In a normal ecg tracing the qrs complex is caused by:
a. ventricular depolarization
b. atrial depolarization
c. ventricular repolarization
d. atrial repolarization
e. none of the above

The answer is A. ventricular depolarization.  

Explanation:

In a normal electrocardiographic plot, the QRS complex is the graphic manifestation of depolarization of the ventricles. It usually has a weevil form in the ECG. This wave appears after the P wave (contraction of the atria) and because the ventricles have greater muscle mass, this QRS complex is larger. In addition to having a very specialized conduction system (His / Purkinje system), it coordinates the depolarization of the ventricles at a very high conduction velocity and, as a consequence, the waves of the QRS complex tend to be very narrow and peak-shaped, in instead of rounded.

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