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The QRS Complex
The QRS complex is one of the components of a typical  electrocardiogram (ECG) tracing of a complete cardiac cycle, normally occuring  after the P wave (atrial depolarization) and before the T wave (ventricular  repolarization). It is a graphical representation of the cardiac impulse  traveling into the ventricular myocardium. To further understand its  morphological characteristics and clinical relevance, a brief discussion of the  QRS complex is provided below.
About  120 to 200 milliseconds (ms) after the generation of an action potential in the  SA node, the cardiac impulse reaches the ventricles through the  atrioventricular (AV) nodal/His bundle system. The cardiac impulse is conducted  to the ventricular mass through the bundle branches and the Purkinje fibers.
The QRS complex represents ventricular depolarization. Normally, the QRS complex is seen as a  “sharp” complex, resulting from the rapid depolarization of the ventricles via  the fast conducting Purkinje fibers of the bundle branches. Compared to the  intra-atrial conduction speed, the conduction speed of the bundle branches is  high, averaging about 4 meters per second.
The first deflection of  the QRS complex is called a Q wave when it is negatively deflected  (pointing downward) and an R wave when it is positively deflected  (pointing upward). A positive deflection following a Q wave is called an R  wave; and a negative deflection following an R wave is designated as an S  wave. A second positive deflection after the R wave is called an R' (R  prime).
A QRS complex may not  always contain a Q, an R or an S. Many combinations of these deflections, as  well as isolated deflections are generally referred to as QRS complexes. It is  important to name the various QRS waves correctly. Upper case and lower case  letters are used to describe not only the type of waves or deflections present  but also the general orientation of the QRS complex.
Examples:
Qr: Large Q, small R, the  QRS complex is mainly negatively deflected.
qR: Small Q, large R, QRS  complex is mainly positively deflected.
Rs: Large R, small S, the  QRS complex is mainly positively deflected.
QrS: Large Q, small R,  large S, the QRS complex is mainly negatively deflected.
qRS: Small Q, large R,  large S, the QRS is as negative as positive.
Duration of the QRS  complex:
The duration of the QRS  complex should not exceed 100 ms. A prolonged duration of the QRS complex  signifies a conduction delay in the ventricles. Causes of intraventricular  conduction delay include: 
Bundle branch blocks
Electrolyte disturbances
Ischemia
Being an important part of the ECG curve, the QRS complex is routinely analyzed when investigating cardiac pathologies. However, all the  components of the ECG must be considered and correlated clinically to ensure an  accurate diagnosis. Healthcare providers of different backgrounds and levels of  expertise will benefit from the i