As a general pediatrician, I see patients who often have irregular heart rates – much of this is common in children and is totally benign. However, occasionally, there are heart rate rhythms that are abnormal, or there is a family history of abnormal heart rhythms. This is when a referral to a pediatric cardiologist is warranted. Salim Idriss, MD, PhD, explains one of these abnormalities in the following discussion. 

--Dennis Clements, MD, PhD, MPH

What is With This QT Interval? 

There has been much in the news lately about the QT interval and stimulant medication for Attention Deficit Disorder (ADD, ADHD, etc). These issues have been raised out of concern that these medications, as well as others, may cause abnormal heart rhythms in people that have an underlying, or hidden, defect in the electrical signals of the heart. Measurement of the QT interval may detect some of these individuals that may be at risk. In the next few paragraphs, I will explain the origin of the QT interval and why abnormalities of the measurement could suggest a problem. 

The Normal Heart Beat 

In order to understand the QT interval, one has to first understand the basic mechanism of a heartbeat. The heart is a pump made primarily of specialized muscle. In order for the heart to beat, a wave of electricity must pass through the muscle in an orderly fashion. A special area of the heart regularly generates the electric wave approximately every second in teens and adults, and more often in younger children. The electric wave is then passed along in the heart muscle from cell to cell in a well-orchestrated series of events. For the wave to move along, sodium, potassium, and calcium must enter in and exit out of the cell in a timed sequence through specialized channels with gates. For each beat, the gates must open for a particular amount of time. The gate must then close, and stay closed, for another amount of time. Then, in order to be ready for the next beat, the gate must reset itself. The time it takes to reset, or repolarize, is an important component in keeping a stable heart rhythm. The whole process takes a fraction of a second. 

The Electrocardiogram 

The electrocardiogram, commonly known as an “EKG” or “ECG”, is a measure of the electrical signals in the body that are generated by the heart. Recording electrodes are placed on the skin surface at specific positions on the chest, arms, and legs in order to measure the signals, which are only a thousandth of a volt. The information contained in the ECG allows us to determine whether the heart rhythm is normal, whether there are areas of the heart that may be too large or small, or whether there may be problems with the electrical resetting process. 

The QT Interval 

The QT interval is a measurement made from the ECG of the time from the onset of the electrical wave in the heart to when the entire heart has reset and is ready for the next beat. The QT interval is shorter at faster heart rates and lengthens at slower rates. Since people’s heart rates vary between individuals, ages, and with activity, the QT interval measurement is adjusted for the heart rate using a mathematical formula. The corrected QT interval, or QTc, is the measurement that we use to determine if a patient has a normal resetting process. 

Problems with Electrical Resetting in the Heart: Long QT Syndrome 

One of the reasons for measuring the QT interval of the ECG is to evaluate for a rare disorder called the long QT syndrome (LQTS). LQTS is a genetic disorder that affects the channels that pass sodium, potassium, or calcium into and out of the heart cells with each beat, as described above. There are at least eleven types of LQTS. Each one is due to a different type of gene defect and each has different ways of affecting people. LQTS alters how the heart resets itself. In many cases, this results in prolongation of the resetting time, and consequently, lengthening of the QT interval in the ECG. Prolongation of the heart’s electrical resetting time, under certain conditions, may lead to dangerous irregularities of the rhythm. Arrhythmias of this type may cause palpitations, fainting, or sudden death. 

LQTS is a rare disorder. However, it is important to identify people with this problem. Many people with LQTS live normal lives without symptoms. Some people require changes in lifestyle to avoid triggers of arrhythmias. This may involve avoiding strenuous exercise, loud noises, overly stressful situations, or certain medications that make the defective heart channels function even more poorly. Others may need to take medication to help with symptoms. Yet, others with LQTS may need to have an implanted defibrillator (ICD). An ICD is a device that continuously monitors the heart’s rhythm. If it detects a life-threatening arrhythmia, the ICD can deliver an electric shock to reset the heart rhythm to normal. 

What if my Child has a Long QT Interval on an Electrocardiogram? 

While LQTS is named for the prolonged QT interval on the ECG, a long QT interval is not the only criterion for diagnosing the disease. This may seem quite confusing. The reason is that the QT interval can vary, and there are borderline values. Some people with LQTS can have a QT interval that is only slightly long or even in the normal range. In addition, accurate measurement of the QT interval can be tricky. Most ECG machines automatically measure a QT interval, and these measurements are frequently incorrect. 

Therefore, if your child needs evaluation for a prolonged QT interval, it is important that it is done by a pediatric cardiologist, or more specifically, a pediatric electrophysiologist with expertise in rhythm disturbances of the heart. For more information about LQTS, there are many online resources such as:  www.sads.org or www.nhlbi.nih.gov/health/health-topics/topics/qt/. 

-- Salim F. Idriss, MD, PhD, is a member of the Duke Pediatric Division of Cardiology and is actively involved in the evaluation, diagnosis and treatment of congenital and acquired heart disease in infants, children, and adolescents. Other clinical specialties include cardiac arrhythmias, syncope, and sudden death; cardiac pacing and defibrillation; electrophysiologic studies and ablation of cardiac arrhythmias.

-- Dennis Clements, MD, PhD, is the chief of primary care pediatrics at Duke Children's Hospital & Health Center.