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Repolarization

Repolarization is the phase of an action potential in excitable cells, such as neurons and cardiomyocytes, during which the membrane potential returns from a depolarized state to the negative resting membrane potential (typically around -70 mV in neurons and -85 to -90 mV in cardiomyocytes), primarily through the efflux of potassium ions (K⁺) via voltage-gated potassium channels.^[1] This process follows depolarization and is essential for restoring ionic gradients across the cell membrane, enabling the cell to prepare for subsequent excitations.^[2] In neuronal physiology, repolarization ensures the rapid reset of the membrane potential, allowing for high-frequency action potential propagation along axons, which is critical for efficient neural signaling and information processing in the nervous system.^[2] The mechanism involves the delayed opening of potassium channels after sodium channel inactivation, often leading to a brief hyperpolarization phase that temporarily inhibits further firing.^[1] In cardiac physiology, repolarization occurs mainly during phase 3 of the action potential, driven by potassium efflux through delayed rectifier channels, and is vital for coordinating ventricular relaxation and maintaining regular heartbeats.^[3] Disruptions in repolarization, such as prolonged durations or heterogeneous timing across the ventricular wall, can lead to life-threatening arrhythmias like torsades de pointes.^[3] Overall, repolarization's precise ion dynamics underpin the excitability of cells in both the nervous and cardiovascular systems, with abnormalities often linked to clinical conditions including epilepsy and sudden cardiac death.^[1]

Fundamentals of Repolarization

Definition and Role in Cellular Excitability

Repolarization refers to the phase of cellular electrical activity in which the membrane potential of an excitable cell returns to its resting state following depolarization, typically restoring the potential to approximately -70 mV in neurons and -90 mV in cardiomyocytes through the efflux of positively charged ions, primarily potassium (K⁺).^[1]^[4]^[5] This process is mediated by the activation of voltage-gated potassium channels, which allow K⁺ to exit the cell down its electrochemical gradient, counteracting the inward sodium (Na⁺) flux that occurs during depolarization.^[1] The repolarization phase ensures the rapid restoration of the transmembrane voltage gradient, which is critical for maintaining the cell's excitability.^[6] The role of repolarization in cellular excitability is fundamental, as it restores the membrane potential, allowing the sodium-potassium ATPase pump to maintain the ionic gradients across the plasma membrane—particularly the high intracellular K⁺ and low intracellular Na⁺ concentrations—and long-term homeostasis.^[1] Without effective repolarization, cells cannot generate subsequent action potentials efficiently, leading to a refractory state that limits firing frequency and prevents exhaustion from prolonged depolarization.^[6] Disruptions in this process, such as delayed or incomplete repolarization, can result in pathological conditions including cardiac arrhythmias due to uneven electrical recovery in the heart or neuronal hyperexcitability culminating in seizures from impaired action potential termination.^[3]^[7] In neurons, repolarization primarily occurs via K⁺ efflux, which not only resets the membrane potential but also contributes to the absolute and relative refractory periods, ensuring unidirectional propagation of signals and preventing chaotic firing.^[6] Similarly, in muscle cells, including cardiomyocytes, this phase restores excitability for coordinated contractions, with the process bridging basic cellular mechanisms to specialized functions like rhythmic cardiac beating.^[1] Modern quantitative understanding of repolarization emerged from the Hodgkin-Huxley model in 1952, which mathematically described the ionic currents underlying action potential dynamics in squid giant axons.^[8]

Repolarization in Action Potentials

Repolarization constitutes the latter portion of the action potential, restoring the membrane potential to its resting state after depolarization. In excitable cells, the action potential is divided into phases: phase 0 involves rapid depolarization driven by sodium (Na⁺) influx through voltage-gated Na⁺ channels; phases 1 through 3 encompass repolarization; and phase 4 represents the resting potential.^[1] Repolarization spans phases 1-3, primarily mediated by potassium (K⁺) efflux, which counters the depolarizing influences and returns the membrane toward the K⁺ equilibrium potential.^[9] Phase 1 marks the initial repolarization, occurring shortly after the peak of depolarization, characterized by a transient outward K⁺ current (I_to) and the inactivation of voltage-gated Na⁺ channels. This phase produces a brief notch in the action potential waveform, particularly prominent in cardiac myocytes, as the outward K⁺ movement begins to hyperpolarize the membrane while Na⁺ conductance declines.^[9] In neurons, this early repolarization is less distinct, blending into the overall falling phase due to the absence of a pronounced plateau.^[1] Phase 2, the plateau phase, features a balance between inward calcium (Ca²⁺) influx through L-type Ca²⁺ channels and delayed outward K⁺ currents, maintaining a relatively depolarized state before gradual repolarization initiates. This equilibrium prolongs the action potential duration, especially in cardiac cells, allowing time for processes like contraction.^[9] The slow activation of delayed rectifier K⁺ channels during this phase contributes to the onset of net repolarization as Ca²⁺ channels begin to inactivate.^[1] Phase 3 completes repolarization through the dominance of outward K⁺ currents, including the rapid delayed rectifier (I_Kr) and slow delayed rectifier (I_Ks), which drive the membrane potential back to rest near -70 to -90 mV. These currents ensure efficient repolarization by driving the membrane potential back to rest near -70 to -90 mV, with I_Kr providing quick adjustment and I_Ks offering sustained efflux.^[9] The driving force for this K⁺ efflux is governed by the Nernst equation for the K⁺ equilibrium potential (E_K):

