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P wave
- represents the electrical impulse starting in the SA node and spreading through the atria.
- the P wave represents atrial depolarization.
- It is normally 2.5 mm or less in height and 0.11 seconds or less in duration.
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QRS complex
- represents ventricular depolarization.
- Not all QRS complexes have all three waveforms!
- normally less than 0.12 seconds in duration.
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T wave
- represents ventricular repolarization (when the cells regain a negative charge; also called the resting state).
- Atrial repolarization also occurs but is not visible on the ECG because it occurs at the same time as ventricular depolarization (i.e., the QRS).
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U wave
- thought to represent repolarization of the Purkinje fibers; although this wave is rare
- it sometimes appears in patients with hypokalemia (low potassium levels), hypertension, or heart disease.
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PR interval
- measured from the beginning of the P wave to the beginning of the QRS complex
- represents the time needed for sinus node stimulation, atrial depolarization, and conduction through the AV node before ventricular depolarization.
- the PR interval normally ranges from 0.12 to 0.20 seconds in duration In adults
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ST segment
- represents early ventricular repolarization, lasts from the end of the QRS complex to the beginning of the T wave.
- beginning of the ST segment is usually identified by a change in the thickness or angle of the terminal portion of the QRS complex
- End is difficult to ID as it merges with T wave
- analyzed to identify whether it is above or below the isoelectric line, which may be, among other signs and symptoms, a sign of cardiac ischemia
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QT interval
- represents the total time for ventricular depolarization and repolarization
- measured from the beginning of the QRS complex to the end of the T wave
- If the QT interval becomes prolonged, the patient may be at risk for a lethal ventricular dysrhythmia called torsades de pointes.
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TP interval
- measured from the end of the T wave to the beginning of the next P wave—an isoelectric period
- When no electrical activity is detected, the line on the graph remains flat; this is called the isoelectric line.
- The ST segment is compared with the TP interval to detect ST segment changes.
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isoelectric
having or involving no net electric charge or difference in electrical potential.
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PP interval
- measured from the beginning of one P wave to the beginning of the next P wave.
- used to determine atrial rate and rhythm.
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RR interval
- measured from one QRS complex to the next QRS complex.
- used to determine ventricular rate and rhythm
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