thaler chapter 1

Naming the Straight Lines

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Naming the Straight Lines

The different straight lines connecting the various waves have also been given names. Thus, we speak of the PR interval , the ST segment , the QT interval , and so on. What differentiates a segment from an interval? A segment is a straight line connecting two waves, whereas an interval encompasses at least one wave plus, in most instances, the connecting straight line.

Segment

Interval

Interval

The PR interval includes the P wave and the straight line connecting it to the QRS complex. It therefore measures the time from the start of atrial depolarization to the start of ventricular depolarization. The PR segment is the straight line running from the end of the P wave to the start of the QRS complex. It therefore measures the time from the end of atrial depolarization to the start of ventricular depolarization. You might be wondering: if the QRS complex has a Q wave, shouldn’t the PR segment be called the PQ segment, and the PR interval the PQ interval? Well, yes, but they’re not. These straight lines are always called the PR segment and PR interval no matter what the configuration of the QRS complex.

PR segment

PR interval

The ST segment is the straight line connecting the end of the QRS complex with the beginning of the T wave. It measures the time from the end of ventricular depolarization to the start of ventricular repolarization.