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Chapter 6: ECG analysis and measurement
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Heart Axis
The cardiac vector (Heart axis) is the average sum of the electrical forces inside the heart, or in other words,
the angle of the result of the vector of the heart's electrical activity.
The electric vector can be calculated for P, QRS and T waves.
Among cardiac angles, the QRS axis has the most clinical use and is easily calculated. Leads I, II and III or
aVF, aVL and aVR leads can be used to calculate the QRS axis. Each of these leads shows the electrical
activity of the heart in a specific direction. The normal cardiac vector is in the range of -30 to +90 degrees.
The following figure shows the angles corresponding to each lead.
QTc Parameter
The QT Interval is from the beginning of the QRS to the end of the T wave, which indicates the duration
of ventricular depolarization and repolarization.
Since QT is affected by heart rate, it needs to be corrected and normalized to the heart rate (normalizing
QT means eliminating its dependence on HR). For example, increasing HR reduces QT, and this
dependence needs to be removed to make a diagnostic comparison with the normal range.
In order to normalize the QT parameter, the following equations are used:
𝑄𝑇𝑐 =
𝑄𝑇
√𝑅𝑅
و
𝑅𝑅 =
60
𝐻𝑅
[𝑠𝑒𝑐]
Figure 6-2 Heart angles related to different leads