E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_o}{[K^+]_i} \right)

where R is the gas constant, T is temperature, z is the ion valence (+1 for K⁺), F is Faraday's constant, [K⁺]_o is extracellular K⁺ concentration (≈4 mM), and [K⁺]_i is intracellular (≈140-150 mM), yielding E_K ≈ -90 mV under physiological conditions.^[5] Repolarization dynamics vary across cell types: in neurons, the process is rapid (lasting ~1-2 ms) without a plateau, relying mainly on voltage-gated K⁺ channels for swift return to rest; in contrast, cardiac myocytes exhibit prolonged repolarization (200-400 ms) due to the phase 2 plateau, which extends the action potential to coordinate contraction.^[1] This difference underscores the adaptive roles of repolarization in signal propagation versus electromechanical coupling.^[9]

Ion Channel Mechanisms

Voltage-Gated Potassium Channels

Voltage-gated potassium (Kv) channels are integral membrane proteins composed of four α-subunits that assemble into a tetrameric structure, each subunit featuring six transmembrane segments (S1–S6). The S4 segment serves as the primary voltage sensor, containing positively charged arginine residues that move outward upon membrane depolarization, initiating conformational changes that open the central pore formed by the S5–S6 segments and their intervening pore loop. This activation mechanism enables selective K⁺ permeation, restoring the negative membrane potential during repolarization in excitable cells. These channels exhibit time- and voltage-dependent gating, with activation occurring rapidly following depolarization to facilitate K⁺ efflux, which counters inward currents and drives membrane repolarization. Inactivation follows activation through mechanisms such as N-type (ball-and-chain occlusion of the pore) or C-type (constriction of the selectivity filter), limiting channel availability and shaping the duration of the outward current. In cardiac myocytes, this gating ensures timely repolarization, preventing excessive prolongation of the action potential. In the heart, several major Kv currents contribute to repolarization: the transient outward current (Ito), mediated primarily by Kv4.3 channels, initiates early repolarization (phase 1); the slow delayed rectifier (IKs), formed by KCNQ1 and KCNE1 subunits, sustains the plateau and supports adaptation to high heart rates; the rapid delayed rectifier (IKr), encoded by KCNH2 (hERG), provides robust phase 3 repolarization with fast inactivation; and the ultrarapid delayed rectifier (IKur), carried by Kv1.5, predominates in atrial tissue for rate-dependent repolarization. These currents collectively ensure efficient restoration of the resting potential.^[10] The biophysical properties of Kv channels are described by the conductance equation:

G = g_{\max} \times n \times h

where G is the instantaneous conductance, g_{\max} is the maximal conductance, n represents the activation gate (often raised to the fourth power for tetrameric symmetry, following Boltzmann-like voltage dependence), and h is the inactivation gate variable. This formulation captures how voltage shifts modulate K⁺ outflow, with typical single-channel conductances ranging from 5–20 pS depending on the isoform.^[11] Genetically, these channels are encoded by the Kv gene family, with specific loci including KCND3 for Ito components, KCNQ1 for IKs, and KCNH2 for IKr. Mutations in these genes, such as loss-of-function variants in KCNH2, disrupt current amplitudes and gating, predisposing to channelopathies like long QT syndrome, though detailed clinical manifestations arise from integrated cellular effects.^[12]^[13] Kv channels display remarkable evolutionary conservation across excitable cells, from invertebrates to mammals, underscoring their fundamental role in action potential termination. In cardiac tissue, adaptations such as auxiliary subunit interactions (e.g., β-subunits modulating gating kinetics) and isoform-specific expression prolong repolarization compared to neuronal counterparts, accommodating the heart's rhythmic demands.^[14]^[15]

Other Ion Contributions to Repolarization

In addition to the primary efflux of potassium ions through voltage-gated channels, repolarization involves the inactivation of sodium channels, which rapidly terminates the inward sodium current during phase 1 of the action potential. This fast inactivation, mediated by the intracellular linker between domains III and IV of the sodium channel, binds to the channel pore shortly after activation, effectively removing the depolarizing influence and contributing to the initial rapid drop in membrane potential.^[1]^[16] Calcium ion handling also plays a supportive role in later phases of repolarization, particularly through the inactivation of L-type calcium channels and the activity of the sodium-calcium exchanger (NCX). L-type calcium channels undergo voltage- and calcium-dependent inactivation during the action potential plateau, reducing inward calcium current and facilitating the transition to phase 3 repolarization.^[17] Concurrently, NCX typically operates in forward mode during phase 3, extruding one calcium ion in exchange for three sodium ions and generating a net inward current that can oppose repolarization; however, under conditions of elevated intracellular sodium, it may produce an outward current that aids repolarization while maintaining calcium homeostasis.^[18]^[16] Chloride currents provide a minor but modulatory contribution to repolarization in certain cell types, primarily via the calcium-activated chloride current (I_Cl,Ca). This current is triggered by intracellular calcium elevation and activates chloride influx, producing an outward current particularly during phase 1, enhancing the early repolarization notch and contributing to rate-dependent adjustments in action potential duration.^[19]^[20] Under conditions of metabolic stress, such as ischemia or hypoxia, ATP-sensitive potassium channels (K_ATP) open to increase potassium conductance, accelerating repolarization and shortening action potential duration to conserve cellular energy by reducing calcium influx and contractility.^[21]^[22] This adaptive mechanism prevents excessive energy depletion but can lead to arrhythmogenic risks if prolonged.^[23] Following repolarization, intracellular processes like the reactivation of the sodium-potassium ATPase (Na⁺/K⁺ ATPase) are essential to restore ionic gradients for subsequent action potentials. This pump actively transports three sodium ions out of the cell and two potassium ions in per molecule of ATP hydrolyzed, counteracting the ion shifts during depolarization and maintaining long-term excitability.^[24]^[25] The overall membrane potential during repolarization reflects the integrated contributions of these ion movements, as described by the Goldman-Hodgkin-Katz equation, which accounts for the permeabilities and concentrations of multiple ions (sodium, potassium, calcium, and chloride) rather than a single species.^[4] This multi-ion framework underscores how secondary currents fine-tune the repolarization trajectory beyond dominant potassium efflux.

