The next step in the orderly approach for 12-lead ECG interpretation is determining the ECG complex intervals.
Intervals
There are important ECG intervals that give clues to underlying cardiac conditions. The key intervals include PR, QRS, and QT intervals .
PR
The PR interval is measured from the onset of the P wave to the onset of the QRS complex, representing conduction from the onset of atrial depolarization until the onset of ventricular depolarization. Normal is 0.12 to 0.20 seconds. In some cases if there is a Q wave then it is a really a PQ interval. Any lead with a clearly defined P wave and a clearly defined QRS complex may be used but a good default lead is lead II. In this illustration, the PR interval is 0.20 seconds and normal.
QRS
The QRS duration is measured from the beginning to the end of the QRS complex, and in this illustration it would be from the onset of the Q wave to the end of the S wave. The QRS represents conduction from the onset to the end of ventricular depolarization. Normal is equal to or less than 0.10 seconds. The QRS is best measured in leads that have a clearly defined onset and offset of the wave forms, and lead II is a reasonable default lead. In this example, the QRS is normal at 0.08 seconds.
QT
The QT interval is measured from the onset of the QRS to the end of the T wave. It represents the onset of ventricular depolarization to the end of ventricular repolarization, that is, systole. The normal QT varies markedly with heart rate, but normal is equal to or less than 0.44 seconds, for a heart rate of 60 beats per minute. The corrected QT is known as the QTc. While there are several formulas to calculate the QTc, a quick technique is to confirm that the QT interval is less than ½ of the R-R interval.
In this illustration, the QT is the red colored portion of the ECG complex that extends from the onset of the Q wave to the end of the T wave. In a lead where the ventricular depolarization begins with an R wave, the QT interval would be from the onset of the R wave to the end of the T wave, but would still be referred to as the Qt interval. In the illustration above, the QT = approximately 0.44 seconds.