thaler chapter 1

The Normal 12-Lead EKG

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The QRS Interval A normal QRS interval, representing the duration of the QRS complex, is 0.06 to 0.1 seconds in duration.

QRS interval

The ST Segment The ST segment is usually horizontal or gently upsloping in all leads. It represents the time from the end of ventricular depolarization to the start of ventricular repolarization.

The T Wave The T wave represents ventricular repolarization . Unlike depolarization, which is largely passive, repolarization requires the expenditure of a great deal of cellular energy (remember the membrane pump). The T wave is therefore highly susceptible to all kinds of influences, both cardiac and noncardiac ( e.g ., hormonal, neurologic), and is therefore variable in its appearance.