Cardiac Repolarization Processes

Atrial Repolarization

Atrial repolarization in cardiomyocytes occurs more rapidly than in ventricular myocytes, with an action potential duration typically ranging from 250 to 350 ms, allowing for efficient restoration of the resting membrane potential to support high-frequency atrial contractions.^[26] This shorter repolarization phase overlaps with ventricular depolarization and is consequently masked by the QRS complex on the electrocardiogram (ECG), making direct observation challenging without specialized techniques.^[27] The rapid timeline contrasts with the longer ventricular repolarization (approximately 200-300 ms), reflecting adaptations to the atria's role in initiating cardiac cycles at rates up to 3-4 Hz during physiological stress.^[28] The process is primarily driven by voltage-gated potassium channels, including the transient outward current (Ito) and the ultrarapid delayed rectifier current (IKur), which exhibit high expression in atrial tissue to facilitate fast phase 3 repolarization.^[29] Ito activates quickly upon depolarization, contributing to early repolarization by efflux of K⁺ ions, while IKur sustains outward current during the plateau phase, enabling triangular action potential shapes characteristic of atrial cells.^[30] In comparison to ventricular myocytes, atrial cells show lower expression of the slow delayed rectifier current (IKs), reducing its contribution to late repolarization and emphasizing reliance on Ito and IKur for brevity.^[31] Physiologically, this swift repolarization enables rapid resetting of atrial excitability, ensuring sequential atrial contraction precedes ventricular filling to optimize diastolic volume and cardiac output, with atrial cells' smaller size and higher intrinsic firing rates further necessitating such efficiency over the more prolonged ventricular plateau for force generation.^[28] Regional variations exist between the right and left atria, where left atrial regions, particularly the pulmonary vein myocardial sleeves, display even shorter action potential durations due to heterogeneous ion channel densities, predisposing them to ectopic activity if repolarization gradients fail.^[32] Patch-clamp studies on isolated human atrial myocytes confirm these dynamics, reporting action potential duration at 90% repolarization (APD90) values of approximately 150 ms under baseline conditions at 1 Hz pacing, underscoring the atrial capacity for rate-adaptive repolarization.^[33]

Ventricular Repolarization

Ventricular repolarization in myocytes of the heart's ventricles is characterized by a prolonged action potential duration (APD) of 200-300 ms in humans, which is essential for coordinating systolic contraction and ensuring effective cardiac output.^[34] This extended timeline contrasts with the briefer repolarization in atrial myocytes, allowing ventricles to sustain force generation during ejection. The phase 2 plateau phase, lasting hundreds of milliseconds, maintains membrane potential near 0 mV through a delicate balance of inward Ca²⁺ influx via L-type channels and outward K⁺ efflux, preventing premature relaxation and facilitating Ca²⁺-induced Ca²⁺ release for contraction.^[35] In phase 3, rapid repolarization is primarily driven by the delayed rectifier potassium currents IKr and IKs, which increase outward K⁺ conductance to restore the negative resting potential.^[36] The transient outward current Ito contributes to initial repolarization differences across the ventricular wall, with higher expression in epicardial cells compared to endocardial cells, leading to a deeper phase 1 notch and accelerated early repolarization in the epicardium.^[37] Transmural gradients arise from these regional variations, where epicardial APD (approximately 317 ms) is shorter than endocardial APD (approximately 360 ms), resulting in faster epicardial repolarization relative to endocardial regions and contributing to the vectorial patterns observed in ventricular recovery.^[38] The APD exhibits rate dependence, shortening at higher heart rates primarily due to accumulation of IKs from incomplete deactivation between beats, which enhances repolarizing K⁺ efflux and reduces the plateau duration.^[39] This adaptation ensures efficient repolarization during tachycardia but can be altered by heterogeneities in IKs expression across cell types. Species differences significantly impact experimental interpretations; rodent ventricular APD is markedly shorter (10-50 ms) and lacks a prominent plateau phase compared to the prolonged human profile, complicating direct translation of drug effects on repolarization in preclinical testing.^[40]

