- The QRS Complex
- In normal sinus rhythm, each P wave is followed by a QRS complex. The QRS complex represents the time it takes for depolarization of the ventricles. Activation of the anterioseptal region of the ventricular myocardium corresponds to the negative Q wave. The Q wave is not always present. Activation of the rest of the ventricular muscle from the endocardial surface corresponds to the rest of the QRS wave. The R wave is the point when half of the ventricular myocardium has been depolarized. Activation of the posteriobasal portion of the ventricles give the RS line. The normal QRS duration range is from 0.04 sec to 0.12 sec measured from the initial deflection of the QRS from the isoelectric line to the end of the QRS complex.
- Normal ventricular depolarization requires normal function of the right and left bundle branches. A block in either the right or left bundle branch delays depolarization of the ventricles, resulting in a prolonged QRS duration.
The QRS complex proceeds ventricular contraction. Ventricular contraction, indicated by a Wiggers diagram as an increase in the ventricular pressure, beginning at about half-way through the QRS complex and continues to the end of the T-wave. Pumping of blood does begins when ventricular pressure exceeds aortic pressure, causing the semi lunar valves to open. This is normally at the end of the QRS complex and start of ST segment.
The animation below shows the QRS complex, noting the heart's electrical activity and the QRS complex recorded on the EKG. Note that the animation had been slowed about 10 times normal heart rate to show the details.