The standard ECG that is recorded by your doctor has 12 leads on the paper. The ECG machine creates these leads from 10 electrodes that are attached to the chest and limbs of the patient.
The ECG electrodes attached to the patient are often mistakenly referred to as ECG leads, which is incorrect.
Each ECG lead requires two electrodes to display an ECG waveform. Lead I is created by the ECG machine from two electrodes placed on the left and right wrists.
ECG watches have two electrodes. One electrode is the back of the watch on the left wrist. And the other electrode is the crown of the watch where we place the finger of the right hand.
ECG watches can display lead I, which is identical to lead I on a standard 12-lead ECG. Because watches also have electrodes on the left and right hand.
In rest, all heart cells on the surface have mainly positive ions, so all heart cells on the surface are positive.
The heart regularly generates electrical impulses in the Sino-Atrial (SA) node, located in the right atrium. The SA node is a specialized electrical tissue with a size of 1 cm, which generates impulses at a frequency of approximately 60/min.
Each impulse spreads through heart cells from the SA node like an electric wave, similar to throwing a stone into water.
The electric wave is called depolarization because it changes the polarity on the surface of cells. Before the wave, cells on the surface are positive, and after the wave, they are negative. After depolarization, about 0.5 seconds, the surface of cells returns to the original positive state, which is called repolarization.
Depolarization and repolarization of the entire heart constitute one cardiac cycle, which lasts approximately 1 second and repeats throughout life.
A voltmeter is a device that measures the difference in electrical potentials between two electrodes. One electrode is positive and the other negative. Let's imagine that the negative electrode is attached to the right part of the heart and the positive one to the left part of the heart.
In rest, all heart cells on the surface are positive, so two electrodes on the heart see the same electrical potential everywhere. The voltmeter shows a zero value.
If a depolarization wave starts to move from the right atrium to the left, it leaves a negative electric charge on the surface of cells behind it. This creates a difference in electrical potentials before and after the wave. The voltmeter shows a positive value in millivolts (mV).
The body is a good electrical conductor, so ECG electrodes do not have to be attached directly to the heart but it is enough if they are attached to the chest or limbs of the patient.
Each ECG lead on standard 12-lead ECG paper is one voltmeter that requires two electrodes. One negative and one positive.
The rule applies that if the depolarization wave spreads towards the positive electrode, a positive ECG curve is created. If the wave spreads from the positive electrode, a negative ECG curve is created.
ECG watches are voltmeters.
The positive electrode is the bottom part of the watch on the left wrist.
The negative electrode is the crown on the watch. When we place the finger of the right hand on the crown, we activate a voltmeter that has a positive electrode on the left wrist and a negative electrode on the finger of the right hand.
The human body is a good electrical conductor, so the positive electrode on the left wrist senses the left part of the heart. And the negative electrode on the right finger senses the right part of the heart.
A standard 12-lead ECG requires two electrodes for lead I, on the left and right wrists. Therefore, the ECG recorded on the watch exactly corresponds to lead I.
The ECG machine has 10 wires. At the end of each wire is one electrode that sticks to the patient's skin. Therefore, the device has 10 electrodes, which are labeled as:
One limb electrode on the right ankle is not used to obtain an ECG curve. The ECG machine uses it as a reference electrical value set to zero. From the reference electrode, the device then compares electrical signals from which it creates an ECG curve.
The ECG machine has 10 electrodes that are attached to the limbs and chest of the patient. These electrodes are often incorrectly referred to as ECG leads, which is a mistake.
The ECG machine creates pairs from 10 electrodes using software, and from each pair, it creates one ECG lead. One electrode can be used multiple times in multiple pairs.
The mentioned reference electrode on the right ankle is not part of any ECG lead. So, it holds that the ECG machine creates 12 leads from 9 electrodes. Essentially, however, it holds that the device needs 10 electrodes.
Thus, the ECG machine creates 12 ECG leads from 10 electrodes, not from 10 leads, as seen on the standard 12-lead ECG paper.
Imagine a 12-lead ECG as 12 eyes that look at the mentioned depolarization wave. Each eye looks at the wave from a different angle. So, a 12-lead ECG printed on paper informs about the spread of the depolarization wave in 3D space.
3 limb electrodes are placed on the right wrist, left wrist, and left ankle. The ECG machine can software-wise make a positive or negative electrode from each electrode as needed.
Limb leads are called Einthoven's leads or standard bipolar leads. Because each lead has two electrodes, positive and negative.
Bipolar limb ECG leads are 3:
Lead I (exactly this lead can be recorded by ECG watches)
Lead II
Lead III
These limb leads are calculated by the ECG machine exactly from the same electrodes as leads I, II, III.
The positive electrode is on one limb (left wrist, right wrist, or left ankle).
The negative electrode is formed by connecting 2 opposite limbs.
Limbs leads (aVR, aVF, aVL) are called Goldberger's leads or unipolar limb leads. Because the positive unipolar electrode is on the limb, and the second negative electrode must be created by connecting 2 opposite limbs.
These leads should have a very low curve on the ECG paper, so the ECG machine augments them. The small "a" at the beginning of each lead means "augmented".
aVR (augmented Vector Right)
aVL (augmented Vector Left)
aVF (augmented Vector Foot)
Each ECG lead on ECG paper requires two electrodes, positive and negative.
The ECG machine creates an electrical point in the center of the heart using software from the 3 mentioned limb electrodes. This point is called the Wiloson Central Terminal (WCT) and it is a virtual negative electrode. Sometimes the WCT is referred to as the electrical center of the heart.
The WCT serves as the negative electrode for unipolar chest leads V1 to V6
Chest electrodes are placed on the chest of the patient. All chest electrodes V1 to V6 are positive.
The negative electrode is the Wilson Central Terminal (WCT), located in the center of the heart.
V1 lead on ECG paper is created from two electrodes. The positive electrode, also called V1, is attached to the chest to the right of the sternum. And the negative electrode is WCT.
Other chest leads V2 to V6 are created exactly like lead V1. Only each electrode is placed in a different place on the chest in a semicircle. Each chest lead V1 to V6 thus looks at the depolarization wave from a different angle.
The ECG machine has 10 electrodes that are attached to the chest and limbs of the patient, they are not leads. One electrode on the right ankle is a reference, does not create an ECG curve.
The ECG machine creates from 3 limb electrodes the Wilson Central Terminal, and then 12 leads that we see on ECG paper. Each lead on ECG paper is created from two electrodes attached to the patient.
The ECG curve recorded on ECG watches is lead I, which exactly matches lead I on the standard 12-lead ECG.
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