Measuring the QTc Pearls and Pitfalls

Have you ever wondered what your physician is referring to when they discuss the "QT interval" or how this interval gets measured?

To explain the QT interval, we first have to start with a standard electrocardiogram (ECG) and its

most common waveforms. An ECG is a graphic representation of the electrical activity of the heart.

The basic pattern of an ECG comprises of three waves: P, QRS, and T. For more information on how an ECG specifically measure the cardiac cycle, please visit Oregon State University's website for a wonderfully in-depth explanation.

The QT interval is the time from the start of the Q wave to the end of the T wave. Long QT syndrome (LQTS) is a disorder of the heart's electrical activity. When the QT interval is too long, it can "trip up" the heart, causing sudden, uncontrollable arrhythmias. Measuring the QT interval isn't as easy as it seems. There are several factors that contribute to this. The QTc is the corrected QT interval after different factors and the correct formula are applied.

To understand how to correctly measure the QTc, check out this lecture by Dr. Michael Ackerman:

You can also use online calculators, such as this one created by Dr. Ackerman and Mayo Clinic.

Meet other people talking about genetic heart disorders on Mayo Clinic Connect. Join the Heart Rhythm Conditions group to join the conversation, share experiences, ask questions, and discover your support network.

For up-to-date information, please follow Dr. Ackerman and the Windland Smith Rice Genetic Heart Rhythm Clinic on Twitter by clicking the links below.