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Long QT syndrome

Updated : August 30, 2023





Background

The QT interval is an important parameter measured on an electrocardiogram that reflects the ventricular action potential duration. This interval represents the time from the start of the Q wave to the end of the T wave and corresponds to the period of ventricular depolarization and repolarization. Several factors, such as age, gender, heart rate, medications, and electrolyte imbalances, influence the QT interval’s duration.

Prolongation of the QT interval can be congenital or acquired. It has been associated with an increased risk of developing life-threatening arrhythmias such as sudden cardiac death and Torsades de Pointes.

Some factors that can cause acquired QT prolongation include medications such as antiarrhythmics, antibiotics, and antipsychotics, electrolyte imbalances such as hypokalemia and hypomagnesemia, and structural heart diseases such as cardiomyopathy. When the QT interval is prolonged, it can lead to delayed repolarization of the ventricles, increasing the likelihood of early afterdepolarizations and triggered activity, ultimately leading to arrhythmias.

Epidemiology

Long QT syndrome, which refers to congenital causes, has a prevalence that is challenging to determine, but it is estimated to occur in 1 out of 10,000 individuals. It is more frequent in females and typically presents with cardiac events during childhood, adolescence, or early adulthood. However, there have been instances where it has manifested in individuals in their fifties.

Around 40% of patients have a positive family history, and 30% have a positive family history of sudden cardiac death. In contrast, acquired causes are relatively more common than congenital causes. Several studies have reported that QT prolongation is prevalent in up to 30% of patients in the intensive care unit.

Anatomy

Pathophysiology

The duration of the QT interval, an indicator of ventricular repolarization, is primarily influenced by the duration of the ventricular action potential. The opening and closing of ion channels in the heart, with positive ions such as calcium and sodium influx causing depolarization and positive ions such as potassium efflux causing repolarization, play a significant role in this process.

Any dysfunction in these ion channels resulting in excess of positive ions intracellularly can cause QT prolongation, which is a rare but potentially life-threatening condition. Long QT syndrome is associated with mutations in 15 genes, with KCNQ1 being the most frequently mutated gene causing Long QT syndrome type 1.

In contrast, QT prolongation is more commonly acquired due to electrolyte disturbances such as hypocalcemia, hypokalemia, and hypomagnesemia, as well as certain medications that block the outward IKr current mediated by the potassium channel encoded by the KCNH2 gene.

Etiology

Genetics

Prognostic Factors

Clinical History

Physical Examination

Age group

Associated comorbidity

Associated activity

Acuity of presentation

Differential Diagnoses

Differential Diagnoses

Brugda Syndrome

Coronary artery anomalies

Cardiac death

Hypertrophic

Short QT syndrome

Seizure

Laboratory Studies

Imaging Studies

Procedures

Histologic Findings

Staging

Treatment Paradigm

The primary objective of managing patients with Long QT syndrome is to prevent lethal arrhythmias like torsade de pointes by controlling the QT interval duration. If a patient is hemodynamically unstable, they should receive non-synchronized electrical defibrillation. The first-line treatment is magnesium sulfate, which has shown benefits regardless of serum magnesium levels.

If magnesium sulfate is ineffective, temporary transvenous overdrive pacing may be considered. Class IB antiarrhythmic drugs such as lidocaine and phenytoin, and isoproterenol may also be used. Beta-blockers are the preferred choice for long-term management of congenital Long QT syndrome as they help prevent ventricular arrhythmias by stabilizing ventricular action potential and blocking sympathetic surges associated with arrhythmias.

An implantable cardioverter defibrillator is recommended if a patient has been resuscitated from a cardiac arrest or has beta-blocker-resistant symptoms. Additionally, it may be necessary for asymptomatic individuals suspected to be at high risk for ventricular arrhythmias.

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Long QT syndrome

Updated : August 30, 2023




The QT interval is an important parameter measured on an electrocardiogram that reflects the ventricular action potential duration. This interval represents the time from the start of the Q wave to the end of the T wave and corresponds to the period of ventricular depolarization and repolarization. Several factors, such as age, gender, heart rate, medications, and electrolyte imbalances, influence the QT interval’s duration.

Prolongation of the QT interval can be congenital or acquired. It has been associated with an increased risk of developing life-threatening arrhythmias such as sudden cardiac death and Torsades de Pointes.

Some factors that can cause acquired QT prolongation include medications such as antiarrhythmics, antibiotics, and antipsychotics, electrolyte imbalances such as hypokalemia and hypomagnesemia, and structural heart diseases such as cardiomyopathy. When the QT interval is prolonged, it can lead to delayed repolarization of the ventricles, increasing the likelihood of early afterdepolarizations and triggered activity, ultimately leading to arrhythmias.

Long QT syndrome, which refers to congenital causes, has a prevalence that is challenging to determine, but it is estimated to occur in 1 out of 10,000 individuals. It is more frequent in females and typically presents with cardiac events during childhood, adolescence, or early adulthood. However, there have been instances where it has manifested in individuals in their fifties.

Around 40% of patients have a positive family history, and 30% have a positive family history of sudden cardiac death. In contrast, acquired causes are relatively more common than congenital causes. Several studies have reported that QT prolongation is prevalent in up to 30% of patients in the intensive care unit.

The duration of the QT interval, an indicator of ventricular repolarization, is primarily influenced by the duration of the ventricular action potential. The opening and closing of ion channels in the heart, with positive ions such as calcium and sodium influx causing depolarization and positive ions such as potassium efflux causing repolarization, play a significant role in this process.

Any dysfunction in these ion channels resulting in excess of positive ions intracellularly can cause QT prolongation, which is a rare but potentially life-threatening condition. Long QT syndrome is associated with mutations in 15 genes, with KCNQ1 being the most frequently mutated gene causing Long QT syndrome type 1.

In contrast, QT prolongation is more commonly acquired due to electrolyte disturbances such as hypocalcemia, hypokalemia, and hypomagnesemia, as well as certain medications that block the outward IKr current mediated by the potassium channel encoded by the KCNH2 gene.

Differential Diagnoses

Brugda Syndrome

Coronary artery anomalies

Cardiac death

Hypertrophic

Short QT syndrome

Seizure

The primary objective of managing patients with Long QT syndrome is to prevent lethal arrhythmias like torsade de pointes by controlling the QT interval duration. If a patient is hemodynamically unstable, they should receive non-synchronized electrical defibrillation. The first-line treatment is magnesium sulfate, which has shown benefits regardless of serum magnesium levels.

If magnesium sulfate is ineffective, temporary transvenous overdrive pacing may be considered. Class IB antiarrhythmic drugs such as lidocaine and phenytoin, and isoproterenol may also be used. Beta-blockers are the preferred choice for long-term management of congenital Long QT syndrome as they help prevent ventricular arrhythmias by stabilizing ventricular action potential and blocking sympathetic surges associated with arrhythmias.

An implantable cardioverter defibrillator is recommended if a patient has been resuscitated from a cardiac arrest or has beta-blocker-resistant symptoms. Additionally, it may be necessary for asymptomatic individuals suspected to be at high risk for ventricular arrhythmias.

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