From the Guidelines
Prolonged QT interval on an ECG is most commonly caused by acquired factors, including electrolyte abnormalities, medications, and underlying heart conditions, rather than congenital factors like Long QT Syndrome (LQTS). The underlying cause of a prolonged QT interval can be complex and multifactorial. According to 1, the QT interval should be measured from the beginning of the QRS complex to the end of the T wave, and a normal QTC is 0.46 second in women and 0.45 second in men.
Key Factors Contributing to Prolonged QT Interval
- Electrolyte abnormalities, particularly hypokalemia, hypomagnesemia, and hypocalcemia, can cause QT prolongation, as noted in 1.
- Medications such as certain antiarrhythmics, antipsychotics, antibiotics like macrolides and fluoroquinolones, and antihistamines can prolong QT by blocking potassium channels, delaying repolarization, as discussed in 1.
- Structural heart disease, bradycardia, hypothyroidism, and hypothermia are also potential causes of QT prolongation.
- Congenital causes, including genetic disorders like LQTS, are less common but important to consider, especially in patients with a family history of sudden cardiac death.
Management and Risk Assessment
When evaluating a patient with prolonged QT, it is essential to check electrolytes, review medications, and consider family history of sudden cardiac death. Management involves correcting electrolyte abnormalities, discontinuing offending medications, and treating underlying conditions. For congenital LQTS, beta-blockers are first-line therapy. A prolonged QT interval is concerning because it increases the risk for torsades de pointes, a potentially fatal ventricular arrhythmia, with values above 500ms considered high risk for arrhythmias, as indicated in 1.
From the Research
Prolonged QTc Interval on ECG: Underlying Causes
The prolonged QTc interval on an ECG is a significant finding that can indicate an increased risk of life-threatening arrhythmias, such as torsades de pointes (TdP) 2, 3, 4. Several underlying causes and risk factors can contribute to QTc interval prolongation, including:
- Demographic factors: Female sex, advancing age, and genetic predisposition 2, 3
- Comorbidities: Heart failure, hypokalemia, hypomagnesemia, and bradycardia 2, 3
- Medications: Use of QT-interval-prolonging drugs, such as antiarrhythmic medications (e.g., quinidine, sotalol, amiodarone) and other medications (e.g., memantine) 2, 3, 4, 5
- Electrolyte imbalances: Hypokalemia and hypomagnesemia 2, 3, 5
- Cardiac conditions: Ischemic heart disease and history of cardiac arrest 6
Risk Factors for QTc Prolongation
Several risk factors can increase the likelihood of QTc interval prolongation, including:
- Hypokalemia: A significant risk factor for QTc prolongation 2, 3, 5
- Use of diuretics: Can contribute to hypokalemia and QTc prolongation 3
- Antiarrhythmic drugs: Can prolong the QT interval as a side effect 2, 3, 4
- QTc-prolonging medications: Can increase the risk of QTc prolongation 2, 3, 4, 5
Clinical Implications
The presence of a prolonged QTc interval on an ECG requires careful evaluation and management to prevent life-threatening arrhythmias, such as TdP. This may involve:
- Close monitoring: Regular ECG monitoring and electrolyte checks 2, 6
- Medication adjustment: Reducing or discontinuing offending medications 2, 4
- Electrolyte correction: Correcting underlying electrolyte imbalances 2, 5
- Patient education: Educating patients to recognize symptoms of TdP and seek immediate medical attention 2