PQ, QRS, QT and RR=CL

• what the basic intervals mean
• how to perform accurate measurements
• that EPs in Hungary are true Jedi Knights :)

PR interval

The PR (or PQ) interval indicates the time from the earliest atrial activation of the sinus node until the beginning of the ventricular depolarization. It is measured between the earliest atrial recorded activity (P wave or high right atrial catheter)

Normal value is between 120 and 200 ms. Shortening of the PR might be present in case of a ventricular preexcitation, while lengthening of the PR indicates AV-node disease.

QRS and QT intervals

The QRS indicates the duration of the ventricular depolarization, while the QT interval represents the whole ventricular depolarization and repolarization:

1. QRS is measured from the first deflection of the QRS until the last deflection of it
2. QT interval starts from the beginning of the QRS until the end of the T-wave. If a U wave is present, it should not be counted in the QT interval. As the QT varies greatly with the heart rate, it is usually expressed as the corrected QT interval (QTc).

Normal value of the QRS: ≤120 ms. A wide QRS complex is the consequence of aberrant conduction (bundle branch block, intraventricular block, or ventricular preexcitation).

The upper limit of QTc is 440 ms in men and 460 ms in women.

Short QTc (very rare): QTc less than 320-350 ms.

Cycle length (CL)

The CL is the time interval between two depolarizations. It is equal to the P-P (or R-R) interval in sinus rhythm. In case of an arrhythmia, the cycle length in the chamber of interest should be measured.