Causes of Prolonged QT Interval

Causes of Prolonged QT Interval

The QT interval on an electrocardiogram (ECG) represents the total time for ventricular depolarization and repolarization. Prolongation of the QT interval indicates delayed ventricular repolarization and is clinically important because it increases the risk of polymorphic ventricular tachycardia, particularly Torsades de Pointes, which can lead to sudden cardiac death.

What is a Prolonged QT Interval?

The QT interval varies with heart rate and is usually corrected using the QTc (corrected QT interval).

General reference values:

QTc > 440 ms in men → prolonged

QTc > 460 ms in women → prolonged

QTc ≥ 500 ms → significantly increased risk of Torsades de Pointes

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Major Causes of Prolonged QT Interval

1. Congenital Long QT Syndromes

These are genetic disorders caused by mutations affecting cardiac ion channels.

Common types include:

LQT1 – KCNQ1 mutation (potassium channel defect)

LQT2 – KCNH2 mutation

LQT3 – SCN5A mutation (sodium channel abnormality)

Key clinical features:

Syncope

Palpitations

Sudden cardiac death in young individuals

Family history of unexplained death

Triggers vary by type:

LQT1 → exercise

LQT2 → emotional stress or sudden noise

LQT3 → events during sleep

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2. Electrolyte Abnormalities

Electrolyte disturbances are among the most common reversible causes of QT prolongation.

Important abnormalities include:

Hypocalcemia

Causes prolonged ST segment

Leads to QT prolongation

Hypokalemia

Delayed repolarization

Associated with U waves

Hypomagnesemia

Predisposes to Torsades de Pointes

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3. Medications

Drug-induced QT prolongation is a very common clinical scenario.

Antiarrhythmic drugs

Sotalol

Dofetilide

Quinidine

Procainamide

Amiodarone

Antibiotics

Macrolides (Azithromycin, Erythromycin)

Fluoroquinolones (Levofloxacin, Moxifloxacin)

Antipsychotics

Haloperidol

Ziprasidone

Quetiapine

Antidepressants

Tricyclic antidepressants

Citalopram

Other drugs

Methadone

Ondansetron

Certain antifungals

These medications typically prolong QT by blocking potassium channels responsible for repolarization.

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4. Cardiac Conditions

Certain structural or ischemic heart diseases can also prolong QT.

Examples include:

Acute myocardial infarction

Myocarditis

Heart failure

Cardiomyopathies

Post-cardiac arrest state

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5. Endocrine and Metabolic Disorders

Metabolic disturbances may alter cardiac ion channel function.

Important causes include:

Hypothyroidism

Severe hypothermia

Starvation or malnutrition

Pheochromocytoma (rare)

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6. Neurological Conditions

Some neurological injuries can produce QT prolongation through autonomic imbalance.

Examples include:

Subarachnoid hemorrhage

Stroke

Traumatic brain injury

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ECG Features of Prolonged QT

Typical ECG findings include:

QT interval exceeding normal limits

Delayed ventricular repolarization

Possible T wave abnormalities

Predisposition to Torsades de Pointes

Torsades ECG characteristics:

Polymorphic ventricular tachycardia

Twisting QRS complexes around the baseline

Often triggered by a premature ventricular beat

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Clinical Importance

Recognizing QT prolongation is crucial because it may lead to life-threatening arrhythmias.

Key clinical steps include:

Reviewing medication history

Checking electrolytes (K⁺, Ca²⁺, Mg²⁺)

Identifying congenital long QT syndromes

Correcting reversible causes immediately

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Key Takeaway

Prolonged QT interval results from delayed ventricular repolarization and can be caused by:

Congenital ion channel mutations

Electrolyte abnormalities

Medications

Cardiac diseases

Endocrine or neurological disorders

Early recognition and correction of reversible causes are essential to prevent Torsades de Pointes and sudden cardiac death.