T waves on electrocardiogram: depolarization abnormalities
When ischemia occurs in one part of the heart, the depolarization of that area decreases disproportionately to the depolarization of other regions. The result is a change in the T wave.
The T wave is positive in all bipolar limb leads, and the T wave is generated by repolarization of the apical region and the outer surface of the ventricles. Thus, the T wave becomes abnormal when repolarization is not taking place. Several factors can alter the depolarization process.
T wave: effect of depolarization conduction delay
As shown in the figure, the QRS complex is significantly prolonged. The reason for this prolongation is the conduction delay of the left ventricle, a consequence of the left bundle branch block. This conduction delay causes the left ventricle to depolarize about 0.08s after the right ventricle, causing the mean voltage vector of the QRS complex to shift to the left. As a result, the right ventricle begins to depolarize before the left ventricle, which makes at the time of the appearance of the T wave, the right ventricle is strongly electrically positive, and the left ventricle is electronegative. In other words, the mean axis of the T wave is skewed to the right, opposite the mean axis of the QRS complex in the same ECG. Thus, when conduction of the depolarizing impulse through the ventricles is delayed, the T wave is always antipolar to the QRS complex.
Abnormal T waves: short depolarization of parts of the ventricles
Figure: T wave image in mild anaemia.
If the ventricular base exhibits an abnormally brief depolarization, the action potential is short—the repolarization of the ventricles will not start from the apex in the same way as it normally does. Instead, the ventricular base will repolarize anteriorly, and the repolarization vector will travel from the apex toward the ventricular base, in contrast to the vector of the normal repolarization potential. As a result, the T waves in all three standard leads will be more negative than the usual positive. Thus, a brief depolarization of the ventricular background is sufficient to induce changes in the T wave (Figure).
Today mild ischemia is the most common cause of short depolarization of the myocardium because this condition increases the potential flow through the K+ channel. When ischemia occurs in one part of the heart, the depolarization of that area decreases disproportionately to the depolarization of other regions. The consequence is a change in the T wave. The anaemia can be the result of acute or chronic coronary occlusion, generally from coronary undernutrition.
One way to detect mild coronary ischemia is to exercise the patient and record an electrocardiogram to see if there are any changes in the T wave. T wave changes need not be obvious because any change in T waves in any lead, either an inversion, in this case, or a biphasic wave is usually a sufficient indication. Whether or not some portion of the ventricles is depolarized is disproportionate to rest, caused by mild or moderate coronary ischemia.
T wave: effect of digitalis
Figure: Double peaked T wave due to digitalis toxicity.
Digitalis is a drug that can be used in coronary ischemia to increase myocardial contractility. However, with overdosage of digitalis, the time to depolarize one part of the ventricle may increase disproportionately to that of the other. Consequently, no specific changes, such as T-wave inversion or biphasic T-wave inversion, can occur in one or more ECG leads. Biphasic T waves caused by digitalis overdose are shown in the figure. Therefore, T-wave changes in the duration of action are often early signs of digitalis toxicity.