Pathophysiological Deviations

Early Repolarization Syndrome

Early repolarization syndrome (ERS) is characterized by J-point elevation of ≥0.2 mV in at least two contiguous inferior or lateral leads on a 12-lead electrocardiogram (ECG), often accompanied by a notched or slurred ST segment, and is more prevalent in young males.^[41] This ECG pattern reflects accentuated transient outward potassium current (Ito) in the epicardium, which creates a voltage gradient during phase 1 of the action potential, leading to early termination of the epicardial action potential dome and resultant J-point elevation.^[41] Unlike normal ventricular repolarization, which maintains uniform repolarization across myocardial layers, ERS exaggerates this transmural gradient, potentially predisposing to arrhythmogenic substrates under certain conditions.^[42] Genetically, ERS is mostly idiopathic, but rare loss-of-function variants in genes encoding potassium or calcium channel subunits, such as KCNJ8 (which codes for the Kir6.2 subunit of ATP-sensitive potassium channels) or CACNA2D (encoding the α2δ subunit of L-type calcium channels), have been associated with the syndrome.^[43] These mutations disrupt ion homeostasis, enhancing Ito prominence or reducing inward currents, thereby amplifying the repolarization abnormality.^[44] While ERS is typically benign, it carries a very low risk of ventricular fibrillation (VF), estimated at approximately 0.07% in affected individuals, particularly when combined with triggers like ischemia or vagal stimulation.^[42] The syndrome was formally recognized in 2008 following studies linking the ECG pattern to idiopathic VF and sudden cardiac arrest.^[45] Recent 2020s research highlights a higher prevalence in athletes, up to 30% in endurance sports, though arrhythmic risk remains low without additional factors.^[46] Diagnosis relies on ECG criteria of J-point elevation ≥0.2 mV with notching in ≥2 leads, confirmed by exclusion of other causes via history and imaging; ambulatory Holter monitoring is recommended to detect dynamic J-wave changes or arrhythmias indicative of higher risk.^[47]

Impaired Repolarization in Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is characterized by recurrent episodes of upper airway obstruction during sleep, leading to intermittent hypoxia and hypercapnia that disrupt normal cardiac repolarization processes. This impairment manifests primarily as prolongation of the action potential duration (APD) and QT interval on electrocardiography, increasing the risk of ventricular arrhythmias. The condition affects repolarization through direct effects on ion channels and indirect autonomic nervous system imbalances, contributing to heightened cardiovascular morbidity in affected individuals.^[48] The core mechanism involves intermittent hypoxia, which prolongs APD by reducing the slow delayed rectifier potassium current (IKs) and other repolarizing outward currents such as IKr and IKur, thereby delaying ventricular repolarization and resulting in QT prolongation. Concurrently, OSA induces heightened sympathetic tone due to recurrent arousals and chemoreceptor activation, further exacerbating repolarization instability by enhancing catecholamine-driven calcium influx and suppressing parasympathetic activity. These changes create a pro-arrhythmic substrate, particularly during nocturnal apneic events.^[48]^[49] At the pathophysiological level, reactive oxygen species (ROS) generated from hypoxia-reoxygenation cycles downregulate key potassium channels, including hERG (mediating IKr), through proteolytic degradation pathways like calpain activation, impairing repolarization efficiency. Additionally, OSA-associated systemic inflammation alters calcium handling by promoting myocardial fibrosis and disrupting sarcoplasmic reticulum function, which prolongs APD and increases repolarization heterogeneity. These molecular alterations are compounded by oxidative stress and endothelial dysfunction, fostering a chronic pro-inflammatory state that sustains repolarization abnormalities.^[50]^[48] Clinical evidence indicates that repolarization impairment is prevalent in OSA, with approximately 30-35% of patients exhibiting abnormal daytime QTc intervals (>450 ms in men, >470 ms in women), particularly in those with severe disease (apnea-hypopnea index >30 events/hour), where QTc is prolonged by about 10-40 ms compared to milder cases or controls. Meta-analyses from the 2010s confirm increased QT dispersion (standardized mean difference 0.57-0.86) correlating with OSA severity, reflecting greater repolarization inhomogeneity. Recent 2024 data from a systematic review and meta-analysis link untreated OSA to a 3.87-fold higher odds of sudden cardiac death (95% CI: 1.09-13.81), partly attributable to these repolarization disturbances, with QT prolongation serving as a key intermediary risk factor.^[51]^[52]^[53] Management strategies emphasize early screening with routine ECG in OSA patients to detect QT prolongation and repolarization heterogeneity, enabling risk stratification for arrhythmias. Continuous positive airway pressure (CPAP) therapy effectively reverses these changes by mitigating intermittent hypoxia, reducing sympathetic overactivity, and normalizing QT dispersion (e.g., decreasing QTcd by 15-20 ms after 3-6 months of use), thereby lowering arrhythmogenic potential and sudden cardiac death risk. Adherence to CPAP is crucial, as untreated severe OSA sustains these impairments.^[54]^[55]

