temporarily raising their threshold for spontaneous depolarization and preventing ectopic rhythms. Such a mechanism ensures stable heart rate control under normal conditions.^[28]^[29]

Electrophysiology

Atrial depolarization and SA node

The sinoatrial (SA) node serves as the primary pacemaker of the heart, initiating the cardiac cycle through its intrinsic automaticity.^[4] Located at the junction of the superior vena cava and the right atrium, the SA node generates spontaneous action potentials that propagate as an electrical wavefront across the atria, coordinating their contraction prior to ventricular activation.^[4] This process ensures efficient atrial emptying and sets the rhythm for the entire heart, typically at a rate of 60-100 beats per minute in adults under normal conditions.^[4] The electrical activity originating from the SA node manifests on the electrocardiogram (ECG) as the P wave, which represents the depolarization of the atrial myocardium.^[4] Normally, the P wave has a duration of 80-100 ms and an amplitude of less than 2.5 mm in the limb leads, reflecting the rapid and synchronous spread of the depolarization wavefront from the right to the left atrium.^[30] This wavefront is facilitated by preferential conduction pathways in the atrial myocardium, sometimes described in anatomical studies as internodal tracts—including the anterior internodal tract (extending from the anterior margin of the SA node around the superior vena cava to Bachmann's bundle), the middle internodal tract (also known as Wenckebach's tract, running along the interatrial septum), and the posterior internodal tract (coursing posteriorly near the inferior vena cava)—though their existence as specialized structures composed of clusters of conducting fibers remains debated, with electrophysiological evidence supporting efficient conduction via ordinary atrial fibers.^[31]^[32] Although the SA node dominates under normal circumstances, ectopic atrial foci—abnormal sites of automaticity within the atrial myocardium—can potentially override it if they exhibit a faster firing rate.^[4] Such foci, often located near the pulmonary veins or crista terminalis, may suppress SA node activity and assume pacemaker control, leading to rhythms like ectopic atrial tachycardia when their rate exceeds that of the sinus node.^[4] Histologically, the high automaticity of SA node cells arises from their specialized structure, including sparse expression of the inward rectifier potassium current (IK1) channels, which reduces hyperpolarization after repolarization and allows for gradual diastolic depolarization.^[33] These cells are smaller, with fewer myofibrils and a higher density of mitochondria compared to working atrial myocytes, supporting their role in spontaneous impulse generation.^[33]

AV conduction and delay

The atrioventricular (AV) node, located at the base of the interatrial septum, serves as the primary site for delaying the cardiac impulse originating from the sinoatrial node, ensuring sequential atrial and ventricular contraction. This AV nodal conduction time is approximately 50-120 ms, allowing for complete atrial systole and emptying of blood into the ventricles before ventricular activation begins, optimizing ventricular preload and cardiac output.^[34]^[35] On the electrocardiogram (ECG), the PR interval—encompassing intra-atrial conduction, AV nodal conduction time, and initial His bundle activation—is measured from the onset of the P wave (atrial depolarization) to the start of the QRS complex (ventricular depolarization), with a normal duration of 120-200 ms. A prolonged PR interval exceeding 200 ms indicates first-degree AV block, reflecting slowed conduction through the AV node without impulse blockage.^[34] Conduction decrements at the AV node manifest as second-degree AV blocks, including Wenckebach (Mobitz I) and Mobitz II types. In Mobitz I, the PR interval progressively lengthens with each successive beat until a P wave fails to conduct, resulting in a dropped QRS complex; this pattern typically arises within the AV node and is often benign, associated with high vagal tone.^[36] In contrast, Mobitz II features a constant PR interval with abrupt non-conducted P waves, usually originating in the infranodal His-Purkinje system and carrying a higher risk of progression to complete heart block.^[36] Following the AV nodal conduction, the impulse rapidly propagates through the His-Purkinje system, a specialized network of fibers including the bundle of His, bundle branches, and Purkinje fibers, which conducts at high speeds (up to 4 m/s) to synchronize ventricular activation. This efficient distal conduction ensures near-simultaneous contraction of the ventricular myocardium, minimizing dyssynchrony.^[37] AV nodal conduction is modulated by the autonomic nervous system, with sympathetic stimulation via beta-adrenergic receptors accelerating impulse transmission by enhancing calcium influx and shortening the refractory period, thereby reducing the PR interval. Conversely, parasympathetic (vagal) input slows conduction through acetylcholine activation of the IK,ACh potassium current, prolonging the action potential and increasing the delay, which can exacerbate blocks during heightened vagal tone.^[38]

