Intervals

PR, QRS, QT and QTc, and whether each is normal for the species.

Understand the measurements

Measure the PR interval

The PR interval is the time the impulse spends crossing the atria and being held at the AV node. Most of it is the AV node delay. A long PR means the node is slow to let the impulse through.

Measure the QRS duration

A wide QRS means the ventricles were activated slowly, either because a bundle branch is blocked or because the beat started in ventricular muscle instead of arriving through the Purkinje system.

Measure the QT interval

The QT is the whole ventricular cycle, depolarisation plus repolarisation. It shortens as the heart speeds up, which is why it is corrected for rate.

Calculate the QTc

Bazett over-corrects at fast rates and under-corrects at slow ones, but it is what every reference range assumes.