Other Repolarization Disorders

Long QT syndrome (LQTS) is a hereditary cardiac disorder characterized by prolonged ventricular repolarization, primarily due to loss-of-function mutations in genes encoding potassium channels such as KCNH2 (LQT2) and KCNQ1 (LQT1), which impair the rapid (IKr) and slow (IKs) delayed rectifier currents during phase 3 of the action potential.^[56] These mutations extend the action potential duration (APD), manifesting as a prolonged QT interval on electrocardiography and increasing susceptibility to torsades de pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation.^[57] The three most common subtypes—LQT1, LQT2, and LQT3 (associated with SCN5A mutations causing enhanced late sodium current)—account for over 75% of cases, with triggers varying by type: LQT1 often provoked by exercise or swimming, LQT2 by auditory stimuli or emotion, and LQT3 by sleep.^[58] Short QT syndrome (SQTS), in contrast, features accelerated repolarization from gain-of-function mutations in potassium channel genes, notably KCNH2 (SQT1), leading to increased IKr conductance and shortened APD, which shortens the QT interval and predisposes individuals to ventricular fibrillation through re-entrant arrhythmias.^[59] This rare condition, with fewer than 100 families reported worldwide, heightens the risk of sudden cardiac death, particularly in young adults, due to the abbreviated refractory period facilitating early afterdepolarizations.^[60] Acquired forms of repolarization disorders mimic congenital LQTS but arise from external factors, such as medications that block IKr, including class III antiarrhythmics like sotalol, which prolong the QT interval in a dose-dependent manner by inhibiting potassium efflux during phase 3.^[61] Electrolyte imbalances, particularly hypokalemia, exacerbate this by reducing IKr availability through enhanced channel inactivation, thereby prolonging APD and amplifying arrhythmia risk in susceptible individuals.^[61] Brugada syndrome exhibits partial overlap with repolarization disorders through accentuation of the transient outward potassium current (Ito) in the right ventricular epicardium, causing a loss of the action potential dome and heterogeneous repolarization that manifests as ST-segment elevation and predisposes to ventricular fibrillation.^[62] This Ito-mediated mechanism, often linked to SCN5A loss-of-function, creates a transmural voltage gradient during early repolarization phases.^[63] The prevalence of congenital LQTS is estimated at 1 in 2000 to 2500 individuals, though underdiagnosis due to asymptomatic carriers may inflate this figure.^[57] Recent advances in gene therapy, particularly from 2023 to 2025, include in vivo base editing of the Scn5a gene in murine models of LQT3, which corrects the mutation and normalizes repolarization, and AAV9-mediated suppression-replacement strategies for KCNH2 variants, as demonstrated in preclinical models of short QT syndrome type 1 (SQT1), with ongoing development for LQT2, showing restored channel function and reduced arrhythmia burden in preclinical studies.^[64]^[65]

Clinical Assessment and Implications

Electrocardiographic Features

Cardiac repolarization manifests on the electrocardiogram (ECG) primarily through specific waveforms and intervals that reflect the recovery of atrial and ventricular myocytes following depolarization. Atrial repolarization is represented by the Ta wave, which typically exhibits opposite polarity to the P wave and has a duration approximately two to three times that of the P wave itself, but it is usually obscured by the overlying QRS complex during normal atrioventricular conduction.^[66] In cases of atrioventricular dissociation, such as third-degree heart block, the Ta wave becomes more visible, appearing as a shallow deflection in the PR segment.^[66] Ventricular repolarization, in contrast, is prominently displayed as the T wave, which corresponds to the final phase of myocyte recovery and reflects transmural dispersion of repolarization across the ventricular wall.^[67] A U wave may follow the T wave when present, potentially arising from delayed repolarization in Purkinje fibers or mid-myocardial M cells, though its physiological significance remains debated.^[67] The QT interval serves as a key measure of ventricular repolarization duration on the surface ECG, extending from the onset of the QRS complex to the end of the T wave.^[68] 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; normal QTc values typically range from 350 to 450 ms in men and 350 to 460 ms in women.^[69] Prolonged QTc indicates delayed repolarization, which can predispose to ventricular arrhythmias, while shortened QTc may reflect accelerated recovery phases.^[68] Abnormalities in repolarization often appear as deviations in the ST segment and T wave morphology, providing diagnostic clues to underlying physiological disruptions. ST-segment elevation or depression signifies alterations in the early phases of ventricular repolarization, commonly linked to imbalances in ion currents or regional ischemia, with elevation typically showing a concave upward morphology in benign cases.^[70] T-wave inversion, where the wave deflects negatively instead of the usual upright configuration in most leads, indicates repolarization abnormalities such as those from electrolyte disturbances, ischemia, or structural heart changes, often accompanying ST shifts.^[71] To assess repolarization heterogeneity, which contributes to arrhythmogenic risk, the Tpeak-Tend (Tp-e) interval—measured from the peak to the end of the T wave—serves as an index of transmural or global dispersion of repolarization.^[72] This interval is rate-dependent and prolonged Tp-e values (>100 ms) have been associated with increased vulnerability to ventricular arrhythmias, as greater dispersion facilitates reentrant circuits.^[73] Recent advances in electrocardiographic analysis incorporate artificial intelligence (AI) algorithms to enhance detection of repolarization abnormalities, enabling automated identification of subtle QT prolongation or T-wave changes with higher sensitivity than traditional methods. These 2020s tools, often leveraging deep learning on large ECG datasets, facilitate early risk stratification for repolarization-related disorders by quantifying dispersion metrics like Tp-e in real-time clinical settings. As of 2025, FDA-approved AI systems, such as those using convolutional neural networks for QTc measurement, have demonstrated improved accuracy in diverse populations.^[74]

