ST Segments, T Waves, QT Intervals, and U Waves

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11Fast & Easy ECGs, 2EFast & Easy ECGs, 2E

ST Segments, T Waves, QTIntervals, and U WavesFast & Easy ECGs, 2nd E – A Self-

Paced Learning Program

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Other Key Steps

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• To thoroughly assess the ECG, examine the ST segments, T waves, QT intervals and U waves



ST Segment

• Isoelectric line that follows the QRS complex and connects it to the T wave

• Represents the end of ventricular depolarization and beginning of ventricular repolarization

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Measuring ST Segments

• PR segment is the baseline from which to evaluate the degree of displacement of the ST segment from the isoelectric line

• Draw a straight line (or use a paper) extending from the PR segment out past the J point

Intervals should be at same level horizontally




• Alternatively, the TP interval can be used – space between

the T wave and the P wave of the beat that follows it

Intervals should be at same level horizontally




• For proper measurement, we use a point 0.04 seconds (one small box) after the J point to identify the presence of elevation or depression

• An elevated ST segment is one that is 1 mm or more above the baseline

• An depressed ST segment is one that is 1 mm or more below the baseline



Abnormal ST Segments

• Abnormal ST segments are indicative of abnormal ventricular repolarization– Two types are ST segment elevation and ST segment depression– ST segments may also be absent or indistinguishable



Abnormal ST Segments

• An elevated ST segment is one that is 1 mm or more above the baseline

• A depressed ST segment is one that is 1 mm or more below the baseline



ST Elevation and Depression

• Elevation or depression of the ST segment is a hallmark feature of myocardial ischemia and injury

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T Waves

• Represent completion of ventricular repolarization (recovery)

• Is a wide, broad waveform

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Measuring T Waves

• To measure the T wave, identify its starting and ending point, then move to the center of the waveform

• Next, count the number of small boxes between the isoelectric line and the top of the waveform

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Abnormal T Waves

• Include those that are tall, peaked or tented, inverted (in leads where they should be upright), biphasic, flat, wide, and heavily notched and those having bumps



Abnormal T Waves

• Indicate presence of abnormal ventricular repolarization



QT Intervals• Extends from

beginning of the QRS complex to the end of the T wave

• Duration varies according to age and sex, but it is usually between 0.36 and 0.44 seconds



Measuring QT Intervals

• First locate the isoelectric line immediately preceding the Q wave (or R wave if there is no Q wave)

• Next, locate the end of the T wave

• Count the number of boxes in between these two points



Abnormal QT Intervals

• An abnormal QT interval duration can indicate myocardial irregularity

• Two types of abnormal QT intervals are prolonged QT intervals and shortened QT intervals



U Waves

• Small wave that follows the T wave.

• Not always seen • Origin is thought to

represent repolarization of the papillary muscles or Purkinje fibers



U Waves

• Most often seen in hypokalemia • May also be present in hypercalcemia, thyrotoxicosis,

digitalis toxicity, epinephrine and Class 1A and 3 antidysrhythmics use, mitral valve prolapse, left ventricular hypertrophy, congenital long QT syndrome and intracranial hemorrhage



Practice Makes Perfect

• In this tracing, determine the type of ST segments, T waves, QT intervals and if U waves are present

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Summary

• Assessing the ST segments, T waves, QT intervals, and U waves is a part of analyzing an ECG tracing

• The ST segment is the isoelectric line that follows the QRS complex and connects it to the T wave. It represents the end of ventricular depolarization and the beginning of ventricular repolarization



Summary

• ST segment depression or elevation can be evaluated by comparing the ST segment with the PR segment. It is considered elevated if it is above the baseline and depressed if it is below it

• Elevation or depression of the ST segment is a hallmark feature of myocardial ischemia and injury



Summary• The T wave represents the completion of

ventricular repolarization• The T wave is larger than the P wave and slightly

asymmetrical. It is oriented in the same direction as the preceding QRS complex

• Abnormal T waves indicate the presence of abnormal ventricular repolarization

• Tall or peaked and inverted T waves (in certain leads) may be seen in myocardial ischemia



Summary• The QT interval represents the time needed for

ventricular depolarization and repolarization• It extends from the beginning of the QRS

complex to the end of the T wave• Its duration varies according to age, sex, and

heart rate, but it is usually between 0.36 and 0.44 seconds

• In certain conditions, such as myocardial ischemia or infarction, a prolonged QT interval can predispose the patient to life-threatening ventricular dysrhythmias



Summary

• When present, the U wave is a small, rounded wave that follows the T wave. It has the same polarity as the T wave.

• Prominent U waves are most often seen in hypokalemia

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ST Segments, T Waves, QT Intervals, and U Waves