Ventricular activation and repolarization

Ventricular activation begins following the atrioventricular delay, with the electrical impulse traveling from the bundle branches into the Purkinje fiber network, a subendocardial system of specialized conducting cells that rapidly distributes the wavefront across the ventricular myocardium. This network ensures near-simultaneous activation from endocardium to epicardium, optimizing synchronized contraction and ejection fraction by minimizing dyssynchrony. The Purkinje fibers facilitate this by conducting at velocities up to 4 m/s, far exceeding the 0.5-1 m/s of myocardial cells, allowing for efficient propagation despite the ventricles' large mass.^[39]^[40] On the electrocardiogram (ECG), ventricular depolarization manifests as the QRS complex, a series of deflections typically lasting less than 120 ms in duration under normal conditions. The sequence of activation proceeds from the interventricular septum (initial leftward Q wave), to the apical regions of both ventricles (prominent R wave reflecting bulk myocardial depolarization), and finally to the basal portions (S wave as the wavefront reaches the upper ventricles). This septal-to-apical-to-basal progression, driven by the Purkinje system's arborization, ensures coordinated ventricular contraction from apex toward base, promoting effective blood ejection.^[41]^[42] Ventricular repolarization follows depolarization and is represented by the T wave on the ECG, which typically peaks 200-300 ms after the QRS complex onset, corresponding to the recovery of the myocardial cells to their resting state. The T wave's polarity—usually upright in most leads—reflects the repolarization sequence, where epicardial cells recover earlier than endocardial cells, creating a vector opposite to depolarization; this transmural gradient results in a positive deflection as the recovery wavefront moves from epicardium to endocardium. The entire process of ventricular electrical activity, from depolarization onset to repolarization completion, is captured by the QT interval, measured from the start of the Q wave to the end of the T wave. To account for heart rate variations, the corrected QT interval (QTc) is calculated using Bazett's formula: QTc = QT / √RR, where RR is the interval between consecutive R waves in seconds, allowing standardized assessment of repolarization duration.^[43]^[43]^[44] Disruptions in the conduction pathway, such as bundle branch blocks, alter ventricular activation and prolong the QRS complex beyond 120 ms. In right bundle branch block (RBBB), the right ventricle is activated belatedly via transseptal spread from the left, producing an rsR' pattern in right precordial leads (V1-V2) and wide S waves in lateral leads. Left bundle branch block (LBBB), conversely, delays left ventricular activation, resulting in broad, notched R waves in lateral leads (I, aVL, V5-V6) and absent septal Q waves, with secondary ST-T changes discordant to the QRS direction. These blocks desynchronize ventricular contraction, potentially impairing hemodynamics, though isolated RBBB often has less clinical impact than LBBB.^[45]^[45]

