ECG Simplified – Part 2

ECG Simplified – Part 2

ECG uses 4 limb electrodes on each of the four limbs, of which the electrode on the right leg is considered electrically neutral while the other three are active electrodes. In addition to this 6 electrodes are placed on specifically designated parts of the chest to get chest lead recordings.
Though there are only 10 electrodes which are used for recording a usual ECG, various electrode combinations can be recorded with these electrodes so that most common recording is a 12 lead ECG. Number of leads may be increased using additional electrodes in specific situations.
Waves noted in a normal ECG are called P wave, QRS complex, T wave and sometimes a U wave. Other waves which can be rarely seen are delta waves and epsilon waves. P wave represents the depolarization of the atria while QRS complex represents the depolarisation of the ventricles. T wave is due to the repolarisation of the ventricles.
PR segment is between the P wave and the QRS complex while the ST segment is between the QRS complex and the T wave. TP segment is between the T wave and the next P wave. TP segment is considered as the true baseline in the ECG. Intervals contain one or more waves and a segment. PR interval contains the P wave and the PR segment and is measured from the onset of the P wave to the onset of the QRS complex.
QT interval contains the QRS complex, ST segment and the T wave. It is measured from the onset of the QRS complex to the end of T wave. PP interval is measured from the onset of one P wave to the onset of next P wave. RR interval is measured from the onset of one QRS complex to the onset of next QRS complex or as the interval between the peaks of two consecutive R waves for simplicity.
Delta waves occur at the onset of QRS complex and Epsilon waves are seen at the end of the QRS complex, in certain pathological conditions. A prominent U wave after the T wave can be seen when the blood potassium is low.
An initial negative deflection which is part of the QRS complex is called Q wave. An initial positive deflection is named R wave. A second negative deflection or a negative deflection following an R wave is called S wave.
A second positive deflection will be termed R’ wave. If there is a negative deflection after the R’ it is called S’. Waves less than 5 mm amplitude may be designated by small letters.