What Does The T Wave Represent On An ECG?

What Does The T Wave Represent On An ECG?

T wave anomalies are incompletely understood and associated with a broad differential diagnosis.

A T wave abnormality can be associated with a life-threatening disease or be a clue to a mysterious illness. T wave abnormalities may also be benign and may be commonly seen in at least half the leads in a 12-lead ECG.

While the T wave is challenging to interpret, it remains a fundamental part of ECG interpretation.

What Is the T Wave in Electrocardiography?

The T wave on the ECG is the positive deflection after the QRS complex. It represents the repolarization of the ventricular myocardium.

To understand the T wave, let’s first briefly go over depolarization and repolarization. Both of these processes represent a shift in the electrical potential and ions of the cells. This is accomplished mainly by sodium, potassium, and calcium ions. At baseline, the outside of these cells are positive in relation to the inside of the cells. As depolarization begins the positive ions flood the cells making the inside positive. Repolarization is the resetting of the ions, restoring the inside to negative and the outside to positive. Both depolarization and repolarization occur from the epicardium to the endocardium (outside of the heart to the inside of the heart)

t wave on an ECG

 

T wave on EKG

 

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On the ECG, depolarization is represented by the QRS complexes, and the T waves represent repolarization. Depolarization is when the ventricles contract and pump blood, and repolarization is when the ventricles relax and fill with blood.

Let’s look at this process in picture form and then show what the ECG machine sees. Here is a piece of cardiac tissue. The endocardium is to the left, and the epicardium is to the right. During the tissue’s resting state, the outside of the tissue is positive, while the inside is negative.

During depolarization, the inside of the tissue becomes positive from left to right. The ECG sees this impulse going toward the electrode and gives a positive or upright deflection. There is then a slight pause in the ion transfer process displayed on the ECG as a flat line (ST segment). This is followed by repolarization (resetting of the ions). Since this process also occurs from the endocardium to epicardium (left to right), the T waves will be in the same direction as the QRS complexes, that is, upright.

What Does a Normal T Wave Look Like?

Normal T waves are typically upright and generally follow the same direction as the QRS complexes. Hence T waves are upright (positive deflection) in leads 1, 2, aVL, aVF, and V2 through V6. The T waves are typically downward (negative deflection) in aVR. In leads 3, and V1, the T wave direction is variable.

Normal T waves are slightly asymmetrical in shape, with a rounded peak that occurs closer to its end than its beginning. The asymmetry is due to a steeper downslope than the upslope of the wave.

The amplitude of the T wave should not exceed 5 mm in limb leads and more than 10 mm in precordial leads, with the amplitude highest in V2-V3 leads. Amplitude diminishes with increasing age.

Negative T waves, also known as T wave inversions, from V1 to V4 leads are frequently found in children and are normal.

T wave inversion is less commonly found in healthy adults but can occur from V1 to V3. The depth of the T wave also becomes progressively shallow from one to the next lead.

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Understanding Abnormal T-Waves

ECG interpretation must be in the context of the individual patient. The T wave is enigmatic, and a holistic interpretation strategy is essential to discern normal from abnormal.

Inverted T Waves

T wave inversions can result from life-threatening events such as acute coronary ischemia, pulmonary embolism, and CNS injury. They can also be completely benign, as in persistent juvenile T-wave inversion and normal variant T-wave inversions.

Commonly T wave inversions are grouped into primary and secondary T wave changes.

For simplicity, a primary change is due to a primary problem in the repolarization of the cardiac tissue. Primary changes describe alterations in the duration or morphology of the spike without simultaneous changes in the orderly activation sequence.

Primary T-wave changes can be benign, such as persistent juvenile T-wave patterns, or serious such as from acute coronary ischemic events, abnormal electrolytes effects, or drug effects.

A secondary change means that the abnormality in repolarization is due to something else, such as an abnormality in depolarization. A good example is the ST and T wave changes that occur in a bundle branch block.

