ECG Challenges: Measurement and Rate Correction of the QT Interval
Gerard B. Hannibal RN, MSN, PCCN; Department Editor
Amer A. Hasanien RN, MSN
David Pickham RN, PhD

$3.95 		AACN Advanced Critical Care
March 2013 
Volume 24  Number 1
Pages 90 - 96
 
  PDF Version Available!

ABSTRACT
The surface electrocardiogram (ECG) is an indirect measure of the heart's electrical activity and is one of the most commonly used diagnostic tools. Measuring the QT interval on the surface ECG is important because when it is long, it is associated with syncope, the arrhythmia torsade de pointes, and death.1-7 The QT interval is an indirect measure of ventricular repolarization and is measured in seconds (s) or milliseconds (ms) from the onset of the QRS complex to the end of the T wave. Including the QRS complex in the QT interval measurement is necessary because the start of a ventricular repolarization is buried somewhere within the QRS complex. Therefore, as the QRS complex is usually less than 120 ms and its onset is easily identifiable, the QRS complex is included. However a second step after the measurement of the QT interval is needed to determine the true ventricular repolarization time.The QT interval is inversely related to heart rate. As heart rate increases, repolarization time decreases, and vice versa. The dependence of QT-interval duration on the underlying heart rate is adjusted by using an equation to derive a QT interval that is independent from heart rate. The derived value is called the heart rate corrected QT interval, or commonly the QTc interval, and represents the expected QT-interval duration when the underlying heart rate is 60/min. Correcting the QT interval for heart rate is necessary for monitoring, because the correction allows the comparison of multiple QT intervals over time and across varying heart rates.The average QTc interval values for men and women differ.8-10 In terms of risk, the upper limits of normal for men and women are 470 ms and 480 ms, respectively11; however, a QTc value greater than 500 ms is considered alarmingly high and substantially increases the risk for the occurrence of adverse events, especially torsade de pointes, in both sexes.11Nurses working in monitored acute and critical care units are required by the practice

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