Clinical significance

Arrhythmias

Arrhythmias arising from the cardiac conduction system disrupt the normal generation and propagation of electrical impulses, leading to irregular heart rhythms that can compromise cardiac output and cause symptoms such as syncope, fatigue, or hemodynamic instability. These disorders primarily involve abnormalities in the sinoatrial (SA) node, atrioventricular (AV) node, His-Purkinje system, or their interconnecting pathways, resulting in bradyarrhythmias, tachyarrhythmias, or conduction delays. The immediate clinical impacts include reduced perfusion to vital organs, increased risk of thromboembolism in atrial arrhythmias, and potential progression to life-threatening events like cardiac arrest.^[46] Sinus node dysfunction, also known as sick sinus syndrome, manifests as impaired SA node pacemaker activity or impulse transmission, leading to severe sinus bradycardia (heart rate <50 bpm), sinus pauses exceeding 3 seconds, or sinoatrial exit block. In this condition, failure of pacemaker cells (P cells) or transitional cells (T cells) due to structural degeneration disrupts the heart's primary rhythm generator, causing chronotropic incompetence where the heart cannot adequately increase rate in response to physiologic demands. A common variant, tachy-brady syndrome, involves alternating bradycardia and paroxysmal supraventricular tachyarrhythmias like atrial fibrillation, affecting over 50% of patients and exacerbating symptoms through erratic rhythm transitions. Clinically, these lead to cerebral hypoperfusion, manifesting as dizziness or presyncope, with an annual incidence of associated AV block ranging from 0% to 4.5%.^[46]^[47]^[48] AV blocks represent interruptions in conduction from atria to ventricles, with second-degree and third-degree (complete heart block) forms being particularly relevant to conduction system pathology. Second-degree AV block type II features constant PR intervals with intermittent non-conducted P waves, often originating infranodally in the His-Purkinje system, while high-grade variants involve multiple consecutive dropped beats. Third-degree AV block results in complete dissociation between atrial and ventricular activity, with no impulses traversing the AV node, forcing reliance on subsidiary pacemakers. Escape rhythms emerge to maintain ventricular rate: junctional escapes from the AV node (rate 40-60 bpm, narrow QRS) or ventricular escapes from Purkinje fibers (rate 20-40 bpm, wide QRS) if junctional fails, but these slower rates often cause bradycardia-induced symptoms like hypotension and fatigue. The immediate impacts include irregular pulses, dyspnea, and risk of syncope due to inadequate cardiac output.^[34] Bundle branch blocks and fascicular blocks constitute intraventricular conduction delays within the His-Purkinje system, prolonging QRS duration and altering ventricular activation sequence. Right or left bundle branch block interrupts impulse flow in respective branches, causing sequential rather than simultaneous ventricular depolarization (QRS >0.12 seconds), while fascicular blocks affect anterior or posterior left fascicles, leading to axis deviation and milder QRS widening (<0.12 seconds). These delays arise from slowed or blocked conduction in the specialized fibers, often progressing to bifascicular or trifascicular involvement in extensive damage. Clinically, they are typically asymptomatic but signal underlying structural disease, increasing vulnerability to complete AV block and reducing synchrony, which impairs ventricular efficiency and elevates cardiac event risk, particularly in left bundle branch block.^[49]^[45] Supraventricular tachycardias (SVT) linked to the conduction system, such as atrioventricular nodal reentrant tachycardia (AVNRT), arise from re-entry circuits within or adjacent to the AV node. AVNRT, the most common SVT (50-60% of cases), involves dual AV nodal pathways—a fast and slow conduit—enabling a reentrant loop where anterograde conduction occurs via the slow pathway and retrograde via the fast, producing rapid rates (118-264 bpm) with near-simultaneous atrial and ventricular activation. Typical slow-fast AVNRT predominates (90%), while atypical fast-slow variants reverse the pathways. These tachycardias cause palpitations, chest discomfort, and lightheadedness due to abrupt hemodynamic shifts, with potential for tachycardia-mediated cardiomyopathy if sustained.^[50] Ventricular arrhythmias originating in the conduction system include premature ventricular contractions (PVCs) and ventricular tachycardia (VT) driven by Purkinje fiber abnormalities. PVCs are ectopic impulses from distal Purkinje sites, often triggered by delayed afterdepolarizations from calcium dysregulation in remodeled fibers post-infarction or in heart failure. VT via Purkinje re-entry forms macro-reentrant circuits involving surviving Purkinje remnants and scarred myocardium, perpetuated by slowed conduction and unidirectional block in the His-Purkinje network. These arrhythmias manifest as palpitations or hemodynamic collapse, with PVCs initiating VT/VF storms in vulnerable substrates, heightening sudden death risk in structural heart disease.^[51]^[52] Key risk factors for these conduction-related arrhythmias include ischemia, which induces electrical instability through oxidative stress and reperfusion injury, promoting re-entry in the Purkinje system. Fibrosis, prevalent in aging and heart failure, replaces conduction tissue with scar, depressing velocity and creating heterogeneous substrates for block and re-entry, particularly in the atria and nodes. Electrolyte imbalances, such as hyperkalemia, prolong conduction by elevating resting membrane potential and reducing excitability, leading to widened QRS, AV block, and ventricular arrhythmias via slowed His-Purkinje propagation.^[53]^[54]^[55]