Therapeutic Approaches

Therapeutic approaches to repolarization abnormalities primarily target underlying pathophysiological mechanisms in conditions such as long QT syndrome (LQTS), short QT syndrome (SQTS), early repolarization syndrome, and repolarization impairments associated with obstructive sleep apnea (OSA). These interventions range from lifestyle modifications and pharmacotherapy to device-based therapies, with selection guided by risk stratification and genotype where applicable. Pharmacotherapy forms the cornerstone for many repolarization disorders. In LQTS, beta-blockers such as nadolol or propranolol are first-line treatments, as they reduce sympathetic drive and thereby mitigate triggers for torsades de pointes.^[75] These agents significantly decrease cardiac events, with evidence showing reductions of approximately 95% in LQT1, 75% in LQT2, and 80% in LQT3 patients according to systematic reviews and guidelines updated through 2023.^[76] For SQTS, which involves accelerated repolarization, potassium channel blockers like quinidine or hydroquinidine are employed to prolong the QT interval and suppress ventricular arrhythmias, serving as an alternative to devices in select cases.^[77] Antiarrhythmics such as amiodarone modulate multiple ion channels, reducing transmural dispersion of repolarization and preventing proarrhythmic effects in structural heart disease with repolarization instability.^[78] Device-based therapies are reserved for high-risk patients unresponsive to or intolerant of medications. Implantable cardioverter-defibrillators (ICDs) are indicated for secondary prevention in survivors of cardiac arrest due to repolarization-related ventricular fibrillation, as well as primary prevention in those with high-risk features like documented syncope or family history of sudden death in syndromes such as LQTS, SQTS, and early repolarization syndrome.^[79] Cardiac pacing, often via dual-chamber pacemakers, shortens the QT interval by increasing heart rate and preventing bradycardia-induced pauses, thereby reducing syncope and arrhythmic events in high-risk LQTS patients.^[80] Lifestyle modifications complement medical management by minimizing triggers. Patients with LQTS or acquired repolarization prolongation should avoid drugs known to extend the QT interval, with comprehensive lists maintained by CredibleMeds.org categorizing agents by risk level (e.g., high-risk includes certain antiarrhythmics and antibiotics).^[81] For repolarization abnormalities linked to OSA, continuous positive airway pressure (CPAP) therapy improves inhomogeneity of ventricular repolarization by enhancing oxygenation and reducing apneic episodes, as demonstrated in randomized studies showing decreased QT dispersion after treatment initiation.^[54] Emerging therapies hold promise for genetic repolarization disorders. Gene editing approaches, including CRISPR-based suppression-replacement constructs targeting KCNH2 mutations in LQT2, remain in preclinical stages, with proof-of-concept studies in animal models as of 2025, aiming to restore normal hERG channel function and normalize repolarization without systemic effects.^[65] These strategies build on foundational gene therapy models, with initial safety data supporting progression toward clinical application in monogenic channelopathies.^[82]

