The ECG waves

Relationship between ECG and electrophysiology.

Now we'll learn about the relationship between ECG waves and cardiac depolarization.

What is the first wave of the ECG?

The P wave

What is a cardiac pacemaker?

It's the area that first depolarizes in the heart, initiating the wave of depolarization.

Where is the cardiac pacemaker usually located?

In the upper posterior region of the right atrium

(after the union between the superior vena cava

and the right atrium)

It's the place called the Sinus Node. Observe:

Start of atrial depolarization

Note that: 

The pacemaker is the first part to depolarize, and may be in the ventricles, the atrioventricular node and even the left atrium!


The pacemaker is usually located in the right atrium (RA).

The RA and later the left atrium will be the first to contract, helping blood to reach the ventricles.

The left atrium contracts just after the RA starts to depolarize, and the two contract together for a while.


What is the P wave?

It's the graphic representation of the depolarization of the atria!

How can we identify the P wave?

It's the first wave of the ECG, with a

rounded shape and a small notch.

Why is there a notch in the P wave?

This is due to depolarization happening first in the right atrium (RA) and then in the left atrium (LA).

Atrioventricular (AV) nodule

What is the AV node?

It's a set of cardiac fibers that delays the passage of the depolarization stimulus from the atria to the ventricles.

Where is the AV node?

Posterior to the tricuspid valve (valve between the RA and the right ventricle), between the L atrium and the R atrium.


Atrioventricular (AV) nodule

OK, but what does the AV node do?

If the stimulus passed immediately to the ventricles, there would be no time for the blood pumped through the atria to reach the ventricles.

The AV node solves this problem by promoting a delay in ventricular contraction, allowing blood to go to the ventricles without them being contracted.

This pause is recorded on the ECG by a line after the P wave (PR segment)

PR interval x PR segment

What's the difference between PR interval and PR segment?

The PR interval accounts for the P wave time; it's the largest and, as we'll see, the most useful.


To memorize it, we can make the analogy that a line segment cannot be curved, that is, the PR segment has no curve (it does not include the P wave).

QRS complex: ventricular depolarization

QRS complex: complex only in name

What is the QRS?

It's the graphical representation of ventricular depolarization (opening of the sodium channels)

What are the QRS waves?

The first, if negative (downward), is the Q wave.

The positive wave (upward) is the R wave.

The negative wave after the R is the... (not too hard to guess) S wave.

ST segment: ventricular systole

What does the ST segment mean?

After sodium enters the cells of the ventricles (depolarization represented by the QRS), there is the entry of calcium promoting ventricular contraction. During the ST segment, the ventricle will be contracting!

ST segment: ventricular systole

Note that the systole corresponds to the entry of Ca+ ions into the myocardial cell.

The cells will now be in phase 2 of depolarization.

T wave: beginning of ventricular diastole

The animation seems wrong, but it's not!

1 - cardiac depolarization = QRS complex

2 - cardiac contraction = ST segment

The T wave appears only after:

3 - Repolarization = T wave

then yes, when we observe repolarization in the drawing

(disappearance of the yellow color), the T wave appears.

T wave: opening of the potassium channels

During the T wave, cardiac muscle cells will open the potassium channels.

Potassium, which has a much higher presence inside the cell, will come out causing the cell to become more negative (repolarized).

U wave: repolarization of the Purkinje fibers?

What is the U wave?

It's the wave that occurs after the T wave, with 5 to 25% of the T wave's voltage.

It's most noticeable in bradycardic individuals (low heart rate).

What does the U wave mean?

It has a dubious origin! (nobody knows)

Figure for interactive activities:


you managed to relate electrophysiology to the name of the waves and the electrocardiographic tracing.