Waves and Intervals on the ECG

Atrial and ventricular depolarization and repolarization are represented on the ECG as a series of waves: the P wave followed by the QRS complex and the T wave.

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The P Wave

The first deflection is the P wave associated with right and left atrial depolarization. Wave of atrial repolarization is invisible because of low amplitude.

Normal P wave is no more than 2.5 mm (two-and-a half1-mm-divisions) tall and less than 120 ms (three 1-mm-divisions) in width in any lead.

In sinus rhythm when the SA node is the pacemaker, the mean direction of atrial depolarization (the P wave axis) points downward and to the left, in the general direction of lead II within a coordinate between 15o and 75o and away from lead aVR. On this count the P wave is always positive in lead II and always negative in lead aVR during sinus rhythm. Conversely, a P wave that is positive in lead II and negative in lead aVR indicates normal P wave axis and sinus rhythm.

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The QRS Complex

The second wave is the QRS complex. Typically this complex has a series of 3 deflections that reflect the current associated with right and left ventricular depolarization. By convention the first deflection in the complex, if it is negative, is called a Q wave. The first positive deflection in the complex is called an R wave. A negative deflection after an R wave is called an S wave. A second positive deflection after the S wave, if there is one, is called the R wave. Some QRS complexes do not have all three deflections. But irrespective of the number of waves present, they are all QRS complexes:

A QRS complex with QRS deflections:
A QRS complex with QR deflections:
A QRS complex with RS deflections:
A QRS complex with only an R wave:
A QRS complex with RSR deflections:
A QRS complex with a QS wave:

(NB: The first wave of the last complex is a negative deflection. Therefore, it qualifies to be called a Q wave. Since all QRS complexes have an R wave, there must be one in this example as well, although it may be so small that it is not visible. A negative deflection following an R wave is an S wave. Hence this single negative deflection deserves to be called a QS wave.)

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QRS duration is the width of that complex from beginning to end, irrespective of the number of deflections present. Normally it lasts no more than 120 ms (three 1-mm-divisions).

The normal QRS axis, like the P wave axis, points downward and to the left within a coordinate between -30o and +90o. This axis is said to be deviated to the left (left axis deviation or LAD) if it lies between -30o and -90o; and deviated to the right (right axis deviation or RAD) if it lies between +90o and 180o. It is either far right or far left axis deviation if it lies between 180o and -90o. The method of determining QRS axis will be explained in a later section.

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The ST Segment

Following the QRS complex is the ST segment, extending from where the QRS ends (irrespective of what the last wave in the complex is) to where the T wave begins. The junction between the end of the QRS and the beginning of the ST segment is called the J point.
ST segment reflects the current flow associated with phase 2 of ventricular repolarization. Since there is no current flow during this plateau phase of repolarization, the ST segment is normally isoelectric with the baseline.

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The T Wave

The T wave represents the current of rapid phase 3 ventricular repolarization (see diagram above). The polarity of this wave normally follows that of the main QRS deflection in any lead. The ventricles are electrically unstable during that period of repolarization extending from the peak of the T wave to its initial downslope. A stimulus (e.g. a run away heart beat called a premature beat) falling on this vulnerable period has the potential to precipitate ventricular fibrillation: the so call R-on-T phenomenon.

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The PR Interval

The PR interval extends from the beginning of the P wave to the beginning of the QRS, whatever the first wave of this complex may be. This interval measures the time from the initial depolarization of the atria to the initial depolarization of the ventricles and reflects a physiological delay in AV conduction imposed by the AV node. Normal range is 120 V 200 ms (3 to 5 1-mm-divisions) and no longer.

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The QT Interval

The QT interval is measured from the beginning of the QRS to the end of the T wave. It represents the time in which the ventricles depolarize and repolarize and is a measure of ventricular action potential (AP) duration.

This interval should be determined in the ECG lead where it is longest. Normal intervals are < 460 ms for women and < 450 ms for men. But QT values are heart-rate dependent and can vary from 270 ms at a heart rate of 150 beats/min to 500 ms at a heart rate of 40 beats/min. Corrected QT interval (QTc), obtained by dividing the measured QT interval by the square root of the RR interval, can be used in place of raw QT interval. Normal QTc is 440 ms or less.

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