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ATP-sensitive potassium channel (KATP) activation can drastically shorten action potential duration (APD) in metabolically compromised myocytes.
[23]
Cardiac KATP channels in health and disease - PMC
Cardiac KATP channels are plasma-membrane proteins crucial for cardiac muscle function under stress, and are required for the adaptive response to stress.
[24]
Physiology, Sodium Potassium Pump - StatPearls - NCBI Bookshelf
Mar 13, 2023 · [1][2] The Na+ K+ ATPase pumps 3 Na+ out of the cell and 2K+ into the cell for every single ATP consumed.
[25]
Regulation of the cardiac sodium pump - PMC - PubMed Central
In excitable tissues, the activity of the plasmalemmal Na pump is vital for the maintenance of normal electrical activity and ion gradients. In cardiac muscle, ...
[26]
Role of IKur in controlling action potential shape and ... - PubMed
The ultrarapid outward current I(Kur) is a major repolarizing current in human atrium and a potential target for treating atrial arrhythmias.Missing: Ito | Show results with:Ito
[27]
Detailed ECG analysis of atrial repolarization in humans - PubMed
The average P wave duration was 124 +/- 16 ms. The PTa duration was 449 +/- 55 ms (corrected PTa 512 +/- 60 ms) and the Ta duration (P wave end to Ta wave end) ...
[28]
Differential Distribution of Cardiac Ion Channel Expression as a ...
Atrial IK1 is 6- to 10-fold smaller than ventricular IK1, explaining the less negative atrial MDP and slower phase-3 repolarization. Ultrarapid delayed ...
[29]
Characterization of an ultrarapid delayed rectifier potassium channel ...
Nov 1, 1996 · Depolarizing pulses positive to 0 mV elicit a transient outward current (Ito) and a sustained 'pedestal' current in canine atrial myocytes.
[30]
Insights into Cardiac IKs (KCNQ1/KCNE1) Channels Regulation
Dec 11, 2020 · IKs channels have been shown as important for the repolarization process of both atrial and ventricular action potentials, but more important in ...
[31]
The role of pulmonary veins in atrial fibrillation: A complex yet simple ...
PV myocytes were shown to have a higher resting membrane potential and a lower action potential amplitude and duration than the left atrium. Much evidence ...
[32]
TASK-1 Channels May Modulate Action Potential Duration of ... - NIH
... action potential duration of human atrial cardiomyocytes, using dynamic patch clamp recordings. ... The APD50 was 31.2 ± 8.9 ms and the APD90 was 147 ± 20 ms (n = ...
[33]
Human Ventricular Action Potential Duration During Short and Long ...
During LAD occlusion, APD for basic beats shortened from 260±4 to 236±4 ms (P<.0001), whereas the control group (RCA occlusion) showed no significant change ( ...
[34]
How does the shape of the cardiac action potential control calcium ...
Towards the end of phase 3 the repolarizing actions of these K+ currents are opposed by an inward INCX produced by the Na+/Ca2+ exchanger operating in its Ca2+ ...
[35]
Characteristics of the Delayed Rectifier Current (IKr and IKs) in ...
Our results suggest that the distinctive phase-3 repolarization features of M cells are due in part to a lesser contribution of IKs and that this distinction ...
[36]
Ventricular repolarization components on the electrocardiogram: Cellular basis and clinical significance
### Summary of Ito Role in Epicardial vs Endocardial Repolarization and Transmural Differences
[37]
Transmural APD gradient synchronizes repolarization in the human ...
A transmural gradient in action potential duration (APD) in the ventricular wall has been suggested to underlie the T wave in humans. We hypothesize that the ...
[38]
Effects of IKr and IKs Heterogeneity on Action Potential Duration and ...
In this report, the effects of heterogeneities of I Kr and I Ks on action potential duration (APD) and its rate dependence (adaptation) are studied
[39]
Quantitative comparison of cardiac ventricular myocyte ... - NIH
Quantitative comparison of AP repolarization, rate-dependence mechanisms, and drug response in human, dog, and guinea pig revealed major species differences.
[40]
Electrocardiographic Early Repolarization | Circulation
Mar 7, 2016 · The early repolarization (ER) pattern (ERP), initially described as elevation of the ST segment of ≥1 leads on the 12-lead ECG, has long been considered a ...
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Early Repolarization Syndrome: Diagnostic and Therapeutic Approach
Nov 27, 2018 · Early repolarization syndrome is diagnosed by the presence of J-point elevation ≥1 mm in ≥2 contiguous inferior and/or lateral leads of a ...
[42]
Early Repolarization Syndrome; Mechanistic Theories and Clinical ...
Jun 30, 2016 · The most frequently reported association has been between the KCNJ8 gene and ERS. KCNJ8 encodes the Kir6.1 subunit of the KATP channel.Missing: CACNA2D | Show results with:CACNA2D
[43]
Clinical and Functional Genetic Characterization of the Role of ...
The present study investigates the role of genetic variants in cardiac calcium-channel genes in the pathogenesis of ERS and probes the underlying mechanisms.Abstract · Introduction · Results · Discussion
[44]
Sudden Cardiac Arrest Associated with Early Repolarization
May 8, 2008 · Case subjects with a repolarization abnormality were at increased risk for recurrent ventricular fibrillation, as compared with those without ...
[45]
Prevalence and Clinical Significance of Early Repolarization in ...
Dec 8, 2024 · Early repolarization (ER) is common in athletes, with a 31.6% prevalence, especially among males and endurance athletes.
[46]
Early Repolarization Syndrome - Cardiovascular Disorders
Early repolarization syndrome is genetic and predisposes to polymorphic ventricular tachycardia (VT), ventricular fibrillation (VF), and sudden death. Consider ...
[47]
Arrhythmogenic mechanisms of obstructive sleep apnea in heart ...
OSA-associated intermittent hypoxia and oxidative stress can also cause cardiac hypertrophy.
[48]
Effects of obstructive sleep apnoea on heart rhythm - ERS Publications
Intermittent hypoxia. In OSA, recurrent apnoea and hypopnoea lead to repeated and marked arterial oxygen desaturations which cyclically recur during the night.
[49]
Calpain activation by ROS mediates human ether-a-go-go-related ...
Mar 1, 2016 · Our results demonstrate that IH decreases hERG protein and hERG-mediated K+ current and these effects are mediated by reactive oxygen species ( ...Missing: K+ | Show results with:K+
[50]
Daytime QT by Routine 12‐Lead ECG Is Prolonged in Patients with ...
Feb 5, 2020 · Abnormal QTc was found amongst 34% of male and 31% of female patients. Patients with severe OSA had longer QTc compared with normal/mild OSA ( ...
