This review explains the ST segment, T wave, and U wave on the ECG/EKG.
Before diving right into these parts of the ECG waveform, we must go back and review the basic anatomy and physiology of the heart’s electrical conduction system.
Electrical Conduction System of the Heart:
It all starts in the:
SA Node (sinoatrial node): this node is located in the upper part the right atrium and is known as the pacemaker of the heart, causing the heart to beat at 60-100 bpm. When this node fires, it sends electrical impulses to the atria causing atrial depolarization of the cells in the right and left atrium (remember depolarization causes the atria to contract). Then the electrical signals go to the:
AV node (atrioventricular node): this node is found in the lower part of the right atrium just above the tricuspid valve and is known as the “gatekeeper”.
The AV node is known for causing a delay in electrical signaling so the atria can fully empty their blood into the ventricles. If there wasn’t a delay, the atria would not fully empty its blood into the ventricles and this would cause problems.
Then it’s time for the ventricles to be depolarized (hence contract). So the electrical signal goes down to the Bundle of His, then the bundle branches (right and left) and lastly the Purkinje fibers, which causes the ventricles to depolarize. Shortly after this process, repolarization occurs and this process repeats itself over and over again.
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What’s the ST Segment, T Wave, and U Wave on the ECG?
ST segment: this represents the completion of ventricular depolarization and beginning of ventricle repolarization. It starts at the end of the QRS and ends at the beginning of the t-wave. The segment should be flat, hence isoelectric (no depression or elevation of more than 1 mm).
T wave: this represents the beginning of ventricular repolarization which leads to ventricle relaxation. The ventricles are so big that when they relax it creates the t wave.
*U-wave: not always present but may indicate hypokalemia or another abnormality in your patient.
What should these parts look like?
ST segment: starts at the end of the s-wave and stops at the start of the t-wave. It should appear flat and should not be elevated or depressed (no more than 1 mm)
T wave: should come after the QRS complex and be round and in the upright position in most leads
You may be interested in more parts of the ECG Waveform.