Diagnostic and therapeutic interventions

Diagnostic methods for assessing cardiac conduction system function primarily include electrocardiography (ECG), electrophysiology studies (EPS), and advanced imaging techniques. The 12-lead ECG serves as the cornerstone for evaluating conduction intervals, such as PR, QRS, and QT durations, enabling the identification of abnormalities like atrioventricular block or bundle branch block through non-invasive surface recordings.^[56] Ambulatory monitoring via Holter ECG extends this assessment by capturing intermittent conduction disturbances over 24-48 hours, particularly useful for detecting paroxysmal bradyarrhythmias or tachyarrhythmias in dynamic settings.^[57] EPS involves invasive catheter-based mapping to measure conduction velocities, localize re-entry circuits, and provoke arrhythmias under controlled conditions, providing precise localization of conduction defects such as accessory pathways or slow pathway involvement in supraventricular tachycardias.^[58] This technique is indicated for risk stratification in patients with syncope or structural heart disease, with high sensitivity for identifying inducible ventricular tachycardia circuits.^[59] Echocardiography complements these by revealing structural correlates to conduction abnormalities, such as valvular disease or chamber enlargement impacting the conduction pathways, through real-time visualization of myocardial motion and strain.^[60] Cardiac magnetic resonance imaging (MRI) excels in detecting fibrosis within conduction tissues, using late gadolinium enhancement and T1 mapping to quantify extracellular volume expansion, which correlates with slowed conduction and arrhythmogenic substrates in conditions like cardiomyopathy.^[61] These imaging modalities are particularly valuable in unexplained conduction disease, where MRI identifies infiltrative processes in up to 30% of cases with preserved ejection fraction.^[62] Therapeutic interventions target conduction disorders through pharmacotherapy and device-based or procedural approaches. Beta-blockers, such as metoprolol, are first-line for supraventricular tachycardias involving enhanced automaticity or re-entry, by prolonging atrioventricular nodal refractoriness and reducing sympathetic drive on the conduction system.^[63] Antiarrhythmic agents are classified into Vaughan-Williams classes I-IV, with Class I sodium channel blockers (e.g., flecainide) slowing conduction in atrial or ventricular tissues, Class III potassium channel modulators (e.g., amiodarone) prolonging repolarization to prevent re-entry, and Class IV calcium channel antagonists (e.g., verapamil) targeting nodal tissues.^[64] These drugs modulate ion channels directly affecting conduction velocity and excitability, though proarrhythmic risks necessitate ECG monitoring.^[65] For bradyarrhythmias due to sinoatrial or atrioventricular node dysfunction, pacemaker implantation restores physiologic conduction by delivering paced impulses, with dual-chamber devices preferred to maintain atrioventricular synchrony and prevent pacemaker syndrome.^[66] Catheter ablation is curative for focal re-entrant arrhythmias like atrioventricular nodal reentrant tachycardia (AVNRT), targeting the slow pathway with radiofrequency energy to eliminate dual nodal physiology, achieving success rates over 95% while minimizing risks like transient atrioventricular block.^[67] In ventricular tachycardia (VT), ablation maps and ablates scar-related circuits, often guided by EPS, reducing recurrence by 70-80% in structural heart disease.^[68] Recent advances emphasize conduction-preserving therapies and molecular interventions. His-bundle pacing, a selective site pacing technique, directly activates the intrinsic conduction system distal to the atrioventricular node, improving ventricular synchrony compared to traditional right ventricular pacing and reducing heart failure progression in patients with conduction delays.^[69] Clinical adoption has grown since 2020, with multicenter trials demonstrating lower pacing thresholds and better long-term outcomes in bradycardic patients.^[58] Gene therapy trials for channelopathies, such as long QT syndrome or catecholaminergic polymorphic VT, have progressed post-2020 using adeno-associated viral vectors to deliver corrective ion channel genes (e.g., SCN5A or RYR2), showing restored conduction and reduced arrhythmia inducibility in preclinical models and initial early-phase human trials.^[70] For instance, in July 2025, the FDA granted fast track designation to SGT-501, an AAV-based gene therapy for CPVT, with first-in-human trials planned for late 2025.^[71] These approaches target genetic defects in conduction proteins, with ongoing phase I/II trials assessing safety and preliminary efficacy in restoring normal electrophysiology based on preclinical data.^[72]

