ECG normal sinus rhythm

Definitions & Key takeaways

Normal sinus rhythm is when the heart's electrical activity is regular and originates in the heart's natural pacemaker, the sinoatrial (SA) node. The (SA) node is located in the right atrium and regularly fires electrical impulses. On an ECG, the normal sinus rhythm is characterized by a P wave corresponding to atrial depolarization wave, and it is followed by an isoelectric line where the depolarization wave goes through the atrioventricular (AV) node. After, there is the QRS complex that represents ventricular depolarization. After QRS follows an isoelectric pause called ST segment, precedes ventricular repolarization represented by T wave.

Chapters:

Watch video only

Introduction0:00–0:52

An electrocardiogram or ECG or electrocardiogram or EKG in German, is a tool that allows us to visualize the heart's electrical activity.
A 12 lead ECG uses multiple electrodes placed around the body which are combined in specific ways to create electrical views of the heart called leads.
There are 6 chest leads and 6 limb leads, and each one captures the heart's activity from a different angle. With each heartbeat, the ECG records the electrical signals, including depolarization, which is the activation of the heart muscle, and repolarization, which is the recovery phase.
Now, to read an ECG, there are a few key elements to keep in mind, and one of the most important ones is determining whether the tracing shows normal sinus rhythm.

Heartbeat0:52–1:21

But before we proceed with what a normal sinus rhythm is, let's look at a single heartbeat from the viewpoint of lead two.
First, here's a quick note about the ECG paper. The horizontal axis represents time, where each small box counts 0.04 seconds, while each bigger box counts 0.2 seconds.
The vertical axis represents voltage. Each small box measures 1 millimeter, which equals 0.1 millivolts.

Depolarization Wave1:21–2:03

At 0 millivolts, we have the baseline voltage, which is known as the isoelectric line, and any movement away from this line reflects electrical activity.
Normally, the heart's natural pacemaker, called the sinoatrial or SA node, initiates each heartbeat. The essay node spontaneously depolarizes, sending an electrical signal across both atria.
First, the right atrium depolarizes, then the left atrium follows. On the ECG, this atrial depolarization shows up as a positive deflection, known as the P wave.

ECG Measures2:03–2:23

Normally, the P wave is less than 0.12 seconds wide and less than 2.5 millimeters tall. Once the atria depolarized, the signal reaches the atrioventricular or AV node, which is located between the atria and ventricles.

P wave2:23–2:50

The AV node briefly slows the signal down, giving the atria just enough time to fully contract and push blood into the ventricles.
Since there's no active depolarization, this pause appears as a flat line right after the P wave. After this brief delay, the signal picks up speed and rushes through the bundle of hiss further down the right and left bundle branches, eventually reaching perkinji fibers and depolarizing the ventricles.

PR Interval2:50–3:42

The first part of ventricular depolarization starts in the interventricular septum and moves from left to right. In other words, it's moving away from the lead too, which creates a small negative deflection called the Q wave.
Next, the signal depolarizes the apex of the heart, which moves toward the lead two, forming the tall R wave. Finally, the signal moves from the apex back to the base of the ventricles, rushing away from lead two, creating a negative deflection called the S wave.
Together, these three deflections create the QRS complex, which represents the ventricular depolarization. Normally, the QRS complex is less than 0.10 seconds wide.
Longer QRS complexes suggest a delay in ventricular conduction, such as a bundle branch block. Now, the interval from the beginning of the P wave to the beginning of the QRS complex is called the PR interval.

QRS Complex3:42–4:18

This interval represents the time it takes for the electrical impulse to travel from the SA node through the atria, through the AV node, and into the conduction system of the ventricles.
Normally, it lasts between 0.12 and 0.20 seconds. If it's longer than that, it could suggest an AV block.
Once the ventricles depolarize, there's a short pause before they begin to reset. This resting phase appears as a flat line called the ST segment.

ST Segment4:18–4:49

The exact point where the QRS complex ends and the ST segment begins, is called the J point. Next comes the ventricular repolarization, which is the process of resetting the electrical charge, so the ventricles can contract again.
This repolarization wave moves in the opposite direction compared to depolarization. But here's the twist, because repolarization involves a wave of negative charge moving away from the lead, it shows up as a positive deflection on the ECG.

T Wave4:49–5:39

Think of it like two negatives making a positive, that's the T wave. The T wave is more spread out than the QRS complex, because repolarization is a slower, more gradual process.
Finally, the part from the beginning of the QRS complex to the end of the T wave, represents the QT interval. The QT interval is the total time the ventricles take to depolarize and re-polarize.
It's worth mentioning that atrial repolarization also takes place, but it happens during the QRS complex, which is hidden by the large ventricular signals, so it's not visible on the ECG.

Review5:39–5:39

Review5:39–8:34

Now that we've broken down what a single heartbeat looks like on the ECG, we can talk about what counts as a normal sinus rhythm.
It's not just about having a heartbeat. We must check a few specific boxes to say that a person has a normal sinus rhythm.
First, it's the heart rate, which refers to how fast the heart is beating. To say someone has a normal sinus rhythm, their heart rate should be between 60 and 100 BPM at rest.
Second, we have heart rhythm, which refers to the regularity of the beats. In a normal sinus rhythm, the heartbeats are evenly spaced, meaning the spaces between R waves on the ECG are consistent with minimal variations.
Third, there should be a P wave before every QRS complex, and every P wave should have a consistent shape throughout the tracing of a single lead.
This confirms that each beat starts in the essay node and depolarizes the atria in a consistent way. Fourth, the PR interval should be between 0.12 and 0.20 seconds, which shows that the electrical signal is moving properly through the AV node without delay.
Fifth, the QRS complex should be narrow, no more than 0.10 seconds wide, indicating that the ventricles are being depolarized as they should.
While it isn't part of the definition of normal sinus rhythm, it's common to check the QT interval. The QT interval should still fall within normal limits, generally less than 0.44 seconds in men, and less than 0.45 seconds in women.
When all these features are present, we can be confident that the electrical signal is following the heart's normal conduction pathway.
The SA node fires the initial signal, which moves through the atria, briefly slows down at the AV node, and travels up the bundle of hiss.
From here, it travels along the right and left bundle branches, spreading through the perkinji fibers to activate the ventricles.
All right, as a quick recap, normal sinus rhythm means the essay note is pacing the heart at a rate of 60 to 100 BPM with regular intervals.
On the ECG there's a P wave, which corresponds to the atrial depolarization wave, and should be less than 0.12 seconds. The interval from the beginning of the P wave to the beginning of the QRS complex is the PR interval, which should last from 0.12 to 0.20 seconds.
Next up is the QRS complex, which represents ventricular depolarization and should be no more than 0.10 seconds.