Long QT Syndrome

You respond to a 22-year-old female complaining of shortness of breath. When you arrive, you find a healthy young female, who says she is feeling light-headed and can’t catch her breath. Upon further questioning, you discover she has been under an increased amount of stress, and you suspect anxiety.

You quickly package the patient and decide to treat her using basic life support, because she isn’t hypoxic and appears otherwise healthy. En route to the hospital, your patient suddenly becomes unresponsive. Her color is quickly changing, she doesn’t appear to be breathing, the pulse oximeter indicates a heart rate of zero, and your pulse check concludes cardiac arrest.

What Happened?

This scenario involving long QT syndrome (LQTS) is more common than you may think. This lesser-known cause of sudden cardiac arrest affects nearly 1 in 2,500 of the healthy population; however, it is not commonly taught in most EMS curricula and may be overlooked.¹ This condition is identified by a long QT interval on an electrocardiogram (ECG) and may be congenital or medication-induced.² An increasing number of sudden cardiac arrests with previously unknown etiology are being attributed to LQTS, including sudden infant death syndrome (SIDS).^3,4

As prehospital providers, we are often the first line of treatment for patients whose outcome we can improve by identifying certain conditions early. In addition to an improved prognosis, we can better prepare for changes in a patient’s condition and triage them to more appropriate receiving facilities.

How Do I Detect LQTS?

As the name implies, simply measure the QT/QTc interval. The QT interval begins at the very end of the PR interval, where your isoelectric line will begin another positive or negative deflection, and ends at the very end of the T wave. Lead II is generally a good place to make your measurement; however, this may change depending on the R axis. Normal QT intervals range from 300 to 450 ms (0.30-0.45 seconds). Females are commonly closer to the higher limit than males.⁵

An easier way to measure is by simply looking at the 12-lead diagnostics that print out with your rhythm strip. While rhythm interpretation may not always be up to par with your monitor, the measurements are usually quite accurate. If your ECG monitor provides a questionable QT interval, double-check it yourself. Find a Q wave that begins on one of the darker lines and measure the QT interval. Remember, the length of each big box (the ones with dark line perimeters) is 200 ms (see Figure 1).

What is the Difference Between QT and QTc?

The c in QTc stands for corrected. The QTc is the QT interval with a formulated correction based on the patient's heart rate (HR). The faster the HR, the shorter the QT interval. The QTc may not always be accurate, but it is commonly used for diagnostic purposes. Bazett's formula is used to determine QTc: QT interval divided by the square root of the RR interval. Fortunately, most ECG monitors formulate this for you.²

How Long Is Too Long?

A QTc greater than 460 ms is considered long. ⁶ As previously stated, females generally fall into the upper limit.

Why Is LQTS Bad?

The simple answer is the prolonged refractory period. The absolute refractory period is the longer section, from the beginning of the QRS to the peak of ventricular repolarization (T wave).⁷ During the absolute refractory period, a second action potential cannot be initiated—hence the term absolute. The relative refractory period is the usually short section of the refractory period that starts after the absolute refractory period and ends with the end of ventricular repolarization (the end of the T wave). The relative refractory period attempts to inhibit a second action potential, but it is possible for a second action potential to initiate. This means another part of the heart may depolarize during repolarization, propagating an arrhythmia. An example of this is commonly referred to as R on T phenomenon. Similar to attacking a sleeping enemy, by depolarizing during the relative refractory period of the previous pacemaker, this second focus may take over. This is a common cause of lethal arrhythmias like ventricular tachycardia.