The QRS Complex

The QRS complex represents ventricular depolarization via the AV node, the bundle of His and the bundle branches. The normal QRS complex should last at least 0.08 seconds (2 small boxes wide) but no more than 0.1 seconds (2 1/2 small boxes wide). If it is shorter than 0.08 seconds then ventricular systole is occurring too rapidly. If it is longer than 0.12 seconds (3 small boxes) this may indicate a bundle branch block. The R-wave may be "squared off" at the apex or may have two peaks (R and R') closely attached indicating a slower transmission of impulses through one of the bundle branches. In reality there are two QRS complexes, a right and left side, slightly out of phase but which appear as one.

In the case of premature ventricular contractions (PVC's) resulting from ectopic ventricular foci, the R-waves may appear like the R-R' of a bundle branch block often resulting in a series of very wide QRS complexes, sometimes with square-topped R-waves. If these occur rapidly (i.e. over 100 beats per minute) it is referred to as ventricular tachycardia. Should a PVC fall on a T-wave, serious dysrhythmias can occur, since the T-wave represents repolarization of the ventricle. If a PVC should occur when the ventricle is only partially repolarized, a new systolic contraction can be activated. This or several similar types of episodes can result in ventricular tachycardia which can then progress to ventricular fibrillation which is fatal if left untreated.

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