[51]
Obstructive sleep apnea and increased QT dispersion - ResearchGate
Aug 10, 2025 · Presumably, abnormal QT dispersion may be related to obstructive sleep apnea (OSA). Objective and methods This work aimed to provide a meta- ...
[52]
Obstructive sleep apnea and the risk of sudden cardiac death
Oct 21, 2025 · This systematic review and meta-analysis aimed to evaluate the association between OSA and SCD and assess the influence of disease severity and ...
[53]
Effect of CPAP on QT interval dispersion in obstructive sleep apnea ...
In OSA patients without hypertension, CPAP therapy improves the inhomogeneity of repolarization via a significant decrease in QTcd.
[54]
Obstructive Sleep Apnea and Circulating Potassium Channel Levels
Aug 19, 2016 · OSA is associated with QT prolongation, and QT prolongation is an independent risk factor for sudden cardiac death.
[55]
Long-QT Syndrome | Circulation: Arrhythmia and Electrophysiology
Aug 1, 2012 · The congenital long-QT syndrome (LQTS) is a life-threatening cardiac arrhythmia syndrome that represents a leading cause of sudden death in the young.
[56]
A comprehensive review of long QT syndrome pathogenesis and ...
Congenital LQTS is caused by genetic mutations in genes encoding cardiac ion channels or their regulatory proteins, resulting in altered channel function and ...
[57]
Genetics of long-QT syndrome | Journal of Human Genetics - Nature
Jun 25, 2015 · Congenital long QT syndrome (LQTS) is an inherited arrhythmia syndrome characterized by a prolonged QT interval in the 12-lead ECG, torsades de pointes.
[58]
Congenital Short QT Syndrome - PMC - NIH
Gain of function mutations in three genes encoding K+ channels have been identified, explaining the abbreviated repolarization seen in this condition: KCNH2 ...
[59]
Short QT syndrome | Cardiovascular Research - Oxford Academic
In short QT syndrome-1, two different missense mutations in two unrelated families led to the same amino acid change and a gain-of-function mutation in HERG.
[60]
Mechanisms, Risk Factors, and Management of Acquired Long QT ...
Hypokalemia is another common risk factor in drug-induced LQTS. Low extracellular potassium paradoxically reduces IKr by enhanced inactivation [42] or ...
[61]
Transient Outward Current (Ito) Gain-of-Function Mutations in the ...
The role of the Ito current remains central to “the repolarization disorder” theory of Brugada syndrome. The use of RV canine wedge preparations over the ...
[62]
Brugada Syndrome | Circulation: Arrhythmia and Electrophysiology
Brugada syndrome (BrS) has originally been described as an autosomal-dominant inherited arrhythmic disorder characterized by ST elevation with successive ...
[63]
AAV9-mediated KCNH2 suppression-replacement gene therapy in a ...
Aug 30, 2025 · To this end, we recently developed a hybrid suppression-and-replacement (SupRep) gene therapy for KCNH2-mediated SQT1 and type 2 long QT ...
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Detailed ECG Analysis of Atrial Repolarization in Humans - PMC
The ECG trace of the atrial repolarization wave (the Ta wave) is under normal circumstances usually obscured by the much larger QRST complex.
[65]
ECG Repolarization Waves: Their Genesis and Clinical Implications
The electrocardiographic (ECG) manifestation of ventricular repolarization includes J (Osborn), T, and U waves.
[66]
The QT Interval | Circulation - American Heart Association Journals
Jun 10, 2019 · To this end, a prolonged QTc, defined as a QTc value >450 ms in males and >460 ms in females measured preferably in lead II or V5 on a standard ...<|separator|>
[67]
QT interval variations and mortality risk: Is there any relationship?
The electrocardiographic QTc is approximately normally distributed in the general population. Normal values for the QTc range from 350 to 450 ms for adult men ...
[68]
ST Segment - StatPearls - NCBI Bookshelf - NIH
T wave changes can also occur with ST depression. Concomitant T waves changes can be seen with ventricular conduction abnormalities and repolarization ...
[69]
ECG T Wave - StatPearls - NCBI Bookshelf
Normal T-wave Etiology. Normally, the T wave is formed at the end of the last phase of ventricular repolarization. · Abnormal T-wave Etiology. Abnormalities in ...
[70]
Tpeak-Tend interval as an index of global dispersion of ... - PubMed
The ECG interval from the peak to the end of the T wave (Tpeak-Tend) has been used as an index of transmural dispersion of ventricular repolarization (DVR).
[71]
Prolonged Tpeak-to-Tend Interval on the Resting ECG Is Associated ...
The interval from the peak to the end of the T wave (TpTe) on the 12-lead ECG is a measure of transmural dispersion of repolarization in the left ventricle and ...
[72]
The 2023 Canadian Cardiovascular Society Clinical Practice ...
Many commonly prescribed drugs such as antidepressants and antibiotics can prolong the QT interval, and recommendations are provided on their safe use.Management Of Clqts · Acquired Long Qt Syndrome · Pediatric Considerations
[73]
A Systematic Review on the Role of Βeta-Blockers in Reducing ...
Sep 1, 2021 · These are highly effective, reducing the risk of cardiac events by approximately 95% in LQT1, 75% in LQT2, and 80% in LQT3. Long-acting β- ...<|separator|>
[74]
Short QT syndrome: pharmacological treatment - PubMed
Conclusions: The ability of quinidine to prolong the QT interval has the potential to be an effective therapy for short QT patients. This is particularly ...
[75]
Amiodarone reduces transmural heterogeneity of repolarization in ...
This study suggests that chronic treatment with amiodarone reduces the transmural dispersion of ventricular repolarization in human heart. This comes from a ...
[76]
Evaluation Of Patients With Early Repolarization Syndrome - NIH
Implantation of a cardiac defibrillator (ICD) is indicated in patients with early repolarization syndrome and previous aborted sudden death due to ventricular ...
[77]
Efficacy of permanent pacing in the management of high-risk ...
The beneficial effects of pacing in high-risk LQTS patients probably relate to the prevention of bradycardia, pauses, and the shortening of long QT intervals-- ...
[78]
Crediblemeds: Home
Changes to the QTdrugs List and List of Drugs to Avoid in CLQTS ... AZCERT's Scientific Review Committee has found substantial evidence that Opipramol ( ...QTDrugs ListsFor Healthcare ProvidersOverview of Long QT ...April 2024 Changes to the ...Members Login
[79]
Congenital Long QT Syndrome: A Focus on Risk Stratification and ...
Jun 27, 2025 · This has prompted the development of a KCNH2 SupRep construct, designed as a potential treatment for LQT 2 patients, with initial safety and ...