An inverted T wave is considered abnormal if the inversion is deeper than 1.0 mm. Inverted T waves found in leads other than the V1 to V4 leads are associated with increased cardiac deaths.

Inverted T waves are seen in the following conditions:

  • Normal findings in children
  • Persistent juvenile T wave pattern
  • Coronary artery disease
  • Myocardial ischemia and infarction (including Wellens Syndrome)
  • Bundle branch block ventricular paced (implanted pacemaker) patterns
  • Pulmonary embolism
  • Hypertrophic cardiomyopathy
  • Raised intracranial pressure

Clues to telling primary from secondary T wave changes

T-wave to QRS axis

Calculating the QRS and T wave axes will help you distinguish primary from secondary T wave changes. With primary T wave changes, the T wave axis is similar to the QRS axis. With secondary changes, the T wave axis is usually opposite to the QRS axis. Another way to look at this is that with primary T wave changes, the T waves follow the same direction as the QRS complexes, with the exception of the abnormal T waves. The abnormal T waves are isolated. With secondary T wave changes, all of the T waves are opposite in direction to the QRS complexes. For example, everywhere the QRS complexes are upright, the T waves are inverted, and everywhere that the QRS complexes are downward, the T waves are upright.

 T-wave morphology

Primary T wave changes typically involve the T wave alone, and the T waves appear symmetrical. Secondary T wave changes usually also involve the ST segments, and the T wave changes are asymmetrical.

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Biphasic T Waves

Biphasic, also known as diphasic T waves, have positive and negative deflections. In other words, two waves are present in opposite directions.

There are two leading causes of biphasic T waves, each with a different biphasic pattern.

  • Myocardial Ischemia – in MI, the T wave is initially positive, moving to terminal negativity.
  • Hypokalemia – the T wave is initially negative, moving to a terminal positivity.

Flattened T Waves

While Flattened T waves are non-specific, they may be seen in patients with myocardial ischemia. A flattened T wave can be completely flat or vary in height from -1.0 mm to + 1.0 mm.

Flattened T waves can be caused by the following:

  • Myocardial ischemia
  • Emphysema
  • Pericardial effusion
  • Hypothyroidism
  • Hypoadrenalism
  • Hypokalaemia
  • Hypocalcemia
  • Digitalis therapy

The typically small-size U wave becomes more prominent in hypokalemia and digitalis therapy. As hypokalemia worsens, the T wave becomes increasingly flattened while the U wave increases in prominence.

In digitalis toxicity, the QT interval shortened and sagged before the flattered T wave and prominent U wave.

Hyperacute T Waves

Hyperacute T waves have an amplitude higher than 10mm in the chest leads and above 5mm in the limb leads. They are broad-based with a peaked, symmetric morphology.

After QT protraction, hyperacute T waves are the earliest ECG sign of acute ischemia.

Hyperacute T waves are most evident in the anterior chest leads and are seen soon after coronary infarction. They tend to be short-lived.

Causes of the hyperacute T waves include acute myocardial infarction, hyperkalemia, and acute myopericarditis.

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Camel Hump T Waves

As the name suggests, camel hump T waves exhibit double peaks. Originally associated with hypothermia, these waves are now deemed non-specific.

Because they arise from several different events, they are challenging to interpret.

Dramatic ECG changes of camel hump T waves have been observed in multiple electrolyte imbalances. For example, elevated U waves at the end of the T wave occur in severe hypokalemia. P waves are embedded in the T wave in various types of heart block and tachycardia.

Importance of Interpreting the T Waves Correctly

T waves are associated with a wide range of diagnoses. 

Changes in the T wave may represent several pathologies, including cardiac disease, pulmonary disease, neurogenic causes, and multiple electrolyte imbalances. However, T wave changes may also be a normal variant.

Interpreting abnormalities in the T wave without clinical history, physical examination, and laboratory studies is a common mistake.

T wave abnormalities are non-specific in the absence of symptoms and clinical history. Therefore, a comprehensive understanding of the T wave is critical for accurate diagnosis and optimal patient care.

Learn To Master The Art Of ECG Interpretation

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