Introduction to ECG Testing & Einthoven’s Triangle:
The electrocardiogram (ECG) represents the heart’s electrical activity, resulting from the contraction (depolarization) and relaxation (repolarization) of the atrial and ventricular muscles. When it comes to understanding the electrical activity of the heart, Einthoven’s Triangle is a fundamental concept in electrocardiography (ECG).
It was developed by Willem Einthoven, a renowned Dutch physician, and it revolutionized the way we analyze and interpret the heart’s electrical signals. In this blog, we will delve into the significance of Einthoven’s Triangle and its role in modern cardiology.
Let’s start with grasping the basics of electrocardiography. An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that records the electrical activity of the heart over a period of time. It is represented graphically in the form of waves, commonly known as ECG waves or complexes.
The ECG Machine starts capturing the electrocardiograph once electrodes are placed on the skin. The electrodes detect the electrical impulses generated by the heart to the EKG Machine. It then amplifies the signals and records the ECG on graph paper for analysis by Medical Professionals.
Medical professionals like cardiologists use EKG to diagnose a wide range of heart conditions, such as arrhythmias, heart attacks, and other abnormalities in the heart’s rhythm and function using Einthoven’s Triangle.
Einthoven’s Triangle is the cornerstone of ECG placement and interpretation. It involves the placement of three electrodes on specific locations of the body to create an equilateral triangle. The three corners of this triangle are named after the respective electrode placements: the right arm (RA), the left arm (LA), and the left leg (LL). By using these leads, there are three standard placements of the electrodes as follows:
Standard Limb Leads
Augmented Limb Leads
Standard Limb Leads I, II, & III
These are utilized to display the potential difference graph between two limbs simultaneously, where one limb holds a positive electrode, and the other limb carries a negative electrode. The three limb electrodes, namely I, II, and III, form a triangle on the right arm (RA), left arm (LA), and left leg (LL).
Lead I: The technique involves placing electrodes on the left and right shoulders along an axis. The negative electrode goes on the right shoulder, and the positive electrode goes on the left shoulder. This positioning results in an orientation angle of 0°.
Ⅰ = LA - RA
Lead II: The orientation of the setup is defined by an axis that extends from the right arm to the left leg, wherein the negative electrode is placed on the shoulder, and the positive electrode is placed on the leg. This arrangement specifically relates to an orientation angle of more than 60 degrees, depicting the positioning of electrodes precisely and accurately.
ⅠⅠ = LL - RA
Lead ⅠⅠⅠ: The orientation of this axis spans from the negative electrode, which is present on the left shoulder, to the positive electrode, which is present on either the right or left leg, resulting in an angle of more than 120°.
ⅠⅠⅠ = LL – LA
Augmented Limb Leads (aVL), (aVR), (aVF):
Goldberger introduced the augmented unipolar limb leads aVR, aVL, and aVF in 1942, and these leads have since become essential components of the 12-lead electrocardiogram (ECG). The ECG includes a total of six limb leads, consisting of three augmented unipolar leads and three standard bipolar limb leads.
The leads are called unipolar as they utilize a single positive electrode, which is compared to a combination of other limb electrodes (as shown in the figure). These leads are named after the location of their positive electrodes, which are placed on the left arm (aVL), the right arm (aVR), and the left leg (aVF).
Together, these leads capture the heart’s electrical activity in the frontal plane by using the axial reference system. In the augmented limb leads, each lead is derived by combining the average of the electrical signals from the two opposite Einthoven electrode positions. The magnitude is given as:
In an ideal scenario where the heart is positioned at the center of an equilateral triangle, taking the average of two electrodes yields the voltage value located exactly midway between them. Therefore, the voltages recorded using these augmented limb leads represent the projection of the cardiac dipole onto the respective lead axes, as shown below, oriented at 30º, 90º, and 150º.
In 1944, Wilson et al. introduced the precordial leads, also known as chest leads (V1-V6), in which V1& V2 should be placed in the anteroseptal region, V3 &V4 should be placed in the anteroapical region, V5&V6 should be placed in the anterolateral region.
These V1-V6 should be perpendicular to all the other leads. The initial depolarization of the ventricles occurs from left to right across the septum, resulting in an initial R-wave in lead V1, followed by an S-wave as the anterior and lateral walls of the left ventricle undergo depolarization.
Leads V5 and V6 show a significant net positive QRS because they are positioned over the anterolateral wall of the left ventricle, which has a large muscle mass experiencing depolarization. The tracings in leads V5 and V6 exhibit polarity opposite to that of V1 since they are observing different sides of the heart. Leads V2-V4 provide intermediate readings due to their electrode placement.
Latest Developments in ECG Recording
In the past, when recording a 12-lead electrocardiogram (ECG), the electrodes for the limb leads were attached to the patient’s wrists and ankles while they lay supine.
However, in 1975, the American Heart Association (AHA) changed its recommendation. They suggested that it’s not necessary to place lead electrodes on the wrist and ankles; instead, it can also be placed on the four limbs.
This new approach allowed for more flexibility in electrode placement and improved the accuracy of ECG recordings. Nowadays, it’s common to record the ECG from the upper arm instead of the wrist to reduce interference from body movement. This is made easier with the use of disposable tab electrodes.
To reduce interference caused by arm and leg movement during ambulatory and exercise ECG, the limb leads can be placed on the torso. This is known as the Mason-Likar lead position. In this position, the arm electrodes are placed in the infraclavicular fossae medial to the deltoid insertions, and the left leg electrode is placed midway between the costal margin and iliac crest in the left anterior axillary line. A prime example of this setup would be the Wellnest 12L Pro 2 ECG Machine and its belt.
Einthoven’s Triangle is a fundamental concept in the field of electrocardiography that provides essential insights into the heart’s electrical activity. By using this simple yet powerful tool, medical professionals can diagnose and monitor various heart conditions effectively. As technology advances, the significance of Einthoven’s Triangle continues to influence the way we understand and care for the human heart, ultimately contributing to improved patient outcomes and better cardiac health worldwide.