ECG normal sinus rhythm

An electrocardiogram, or ECG, or elektrokardiogramm 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 six chest leads, six 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.

But before we proceed with what a normal sinus rhythm is, let’s look at a single heartbeat from the viewpoint of lead II. 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. At 0 millivolts, we have the baseline voltage, which is also 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 SA 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. Normally, the P wave is less than 0.12 seconds wide and less than 2.5 millimeters tall.

Once the atria depolarize, the signal reaches the atrioventricular or AV node, which is located between the atria and ventricles. 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 His, further down the right and left bundle branches, eventually reaching Purkinje fibers and depolarizing the ventricles. 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 II, which creates a small negative deflection called the Q wave. Next, the signal depolarizes the apex of the heart, which moves toward the lead II, forming the tall R wave. Finally, the signal moves from the apex back to the base of the ventricles, rushing away from lead II, 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.

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. 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. 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 repolarize.

It’s worth mentioning that atrial repolarization also takes place, but it happens during the QRS complex and is hidden by the large ventricular signals, so it’s not visible on the ECG.

Normal Sinus Rhythm Characteristics