From the Guidelines
Prolonged QT interval is a cardiac condition where the heart takes longer than normal to recharge between beats, specifically affecting the QT interval on an electrocardiogram (ECG), with a QTc >500 ms associated with a 2- to 3-fold higher risk for Torsades de Pointes, as indicated by the American Heart Association and the American College of Cardiology Foundation 1.
Definition and Risk
The QT interval represents the time from when the ventricles start to contract until they finish relaxing and prepare for the next heartbeat. A prolonged QT interval can be congenital (Long QT Syndrome) or acquired from medications, electrolyte abnormalities, or other medical conditions. It's concerning because it increases the risk of developing a dangerous heart rhythm called Torsades de Pointes, which can lead to sudden cardiac death.
Normal QT Intervals and Prolongation Thresholds
Normal QT intervals are generally less than 450 milliseconds for men and 470 milliseconds for women, with values above these thresholds considered prolonged, as supported by the American College of Cardiology 1.
Causes and Management
Many medications can cause QT prolongation, including certain antibiotics (such as azithromycin), antipsychotics, antidepressants, and anti-arrhythmic drugs. Management involves identifying and addressing underlying causes, avoiding QT-prolonging medications, maintaining normal electrolyte levels (especially potassium and magnesium), and in some cases, using beta-blockers or implantable defibrillators for high-risk patients, as recommended by the American Heart Association 1.
Clinical Importance and Monitoring
The clinical importance of QT-interval prolongation necessitates careful monitoring, especially in patients with known risk factors or those taking medications that can prolong the QT interval, with the American Heart Association emphasizing the need for continuous ECG monitoring with immediate access to defibrillation in certain conditions 1.
Key Considerations
- QT Interval Measurement: The QT interval should be measured from the beginning of the QRS complex to the end of the T wave, with consideration for the challenges in determining the end of the T wave, especially in cases of T-wave distortion or the presence of U waves.
- Correction for Heart Rate: The QT interval is often corrected for heart rate (QTc) to assess trends over time, although the best correction formula and the threshold for dangerous QT prolongation are subjects of ongoing debate.
- Risk Assessment: A QTc >500 ms is associated with an increased risk for Torsades de Pointes, but the risk also exists at lower QTc values, especially in the presence of other risk factors such as hypokalemia or the use of QT-prolonging medications.
From the Research
Definition of Prolonged QT Interval
- The QT interval is an electrophysiological entity characterized by an extended duration of the ventricular repolarization, reflected as a prolonged QT interval in a surface ECG 2.
- A prolonged QT interval signifies an increased risk of the life-threatening arrhythmia torsades de pointes (TdP) 3.
- Normal QTc intervals are typically <450 ms for men and <460 ms for women, and for every 10 ms increase, there is a ~5% increase in the risk of arrhythmic events 4.
Causes of Prolonged QT Interval
- The most common acquired cause of Long QT syndrome (LQTS) is drug-induced QT interval prolongation 2.
- Antiarrhythmic drugs such as amiodarone, sotalol, quinidine, procainamide, verapamil, and diltiazem are associated with the prolongation of the QTc interval 2, 5.
- Noncardiovascular drugs such as ondansetron, macrolide and fluoroquine antibiotics, and typical and atypical antipsychics agents can also prolong the QTc interval 2.
Risk Factors for Prolonged QT Interval
- Risk factors for drug-induced TdP include hypokalemia, female sex, drug-drug interactions, advancing age, genetic predisposition, hypomagnesemia, heart failure, bradycardia, and corrected QT (QTc) interval prolongation 3.
- Patient-related risk factors, such as age >65 years, uncorrected electrolyte disturbances, and co-prescribed medicines that could increase the risk of QT prolongation, should be considered when prescribing drugs associated with QT prolongation 4.
Management and Monitoring of Prolonged QT Interval
- Close monitoring of the QT interval is necessary to prevent TdP, and patients at risk should be educated to go directly to the emergency room if they experience palpitations, lightheadedness, dizziness, or syncope 3.
- When the QTc interval is prolonged, dose reduction or discontinuation of the offending drug should be considered, and electrolytes corrected as needed 3.
- A consistently prolonged QTc interval is associated with an increased risk of sudden cardiac death (SCD), and repeated measurements of the QTc interval could enhance risk stratification 6.