The Electrocardiogram (Ecg)

Front 1

Describe the sequence of depolarizations underlying the ECG

Back 1

The ECG measures changes in electrical potential energy on the body surface that result from the movement of a wave of depolarization and depolarization across the heart.

1) P wave: atrial depolarization moving out of the SA node from right to left, top to bottom. P wave ends when both atria are depolarized.
- Time in between the end of the P wave and the beginning of the QRS complex represents the time it takes for the impulse to travel through the AV node and be transmitted to the ventricles through the bundle of his and the purkinje fibers.

2) PR interval: period from beginning of P wave to the beginning of the QRS complex which encompasses both atrial depolarization and impulse transduction through the AV node.

3) QRS complex: ventricular depolarization. First depolarization of the ventricles usually occurs in the left ventricle and moves from left to right. This is responsible for the brief downward deflection known as the Q wave, although it is not seen in all ECGs.

4) R wave: as depolarization moves from the purkinje fibers into the ventricular muscles both he right and left sides of the heart are depolarizing simultaneously. However, since the mass of the left ventricular muscle is so much greater than that of the right ventricle the net dipole is towards the left and the deflection is upward.

5) S wave: last portion of the ventricle to depolarize is usually the basal part of the left ventricle. On some leads this may appear as a downward deflection if the net dipole vector points upward or to the right. May only be seen in some leads. During this QRS period the atria are repolarizing, but this is not seen on the ECG as the depolarization is generally hidden by the much larger dipole of ventricular depolarization.

5) ST segment: when boy ventricles are completely depolarized there is no net dipole.

6) T wave: the ventricles repolarize from left to right causing an upward deflection in the ECG wave