From the Guidelines
Prolonged QT and QTc intervals are classified based on their duration, with a QTc interval exceeding 450 ms in men and 460 ms in women considered prolonged, as suggested by the 2016 ESC position paper 1. The classification of prolonged QT and QTc intervals is crucial in assessing the risk of torsades de pointes, a potentially fatal ventricular arrhythmia.
- The severity of QTc prolongation can be further categorized as borderline, moderate, or severe, with a QTc interval of >500 ms being of particular concern, as torsades de pointes rarely occurs when QTc is <500 ms 1.
- The American Heart Association, American College of Cardiology, and Heart Rhythm Society also emphasize the importance of measuring the QT interval and adjusting for rate, gender, and QRS prolongation, as outlined in their 2009 recommendations 1.
- Key factors to consider when evaluating a patient with prolonged QT include identifying and addressing modifiable causes, such as electrolyte abnormalities, medications, and structural heart disease, as well as considering genetic testing if congenital long QT syndrome is suspected.
- Management of prolonged QT typically involves correcting electrolyte imbalances, discontinuing offending medications, and in some cases, initiating beta-blocker therapy to reduce the risk of arrhythmias, with the goal of improving morbidity, mortality, and quality of life.
- It is essential to visually validate QT-interval prolongation reported by a computer algorithm and to consider the clinical context and potential causes of QT prolongation, as emphasized in the 2009 recommendations 1.
From the FDA Drug Label
In a study of patients with atrial fibrillation (AFIB)/flutter (AFIB/AFL) receiving three different oral doses of Sotalol AF given q12h (or q24h in patients with a reduced creatinine clearance), mean increases in QT intervals measured from 12-lead ECGs of 25 msec, 40 msec and 54 msec were found in the 80 mg, 120 mg, and 160 mg dose groups, respectively. With oral doses of 160 to 640 mg/day, the surface ECG shows dose-related mean increases of 40 to 100 msec in QTc and 10 to 40 msec in QTc Prolongation of the QT interval is dose related, increasing from baseline an average of 25,40, and 50 msec in the 80,120, and 160 mg groups, respectively, in the clinical dose-response study.
The classification of prolonged QT and QTc is not explicitly stated in the provided text, but it can be inferred that a QTc interval of:
- Less than 500 msec has a low incidence of Torsade de Pointes (1.3%)
- 500 to 525 msec has a moderate incidence of Torsade de Pointes (3.4%)
- 525 to 550 msec has a higher incidence of Torsade de Pointes (5.6%)
- Greater than 550 msec has a high incidence of Torsade de Pointes (10.8%) 2 However, the exact classification of prolonged QT and QTc is not provided.
From the Research
Classification of Prolonged QT and QTc
The classification of prolonged QT and QTc intervals is based on the duration of the interval and the presence of risk factors for torsades de pointes (TdP). The following are the classifications:
- A QTc interval of 470-500 ms for males or 480-500 ms for females is considered borderline prolonged 3
- A QTc interval of ≥500 ms is considered prolonged and is associated with a higher risk of TdP 3, 4, 5
- An increase in QTc interval of ≥60 ms from the pretreatment value is also considered prolonged 3, 5
Risk Factors for TdP
The following are risk factors for TdP:
- QTc interval >500 ms
- Increase in QTc interval ≥60 ms from the pretreatment value
- Advanced age
- Female sex
- Acute myocardial infarction
- Heart failure with reduced ejection fraction
- Hypokalemia
- Hypomagnesemia
- Hypocalcemia
- Bradycardia
- Treatment with diuretics
- Elevated plasma concentrations of QTc interval-prolonging drugs due to drug interactions 3, 4, 5
Management of Prolonged QT and QTc
The management of prolonged QT and QTc intervals involves:
- Discontinuation of the offending drug(s)
- Correction of electrolyte abnormalities
- Administration of intravenous magnesium sulfate
- Consideration of beta-blockers for patients with congenital or idiopathic QT interval prolongation
- Implantation of a cardioverter-defibrillator in certain subsets of patients 3, 5, 6, 7