Long QT on ECG: a pediatric case of exertional syncope and cardiac arrest

Long QT on ECG is the diagnosis at the center of this pediatric case of exertional syncope and cardiac arrest. This lesson walks through a full ECG review—confirming sinus rhythm, checking the PR interval and QRS duration, then applying Bazett's formula to calculate the corrected QT interval (QTc). See how to use ECG, step by step, to reach a long QT diagnosis.

In this lesson from our Pediatric ECG course, you'll learn how to:

• Recognize syncope in children as a warning sign of an underlying cardiac cause
• Systematically review a pediatric ECG: rhythm, rate, axis, and intervals 
• Calculate QTc with Bazett's formula to identify a long QT interval on ECG
• Understand why long QT syndrome puts young athletes at risk of sudden cardiac arrest during exertion
   

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In this Medmastery lesson, we'll explore long QT syndrome and how it presents on an ECG. This case-based lesson will help you practice recognizing key ECG features and understand their significance when diagnosing long QT syndrome in children.

Exertional syncope: a pediatric case

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Our patient is a 17-year-old soccer player that experienced exertional syncope, a sudden loss of consciousness during physical activity while running on the field. Immediately after her syncopal event, she was noted to be without a pulse, and she was not breathing. 
Her coach reacted quickly, called for an AED, or automated external defibrillator, and began chest compressions. Fortunately, our patient started waking up as soon as the AED arrived. The patient was taken to the emergency room, and an ECG was obtained. 

Let's take a look at that ECG and review the rhythm, rate, and axis.

Assessing sinus rhythm on ECG

[0:55]
Rhythm assessment shows a P wave before every QRS complex and a QRS complex after every P wave. The P wave axis is upright, or positive, in leads I and aVF, indicating sinus rhythm. Using the gridlines on the ECG, calculating the heart rate reveals a rate of about 85 beats per minute. Perfectly normal.

Mild right axis deviation on ECG

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Now we examine the QRS complexes, which are downward in lead I and upright in aVF. These findings suggest mild right axis deviation.

QT interval assessment

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Now let's look at the intervals. The duration of the PR interval and QRS complex is normal, but the QT interval, even to the naked eye, looks longer than normal. The QT interval reflects the total time taken for the ventricles to contract (depolarize) and then relax (repolarize) in a single heartbeat. On this ECG, the QT interval is measured to be 480 milliseconds, with an RR interval of 700 milliseconds. 

Applying Bazett’s formula

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If we apply Bazett's formula, we can calculate the corrected QT by dividing the measured QT interval by the square root of the measured RR interval. Let's start by converting the RR interval and the QT interval from milliseconds to seconds. Now, using Bazett's formula, divide the QT interval of 0.48 seconds by the square root of the RR interval, 0.7 seconds, which gives us a corrected QT of 0.57 seconds, or 500 milliseconds. Wow. 

Diagnosing long QT on ECG

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If you remember, the QTc should be less than 470 milliseconds. This degree of QTc prolongation is diagnostic of long QT syndrome. Long QT syndrome is an inherited heart rhythm disorder that can cause sudden death from malignant ventricular arrhythmias.

Long QT syndrome pathophysiology

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Ion channels, tiny little pores in the cell membrane, tightly regulate heart muscle depolarization and repolarization. When ion channels do not function normally, which is the case in long QT syndrome, the flow of ions in and out of the heart cells is disrupted. This delays the heart's ability to repolarize, or recharge, which we see as a prolonged QT interval on the ECG.

Ventricular arrhythmias during exertion

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This condition causes the heart to be vulnerable to developing very fast ventricular arrhythmias that can lead to sudden death. These arrhythmias typically occur during exertion, as was the case in our patient.

Recovery after long QT syndrome diagnosis

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Fortunately for our patient, her arrhythmia spontaneously converted, and no immediate intervention was required. She recovered fully after her event. Once we diagnosed her with long QT syndrome in our office, we quickly started her on medication, and she has done great since, with no recurrence of symptoms for the last 12 years.