ECG Level 1 Tutorial: Basic Electric Stuff
Tutorial: Basic Electric Stuff
This module will teach the basics of electrical physiology in the heart.
How to level up?
Develop your skills by completing our Practice Cases!
Tutorial: Basic Electric Stuff Drawing the ECG!
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Drawing the ECG!
In this lesson, we will describe how the vector generates all the waveforms on the ECG tracing. To understand this, you will also need to understand all previous lessons in this module. This lesson will put it all together for you.

In summary, remember that a depolarization charge moving toward an electrode will create a positive deflection. In the video that follows, you will see how the vectors create the ECG tracing. We have color coded the ECG to help explain each step of the way. Video has no sound.


Tissues that are very small, like the SA node and the AV node do not have a strong enough signal to generate a waveform on the ECG. Therefore, when the SA node fires, it does not show up on the ECG. We indicated this using the green color.
Sinus node has no deflection

In the following image, we can see how the vector for the atrial depolarization creates a downward moving electrical charge. The added black circle in the image below represents the positive ECG electrode and in this case, we are using lead II, which would be pointing from about the top left corner of the picture down toward the black circle. Because the atrial waveform is moving toward the positive electrode, the ECG tracing in yellow is an upward deflection.
Atrial depolarization creates upward deflection which is the P wave

In the following image, atrial depolarization is complete and the electrical activity is inside the AV node. The AV node is too small to cause a deflection on the ECG. This is represented in purple.
AV node depolarization creates no deflection

Similar to the AV node, the purkinje fibers also do not create any deflection on the ECG tracing (also purple).
Conduction down the bundle branches creates no deflection

This is a very important detail: the ventricular septum is depolarized from the left bundle branch (we drew little tiny branches on it to show that it releases the electrical signal here). Therefore, the septum depolarizes from left to right. Look closely at the vector arrow. It is pointing to the left of the image (toward the right ventricle) and therefore, is pointing AWAY from the positive ECG electrode. Since the electrical activity at this moment in time is AWAY from the ECG electrode, the deflection is downgoing. This is shown in the blue tracing.
Septal depolarization causes depolarization toward the RV, which is slightly away from lead II positive electrode, so a downward deflection occurs

Very quickly, the septal depolarization is complete and the remaining ventricle depolarizes from endocardium to epicardium, resulting in a large electrical signal moving toward the ECG electrode, generating a large upward deflection shown in pink.
Depolarization from endocardium to epicardium in the ventricle

When this wave of depolarization is completed, the ECG tracing falls back to baseline (orange).
Depolarization is completed so there is no movement of electrical activity and the deflection returns to baseline

The ventricles remain depolarized for a short duration. This results in no movement of electrical activity and so we have a flat segment on the ECG (green).
There is still no movement of electrical activity. This creates the ST segment

When the ventricles begin to repolarize (blue), the repolarization event is moving away from the ECG electrode. However, the deflection is UPWARDS! This is important. Repolarization is the electrical OPPOSITE to depolarization. Therefore, the deflection that repolarization produces is opposite. Thus, repolarization moving toward the electrode causes a downward deflection, but moving away, causes an upward deflection in blue.
REpolarization now occurs

Congratulations! This is the end of this module. There were no "practice" sessions in this module, but there will be in the next module as you practice your skills at waveform identification in both normal and abnormal ECGs. This is a difficult skill, so if you have never looked at ECGs before, it might be a little frustrating at first. This is normal.