The P WaveP waves are caused by atrial depolarization. In normal sinus rhythm, the SAnode acts as the pacemaker. The electrical impulse from the SA node spreadsover the right and left atria to cause atrial depolarization. The P wavecontour is usually smooth, entirely positive and of uniform size. The P waveduration is normally less than 0.12 sec and the amplitude is normally less than0.25 mV. A negative P-wave can indicate depolarization arising from the AVnode.Note that the P wavecorresponds to electrical impulses not mechanical atria contraction. Atrialcontraction begins at about the middle of the P wave and continues during thePR segment. The PR Segment PR segment is the portion on the ECG wave from theend of the P wave to the beginning of the QRS complex, lasting about 0.1seconds. The PR segment corresponds to the time between the end of atrialdepolarization to the onset of ventricular depolarization. The PR segment is anisoelectric segment, that is, no wave or deflection is recorded. During the PRsegment, the impulse travels from the AV node through the conducting tissue(bundle branches, and Purkinje fibers) towards the ventricles. Most of thedelay in the PR segment occurs in the AV node. Although the PR segment isisoelectric, the atrial are actually contracting, filling the ventricles beforeventricular systole.The QRSComplexIn normal sinus rhythm, each P wave isfollowed by a QRS complex. The QRS complex represents the time it takes fordepolarization of the ventricles. The Q wave is not always present. The R waveis the point when half of the ventricular myocardium has been depolarized. The normalQRS duration range is from 0.04 sec to 0.12 sec measured from the initialdeflection of the QRS from the isoelectric line to the end of the QRS complex.Normal ventriculardepolarization requires normal function of the right and left bundle branches. Ablock in either the right or left bundle branch delays depolarization of theventricles, resulting in a prolonged QRS duration.The STSegmentThe ST segment represents the period fromthe end of ventricular depolarization to the beginning of ventricularrepolarization. The ST segment lies between the end of the QRS complex and theinitial deflection of the T-wave and is normally isoelectric. Although the STsegment is isoelectric, the ventricules are actually contracting.The T WaveThe T wave corresponds to the rapid ventricular repolarization. The wave isnormally rounded and positive.