T Wave, ST Segment, and QT Time

In each ECG, the T wave and the ST segment must be looked at closely in all leads. The ST segment should always be considered in combination with the T wave.

The J point marks the transition of the S wave to the ST segment. If the S wave is missing, then the J point marks the transition of the descending R wave to the ST segment.

The ST segment reflects the time segment after the excitation propagation. For the duration of the ST-segment, both ventricles are fully excited. The ST segment should be isoelectric under physiological conditions. It lasts from the J point to the start of the T wave.

Assessment of the ST segment is of crucial importance for ECG interpretation. The following chapter will teach you how to assess repolarization. Starting today, you should look closely at the ST segment in every lead of every single ECG you see!

The feared ST elevation myocardial infarction got its name from the ST segment. It is very important to detect a clinically-relevant ST segment elevation, and to avoid the reporting of false-positive findings.

If the ST segment is not isoelectric, this means that ST segment depression or ST segment elevation is present. ST segment elevation can emerge from the descending R, which is a highly pathological finding and suggestive of an acute myocardial infarction! Alternatively, ST segment elevation can emerge from the ascending S, which may be harmless. However, the finding must be interpreted within the overall clinical context, that is with respect to the patient’s symptoms and medical history. You will learn more about this in the chapter on coronary heart disease and myocardial infarction. The course of the ST segment must also be assessed, that is, you must decide whether it is horizontal, ascending, or descending.

In practice, thorough training is key through observing a large number of real STEMI patients. We have therefore built ECG trainer modules, where you can look at endless ECGs with and without clinically-relevant ST segment elevation. As in the other chapters, you find the links to our ECG trainers below this video. Don’t worry: After completing the entire ECG course, you will never feel nervous about interpreting the ST segment ever again.

It is important to understand that when depolarization is abnormal,repolarization will be abnormal as well. This explains why repolarization may look abnormal in patients with bundle branch block.

In left bundle branch block, repolarization is altered across the entire 12-lead ECG. In right bundle branch block, depolarization and repolarization are altered in the right precordial leads, usually v1, v2, and sometimes in v3 also.

The T wave depicts the repolarization of the ventricular myocardium.

As said before, in each ECG, the T wave must be looked at closely in all leads.

In general, T waves can be particularly high, particularly flat, positive, negative, and biphasic, meaning both positive and negative. The normal T wave has a rather slow rise and a faster decay, and its peak is rounded.

T wave inversion is suspicious and must be interpreted correctly. Here, it is important to distinguish a terminal T wave inversion from a preterminal T wave inversion.

A terminal T wave is an important sign of acute coronary ischemia. In terminal T wave inversion, the bisector is perpendicular to the isoelectric line, or pointed away from the QRS complex. We will look at this in more detail in the chapter on coronary heart disease and myocardial infarction.

In preterminal T-wave inversion, the bisector line is pointed through the tip of the T-wave towards the QRS-complex.

Physiologically, T wave inversion can occur in adults in v1; in young adults in v2.

Oftentimes, you will find T wave inversion, if the main vector of the ventricular excitation is negative. The T wave is then oriented in the same direction as the QRS complex. This is termed T wave concordance.

Classical pathological findings are peaked T waves, which are an early sign of ischemia in patients with a myocardial infarction, and tented T waves, which are caused by hyperkalemia. You will learn more about pathological T waves in the following chapters.

The QT interval depicts the systole, which is the period of ventricular is contraction. This is measured from the beginning of the Q wave to the end of the T wave. The duration of systole, and thus the width of the QT segment, decreases in tachycardia and increases in bradycardia. For this reason, we should not consider the absolute time of the QT segment, but rather evaluate the time after correction for heart rate. Various formulas are available for this purpose. The most common one is Bazett’s formula. For this formula, the QT interval is evaluated in lead 2, or in leads v4 or v5. The Bazett formula is only valid for heart rates of between 60 and 100, and therefore should not be used in patients with bradycardia or tachycardia.

In bundle branch block, the QRS complex widens, and the QT interval is extended. To correct this false extension of the QT interval, we first calculate the difference between the duration of the normal QRS complex --- which has a maximal duration of 120 milliseconds --- and the duration of the widened QRS complex. We then subtract this value from the QT interval.

This sounds more complicated than it is. Let’s consider lead 2. The QRS complex is significantly broadened, with a width of 160 ms. The corrected QT interval is 460 ms. We take the absolute time of the QRS, which is 160 ms, and subtract the normal value of 120 ms, which gives us 40 ms. So, 460 minus 40 is 420 milliseconds, which is normal.

Most modern ECG machines calculate the corrected QT interval, often abbreviated as QTc, and show it in the printout. The upper normal value for the corrected QT interval is between 440 and 460 ms, depending on the formula that is used. We call this “long QT” when prolonged, and “short QT” when shortened.

There are various pathomechanisms for both scenarios. However, for now, only the identification of an extension or a shortening of the QT interval is important.

The U wave is a positive or negative wave following the T wave. Positive U waves occur, for example, in hypokalemia.

Cardiologists are still not sure whether the U wave really has any pathological significance in the absence of hypokalemia.