Why is it called a 12-Lead ECG when there are only 10 Leads?
An electrocardiogram (ECG) is one of the most important tests to determine heart health. However, it can leave anyone thinking, 'Why is it called a 12-lead ECG when there are only 10 electrodes?' The answer lies in how the 10 electrodes capture 12 different electrical heart views.
Whether you want to define your skills or seek to understand this important medical procedure, Gauze will help you gain sufficient knowledge of how ECGs work. In this blog, we will explain how our 12-lead ECG works despite using only 10 electrodes and how the correct placement of these electrodes helps us evaluate heart conditions.
What is an ECG?
ECG is a quick and non-invasive test that healthcare professionals commonly do. An electrocardiogram (ECG) measures the heart's electrical activity. The electrodes are connected to the leads, and then the ECG machine captures the electrical signals generated by the heart as waveforms on the monitor. These waveforms help doctors notice.
- Heart’s rhythm.
- Abnormalities such as arrhythmias, heart attack, or other cardiac issues.
ECG lead placement:-
It involves attaching electrodes to the patient's body to measure and record the heart's electrical activity.
It can detect
- Various heart conditions.
- Previous heart attacks.
Correct lead placement ensures quality ECG readings and provides insight into a patient's heart health. The number of leads and their placement can vary depending on the ECG.
Who requires a 12-lead ECG?
12-lead ECG, screens for ischemia when patients experience any of the following:-
- Shortness of breath.
- Palpitations.
- Sudden chest pain.
- Dizziness.
Diseases and diagnosis with 12-lead ECG:-
A 12-lead ECG helps in diagnosing and monitoring diseases like:
- Prinzmetal’s angina.
- Myocardial infarction (MI).
- Pericarditis.
- Left ventricular hypertrophy.
In conditions like angina, ECG changes may be fleeting, but in other conditions, such as MI, the ECG changes could signal life-threatening complications.
12-Lead ECG Standard Position and Placement:-
1. Chest precordial electrodes
- V1 in the fourth intercostal space on the right sternal edge.
- V2 in the fourth intercostal space on the left sternal edge.
- V3 midway between V2 and V4.
- V4 in the fifth intercostal space at the midclavicular line.
- V5 at the Left anterior axillary line at the same horizontal level as V4.
- V6 at the Left mid-axillary line at the same horizontal level as V4 and V5.
2. Limb Leads
- Right Arm (RA).
- Left Arm (LA).
- Right Leg (Neutral).
- Left Leg (LL).
They are attached to the arms and legs in the region proximal to the wrist and ankle.
- ECG readings are taken on the vertical plane (frontal leads) and the horizontal plane (transverse leads).
3. Heart’s vertical plane (frontal leads)
The bipolar lead positions measure voltage between two electrodes.
- Lead I (LA to RA).
- Lead II (LL to RA).
- Lead III (LL to LA).
The unipolar lead positions are:
- aVR (augmented vector right).
- aVL (augmented vector left).
- aVF (augmented vector foot).
4. Heart’s horizontal plane (transverse leads)
- V1, V2, V3, V4, V5, and V6 are the six chest precordial electrodes, providing six different viewpoints.
Understanding Einthoven’s Triangle in ECG:-
Einthoven’s triangle forms the basis of electrocardiography. The imaginary triangle is found by placing electrodes on:
- Right Arm(RA).
- Left Arm (LA).
- Left Leg (LL).
These points record electrical activity through the augmented limb leads and with each other to form standard limb leads.
- Lead I: LA (+) to RA (-).
- Lead II: LL (+) to RA (-).
- Lead III: LL (+) to LA (-).
They measure the voltage difference between the two limbs forming a triangle's side.
Augmented limb leads:-
Each uses one limb as a positive electrode and the average of the other two as the negative reference
- aVR: RA (+) compared to (LA+LL) as(-).
- aVL: LA (+) compared to (RA+LL) as(-).
- aVF: LL (+) compared to (RA+LA) as(-).
Electrodes and Leads
What are electrodes?
Electrodes are conductive pads attached to the skin surface. When placed on the skin of the chest, arms, and legs, they detect the electrical current generated by the heart and affect the ECG.
How are waveforms created?
1. The depolarization wave moves
- Towards the positive electrode results in a positive deflection, causing an upward wave in the ECG. Away from the positive electrode results in a negative deflection, causing a downward wave in the ECG.
2. Repolarization wave
- Towards the positive electrode causes a negative deflection.
- Away from the positive electrode leads to a positive deflection.
