Long QT in children and teens is a particularly concerning condition characterized by a prolonged QT interval and arrhythmias stemming from torsades de pointes (twisting of the points), which can cause syncope and may progress to ventricular fibrillation, resulting in cardiac arrest or sudden death. A 12-lead ECG provides key information for diagnosing and monitoring prolonged QT, and it plays a central role in managing patients with long QT.
Types of Long QT and How to Manage The Condition
The true prevalence of long QT syndrome (LQTS), which is either congenital or acquired, is unknown because the condition often goes unnoticed. One widely cited study in Circulation from 2009 estimated a prevalence of congenital LQTS of about 1 in 2,000. The National Heart, Lung, and Blood Institute (NHLBI) says experts estimate that LQTS can be found in about 1 in 7,000 people and that it causes 3,000 to 4,000 sudden deaths in American children and young adults each year. Cardiac arrest or sudden death will be the first indication of a problem in about 10% of those who are affected.
According to the NHLBI, there are several known types of congenital LQTS: type 1 (arrhythmias triggered by emotional stress or exercise, which are the most common), type 2 (arrhythmias sparked by extreme emotions), and type 3 (arrhythmias related to bradycardia during sleep).
LQTS can also be acquired, either as a side effect of QT-prolonging medications or as a consequence of electrolyte imbalances induced by various medical conditions, including severe diarrhea or vomiting, eating disorders, and some thyroid disorders.
There are several options for management, including avoiding medications that prolong QT. The NHLBI says more than 50 drugs, including antihistamines/decongestants, diuretics, antibiotics, antiarrhythmics, antidepressants, antipsychotics, and some cholesterol-lowering and diabetes medications, have been associated with LQTS. Of note, an editorial in Circulation Journal notes that attention deficit/hyperactivity disorder (ADHD) is relatively common in children, and that stimulant therapy used to treat it "is relatively contraindicated in patients with LQTS because of the possibility of LQTS-triggered cardiac events."
CredibleMeds.org maintains a list of medications associated with QT prolongation.
Diagnosing Long QT in Children and Others
Establishing a diagnosis of LQTS requires a three-pronged approach that includes a medical history/physical exam, genetic testing, and ECG. According to a consensus statement from the Heart Rhythm Society (HRS), European Heart Rhythm Association, and Asia Pacific Heart Rhythm Society published in EP Europace in 2013, LQTS is diagnosed when one or more of the following conditions is met:
- The LQTS score, which incorporates age, medical and family history, symptoms, and the QT interval corrected for heart rate (QTc), is 3.5 or greater, and there is no secondary cause of QT prolongation.
- There is a pathogenic mutation in one of the LQTS genes (the authors note that 20% to 25% of these patients may have a QTc in the normal range).
- QTc interval is 500 ms or greater on repeated 12-lead ECGs, and there is no secondary cause of QT prolongation; or QTc is between 480 and 499 ms in a patient with unexplained syncope in the absence of a secondary cause or pathogenic mutation.
According to the NHLBI, because the QT may not be prolonged at all times in a patient with LQTS, a stress test or use of a Holter monitor may be necessary to make the diagnosis.
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Challenges in ECG Interpretation
In their 2015 guidance on cardiac channelopathies in competitive athletes, the American Heart Association (AHA) and American College of Cardiology (ACC) pointed out that diagnosing LQTS can be difficult, with much variability between pediatric and adult heart rhythm specialists. In a study published in Circulation, for instance, 40% of patients previously diagnosed with LQTS were deemed normal after subsequent evaluation at a specialty LQTS center.
Recommendations from the AHA, ACC, and HRS regarding standardization and interpretation of ECG, published in 2009 in Circulation, detail some specific findings to watch out for in LQTS, noting that the U wave can fuse with the T wave when the QT interval is "markedly" prolonged—as is seen in both congenital and acquired LQTS—and that QT and ST-T patterns can vary widely across LQTS genotypes.
The groups also highlighted problems associated with measuring the QT interval, and recommended adjustments not only for rate, but also for sex and age because of demonstrated differences between males and females, which become smaller as both groups grow older. The authors recommended using an adjusted QT of at least 460 ms for women and 450 ms for men to define a prolonged QT interval.
Relevant Pediatric ECG Issues for Follow-up
An article from the Emergency Medicine Residents' Association (EMRA) details important differences between pediatric and adult ECGs that are relevant to evaluating long QT in children. Rate is a key consideration, as children can tolerate a much higher heart rate and rely on rate increases to boost cardiac output. Moreover, resting heart rate values in children don't overlap with the adult range until the age of five or older, and children can also have a wider range of rates.
There are also pertinent differences between pediatric and adult ECGs in axis, precordial T waves, Q waves, and intervals. "Identification of differences in the pediatric and adult ECG aid in distinguishing potentially life-threatening abnormalities from a normal ECG, with one of the most notable and vastly overlooked conditions being prolonged QT interval," according to the EMRA article.
Because children with LQTS will require a lifetime of care, it is crucial for cardiologists to consider potential issues related to interpretation of pediatric ECG as they conduct their ongoing checkups.
