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Here is a recording of a normal beat, with important
elements labeled. The first element is the P-wave,
which is a recording of the electrical activity of the atria. The
larger deflection, showing larger voltages caused by the electrical activity
of larger muscle masses, is called the QRS complex, and
is a recording of ventricular depolarization.
Then follows the T-wave, a recording of ventricular repolarization.
Here is the same beat with other important segments labeled.
The PR interval is
the time elapsed during atrial depolarization and A-V nodal delay.
Here it lasts about three and three-quarter small boxes, or 3.75 x 0.04 = 0.15 seconds.
The QRS duration is the time during which the ventricles depolarize.
Here it lasts about one and one-half small boxes, or 1.5 x 0.04 = 0.06 seconds.
The QT interval is the time during which the
ventricles depolarize and repolarize. Here that interval
is two small boxes, then a big box, then three more small boxes, or 2 x
0.04 plus 0.2 plus 3 x 0.04 = 0.40 seconds. The ST segment is
not clearly demarcated, but is very important for evaluating certain disease