3. No electrical activity
- The ECG records a flat line when there is no depolarisation or repolarisation.
4. Biphasic waves and electrode placement
- When a depolarization wave approaches an electrode, a positive deflection is recorded.
- When the wave is under the electrode, the tracing returns to baseline.
- As the wave moves away from the electrode, a negative deflection is formed.
What are leads?
An ECG lead is a graphical representation of the heart’s electrical activity. Multiple leads are used because the heart functions in a three-dimensional space. To capture its electrical signals accurately, we must observe them from horizontal, vertical, and anterior-posterior planes.
Why is it Called a 12-Lead ECG When There are Only 10 Electrodes?
The ten electrodes, also known as leads, measure the heart's electrical activity from different angles, obtaining twelve distinct views. Electrodes are adhesive patches applied to the patient’s skin to detect electrical signals. These electrodes are connected to the ECG machine using wires, called leads. Here, the word lead refers to the views, not the wires. To simplify:
Out of the 10 electrodes
- 4 are limb electrodes: RA, LA, RL, LL.
- 6 are chest electrodes, V1 to V6.
These electrodes generate
- 6 limb leads I, II, III, aVR, aVL, aVF.
- 6 chest leads, V1 to V6.
Thus, the 10 physical electrodes used create 12 different electrical views into the heart, hence the 12-lead ECG.
The 12-lead ECG is efficient because it uses:-
- Grounding: The right leg electrode stabilizes the ECG and reduces noise.
- Signal Averaging: The three augmented limb leads (aVR, aVL, aVF) don’t need extra wires. They’re calculated from the existing limb electrodes by combining signals, providing extra views without additional electrodes.
Colour Coding Standards for a 12-Lead ECG
The IEC (International Electrotechnical Commission) and AHA (American Heart Association) systems are prominent colour-coding systems for 12-lead ECGs. Each lead is assigned a specific colour and position for cardiac monitoring.
- The IEC system is a globally recognised standard for ECG lead placement for clear identification.
- The AHA system is used in North America and is known for its simplicity and ease of use.
12-lead ECG placement mnemonic
AHA system for four electrode placement.
- Smoke over fire, black lead, and red lead on the left arm and leg, respectively.
- Snow over grass, white lead followed by green lead on the right arm and leg, respectively.
IEC system, for four electrode placement.
- Ride Your Green Bike- Red (right arm), Yellow (left arm), Green (foot), Black (neutral).
Best Practices for Lead Placement
- Count the intercostal space: the first intercostal space is just below the clavicle.
- Wipe the surface and clean the dead tissue from the chest using a dry gauge. Remove hair from the chest where electrodes are to be applied.
- In women with large breasts, we need to place the leads in the crease underneath the breast, because there is too much tissue to get a clear reading.
- Review the patient’s old medical records for trends.
Reducing artifacts
We should minimise artifacts in a 12-lead ECG. It can be accomplished by following the guidelines:-
- The patient should lie in a supine position.
- The patient’s arm should be by his side, and his shoulders should be relaxed.
- All electrical devices, such as a mobile phone, should be kept away from the patient and the machine itself, as they can interfere.
- Dry the skin if it is moist.
- Shave hair that interferes with electrode placement.
- The electrode gel should be moist.
- Electrodes should not be placed over bones or other areas with significant muscle movement.
Conclusion
Gauze provides you with all the necessary information and knowledge to keep you updated. We recommend following best practices when taking ECG readings. Proper skin preparation, electrode placement, and the right electrode in the right position provide an accurate reading.
It helps eliminate misdiagnosis and improve patient outcomes. Knowing the principles on which the ECG works forms the basis for doing it appropriately.
Here to answer all your questions
Each lead gives us a view of the heart from a different angle.
The skin should be cleaned with an alcohol swab to remove oil and dirt, ensuring proper contact and reducing artifacts in the ECG.
Yes, 1, 3, and 5 lead ECGs are possible. They have the benefits of convenience and simplicity, but a drawback is that they provide limited information. They provide a narrow view of the heart's activity. They may not be sufficient for diagnosing complex conditions.
A 12-lead ECG uses 10 electrodes and provides 12 electrical views of the heart, highlighting the tool's efficiency.
The skin should be cleaned with an alcohol swab to remove oil and dirt, ensuring proper contact and reducing artifacts in the ECG.
Correct electrode placement is important for obtaining accurate readings. Improper placement can lead to misdiagnosis and inaccurate assessment of heart health.
