ECG (u6 bio notes)
The electrocardiogram, or ECG, is a recording of the heart’s electrical activity via leads that are placed at
standardized points on the subject’s body. These animations show how the different ECG waves are
produced during a cardiac cycle. The P wave is produced as the electrical impulse travels through the
atria, causing them to contract. The P wave corresponds to Arial Depolarization. The impulse then
travels through the Atrioventricular Node, producing a flat section on the ECG, known as the PR
segment. The PR segment is flat because no current is flowing through the cardiac muscle cells. As the
impulse travels from the AV node to the Bundle of His and along the upper part of the septum, the Q
wave is produced. The passage of the electrical signal through the ventricles produces a large spike
called the QRS complex. The QRS complex is much larger than the P wave because the ventricles are
much larger than the atria and so produce more electrical activity. Most of the QRS complex represents
depolarization of the left ventricle, which has much greater mass than the right. The impulse remains
unchanged for a moment as the ventricle remains depolarized. This gives rise to another flat section,
called the ST segment. As the impulse fades away and the ventricles repolarize, the T wave is formed.
Note, this animation shows the ECG recorded from a lead placed at position V6, one of the standard
electrode positions to the left of the heart. Leads at other positions detect the electrical activity of the
heart from a different point of view. And therefore produces differently shaped ECGs. As you will see,
during atrial fibrillation, the electrical activity of the atria becomes disorganized. As a result, there is no P
wave on the ECG of a patient in atrial fibrillation. The absence of P waves is one of the most important
features used to diagnose atrial fibrillation.