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A short stimulation at high frequency (10-. 100 impulses/sec) applied directly to the sinus node led to hyperpolarization, temporary suppression of pacemaker.
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It is believed to involve the perinodal (T) cells. Similar to sinus pause and arrest, the atria do not receive the proper signal to contract, and thus, the ECG ...
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Prevalence and relation to training of electrocardiographic findings ...
Training frequently induces electrocardiographic (ECG) changes that mimic heart diseases, requiring specific criteria for interpretation.
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Surgical and Electrical Anatomy of the Inter-Nodal and Intra-Atrial ...
Three routes are described in this paper; anterior (#1), middle (#2), and posterior (#3). Branches of the route are described by adding sub-numbers to the route ...
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Differential Distribution of Cardiac Ion Channel Expression as a ...
This article reviews the major regionally determined features of cardiac electrical function, discusses underlying ionic bases, and summarizes present knowledge
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Feb 12, 2024 · Atrioventricular block represents a delay or disturbance in the transmission of an impulse from the atria to the ventricles.
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Normal and Abnormal Electrical Conduction - CV Physiology
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Second-Degree Atrioventricular Block - StatPearls - NCBI Bookshelf
Mobitz type I (Wenckebach) is often a normal variant and seen in individuals with a high vagal tone without evidence of structural heart disease. However, this ...
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Conducting System of the Heart - Bundle of His - SA Node - TeachMeAnatomy
### Summary of His-Purkinje System's Role in Rapid Conduction After AV Node
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Mar 14, 2014 · The autonomic nervous system plays an important role in the modulation of cardiac electrophysiology and arrhythmogenesis.
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Heart Conduction System (Cardiac Conduction) - Cleveland Clinic
The Purkinje fibers to your ventricles (bottom heart chambers), which causes them to contract and deliver blood out to your body. Advertisement. These steps ...
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Apr 9, 2009 · Conduction through the His-Purkinje system is an important determinant of the sequence of ventricular activation and contraction. Cardiac ...
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Conquering the ECG - Cardiology Explained - NCBI Bookshelf
QRS wave complex. The three waves of the QRS complex represent ventricular depolarization. For the inexperienced, one of the most confusing aspects of ECG ...Electrocardiography · Depolarization of the heart · The ECG trace
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Changing Activation Sequence in the Embryonic Chick Heart
The main finding of the present study is that the mature ventricular apex-to-base activation sequence begins between stages 29 and 31, a period in heart ...
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Electrocardiographic T Wave and its Relation With Ventricular ...
May 16, 2014 · The T wave on the ECG (T-ECG) represents repolarization of the ventricular myocardium. Its morphology and duration are commonly used to ...
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Corrects the QT interval for heart rate extremes (choose from Bazett, Fridericia, Framingham, Hodges, or Rautaharju formulas).
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Left Bundle Branch Block - StatPearls - NCBI Bookshelf
Oct 5, 2024 · LBBB appears as a widened QRS complex with certain characteristic features on electrocardiography, described below.
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Sick Sinus Syndrome - StatPearls - NCBI Bookshelf - NIH
Sick sinus syndrome, also known as sinus node dysfunction, is a disorder of the sinoatrial node caused by impaired pacemaker function and impulse transmission.Continuing Education Activity · Etiology · Pathophysiology · History and Physical
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Sinus Node Dysfunction - StatPearls - NCBI Bookshelf
Sinus node dysfunction is a disease that is characterized by the inability of the sinoatrial node of producing an adequate heart rate that meets the physiologic ...
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Sinus node dysfunction: current understanding and future directions
Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical ...
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Bundle Branch Block and Fascicular Block - Cardiovascular Disorders
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Common Types of Supraventricular Tachycardia: Diagnosis ... - AAFP
Oct 15, 2010 · AVNRT and AVRT are electrical aberrancies that occur mainly as a result of reentry. Less commonly, increased automaticity or triggered activity ...
