E.g. The PR segment vs. the PR interval.
Segment
A segment in an ECG is the region between two waves. PR segment starts at the end of the P wave and ends at the start of the QRS complex. The ST segment starts at the end of the QRS wave and ends at the start of the T wave. The TP segment is found between the end of the T wave and the beginning of the next P wave; It is the true isoelectric segment in the ECG. ** With segments, you talk about morphology: elevation or depression or progression of segments.
Intervals
An interval in an ECG is a duration of time that includes one segment and one or more waves. The PR (or PQ) interval starts at the start of the P wave and ends at the start of the QRS. It denotes the conduction of the impulse from the upper part of the atrium to the ventricle. The QRS interval covers the QRS complex from beginning to end. [The QRS complex also covers an interval]. The QT interval starts at the start of the QRS and ends at the end of the T wave. It denotes the electrical systole of the heart.**Intervals are only described based on their duration of time. You speak of it as a duration and so cannot talk about the morphology or depression or elevation of an interval.
- P wave = Atrial depolarization. The positive wave of depolarization spreads from the SA node and is conducted throughout the cells of the atria through gap junctions in that connect these cells.
- PR segment = depolarization of the AV node. I.e. When current is passing through the AV node. It’s a flat line because the wave is not strong enough to be recorded on the voltmeter.
- PR interval = Wave goes over the atrium and through the AV node and ends just before it activates the ventricles to depolarize.
- Q wave = Ventricular Septal Depolarization
- R wave = Resultant or major ventricular muscle depolarization. The resultant vector is directed downward and leftward.
- S Wave = Basal Ventricular depolarization, i.e. depolarization of the base of the ventricles. Note the apex of the heart is the L. pointed end. The base of the ventricles connects to the atria.
- ST segment = During the ST segment, all the ventricular myocardium is depolarized. All have positive charges. So there is nothing potential difference to be recorded by the voltmeter (ECG machine). So you have a flat line.
- T wave represents ventricular repolarization.
- QT interval = Important because it captures the beginning of ventricular depolarization through the plateau phase to the ventricular repolarization. It covers the entire ventricular activity. During this time, the action potential was generated and terminated in the ventricular tissue. The beginning of the QRS complex is the start of ventricular systole and that goes until the end of the T wave. Ventricular diastole starts when the T wave ends.
- U wave. Sometimes the electrical activity of the ventricular papillary muscle is out of phase with the rest of the ventricles and will record as a “U” wave that shows after the T wave.
In the ventricles, the last cells to depolarize are the first to repolarize. Why?