Etiology
The His/Purkinje specialized muscle cells coordinate the depolarization of both ventricles, and if they are working efficiently the QRS complex is 80 to 120 ms in duration (represented by three small squares or less at the standard paper speed of 25mm/s). Any abnormality of conduction takes longer and causes "widened" QRS complexes. In bundle branch block there can be an abnormal second upward deflection within the QRS complex, and in this case the second upward deflection is referred to as R' (pronounced "R prime"). This would be described as an RSR' pattern.
Ventricles contain more muscle mass than the atria; therefore the QRS complex is considerably larger than the P wave. The QRS complex is often used to determine the axis of the electrocardiogram (although it is also possible to determine a separate P wave axis).
The duration, amplitude, and morphology of the QRS complex are useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
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