From the Guidelines
Interval resolution in the context of electrocardiography refers to the measurement of the QT interval, which is the time from the start of the QRS complex to the end of the T wave, and its correction for heart rate to assess trends in a given patient over time. This process is crucial in evaluating the risk of potentially lethal ventricular arrhythmias, such as torsades de pointes. The QT interval should be measured from the beginning of the QRS complex to the end of the T wave, and it is essential to determine the appropriate lead(s) in which to measure the QT interval and adjust it for increases in QRS duration, gender, and rate 1.
Some key considerations in interval resolution include:
- Recognizing the onset of the QRS complex and the end of the T wave
- Determining the appropriate lead(s) in which to measure the QT interval
- Adjusting the QT interval for increases in QRS duration, gender, and rate
- Using the lead showing the longest QT interval, usually V2 or V3, and considering measurements from adjacent leads if the difference is more than 40 ms 1
It is also important to note that the QT interval can be affected by various conditions, such as hypokalemia and hypocalcemia, and that its prolongation can be induced by administration of QT-prolonging cardioactive drugs 1. Visually validating QT-interval prolongation reported by a computer algorithm is essential in view of the clinical importance of QT-interval prolongation. A normal QTC is 0.46 second in women and 0.45 second in men, and a QTC of 0.50 second in either sex has been shown to correlate with a higher risk for torsades de pointes 1.
In clinical practice, the most commonly used QT correction formula is the one introduced by Bazett, QTC = QT interval divided by the square root of the R-R interval measured in seconds 1. However, the adequacy of Bazett’s formula has been questioned, and some evidence exists that the formula overcorrects the QT interval at fast heart rates and undercorrects it at low heart rates 1. Therefore, it is crucial to carefully evaluate the QT interval and its correction in each patient to assess the risk of potentially lethal ventricular arrhythmias.
From the Research
Definition of Interval Resolution
- Interval resolution refers to the process of measuring and interpreting the QT interval, which represents the time taken for the ventricles to completely repolarize after activation 2.
- The QT interval is an essential aspect of ECG interpretation, and its abnormal prolongation can lead to torsades de pointes, a form of potentially life-threatening polymorphic ventricular tachycardia (VT) 2, 3, 4.
Measurement and Correction of QT Interval
- The normal QT interval is typically measured relative to the heart rate, and it often requires "correction" for the current heart rate to correctly stratify the risk of torsades de pointes 2.
- A wealth of correctional formulae have been derived, but none has proven superior, making the measurement and correction of the QT interval a challenging task 2.
- Automated ECG printouts cannot be relied upon to diagnose QT interval prolongation, and the onus is on the clinician to identify it 2.
Clinical Significance of QT Interval Prolongation
- QT interval prolongation can be caused by various factors, including errors in the use of medications that may prolong this interval 5.
- A QT interval of at least 500 milliseconds generally has been shown to correlate with a higher risk of torsades de pointes, but there is no established threshold below which prolongation of the QT interval is considered free of proarrhythmic risk 5.
- The risk of torsades de pointes should be assessed in patients who are about to begin taking a QT-prolonging medication, and proper monitoring of the QT interval is essential in these patients 5.
Treatment Strategies for QT Interval Prolongation
- Combination therapy with a beta-blocker and cardiac pacing appears to be a highly effective primary therapy for symptomatic patients with the long QT syndrome 3, 4.
- Atrial pacing combined with beta-blocker therapy may be a favorable approach in young high-risk patients with long QT syndrome, particularly those with Jervell and Lange-Nielsen syndrome 6.
- The use of QT-prolonging medications can predispose to torsades de pointes, and clinicians should be aware of the potential risks and take steps to minimize them 5.