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Ventricular Arrhythmias Involving the His-Purkinje System in the ...
Perturbation of the His-Purkinje system can lead to life-threatening ventricular arrhythmias, including ventricular tachycardia (VT) or ventricular fibrillation ...
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Sep 2, 2024 · Ventricular premature complexes (VPCs) are ectopic impulses originating from an area distal to the His Purkinje system. VPCs are the most ...
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Mechanisms of Arrhythmias and Conduction Disorders in Older Adults
Aging is associated with an increased prevalence of cardiac arrhythmias, which contribute to higher morbidity and mortality in the elderly.
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Mar 17, 2017 · In this article, we discuss the basic science underlying the effects of both hypo- and hyperkalemia on cardiac excitability and arrhythmias.
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The electrocardiogram (abbreviated as ECG or EKG) represents an electrical tracing of the heart and is recorded non-invasively from the surface of the body.Introduction · Anatomy and Physiology · Equipment · Clinical Significance
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Conduction Disorders: The Value of Surface ECG - PMC
Surface ECG allows the diagnosis of atrioventricular and intraventricular conduction disorder and its anatomical block site most of the time.
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Electrophysiologic Study Interpretation - StatPearls - NCBI Bookshelf
Mar 28, 2025 · EPS involves using intracardiac catheters to assess electrical conduction, identify arrhythmia mechanisms, and guide therapeutic interventions.
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Electrophysiologic Study Indications and Evaluation - NCBI - NIH
Apr 3, 2023 · An electrophysiology study is performed for the identification of arrhythmias and risk stratification of patients at risk of sudden cardiac death.
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State-of-the-Art Imaging of Infiltrative Cardiomyopathies
Nov 2, 2023 · This review will highlight the role of cardiac imaging, including echocardiography, cardiac magnetic resonance imaging (CMR), and nuclear ...
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Imaging tools for assessment of myocardial fibrosis in humans
In this article, we will first review current medical imaging technologies for assessing fibrosis including echo and MRI.
[61]
Prevalence of cardiac fibrosis and infiltrative cardiomyopathy in ...
Jul 6, 2025 · Cardiac MRI may be an important adjunctive tool for the investigation of conduction disease, particularly in younger patients.
[62]
Beta-blockers in cardiac arrhythmias–Clinical pharmacologist's point ...
β-blockers can also be used in regular wide–QRS complex tachycardias, but only in case of SVT with aberrant conduction. When the mechanism of arrhythmia is ...
[63]
Antiarrhythmic Medications - StatPearls - NCBI Bookshelf - NIH
Antiarrhythmic medications play a pivotal role in managing various cardiac rhythm disorders, and their classification under the Vaughan-Williams system ...Missing: pharmacotherapy | Show results with:pharmacotherapy
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Modernized Classification of Cardiac Antiarrhythmic Drugs
Oct 22, 2018 · Class II drugs, comprising β-adrenergic inhibitors, reduce sino-atrial node (SAN) pacing rates and slow atrioventricular node (AVN) AP ...
[65]
Pacemaker Insertion - StatPearls - NCBI Bookshelf
Oct 6, 2024 · [11]. Bundle of His: The bundle of His is a thin structure with specialized conduction cells connecting the distal part of the compact ...
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Electrophysiology Study and Ablation of Atrioventricular Nodal ...
Jul 30, 2023 · This activity reviews the role of electrophysiology study and catheter ablation in the diagnosis and treatment of AVNRT.
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Atrioventricular Block During Slow Pathway Ablation | Circulation
Jun 1, 2015 · The patient underwent permanent pacemaker implantation after the ablation procedure as there was no recovery of AV conduction. Open in ...
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Conduction System Pacing: Where Are We in 2025? - PMC - NIH
In particular, LB pacing appears to be the most clinically applicable therapy as it avoids the disadvantages of His-bundle pacing (sensing challenges, higher ...Missing: 2020-2025 | Show results with:2020-2025
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Gene Therapy in Cardiovascular Disease: Recent Advances and ...
Nov 11, 2024 · Multiple early-phase clinical trials are ongoing to examine the safety and efficacy of gene replacement and genome editing through viral and ...
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Gene Therapy for Cardiac Arrhythmias: Mechanisms, Modalities and ...
Jul 30, 2025 · Gene therapy has emerged as a transformative approach, offering precise and durable interventions that directly target the arrhythmogenic ...