ECG | EKG | Unipolar Limb Leads | Electrocardiography | Cardiology

Understanding Bipolar and Unipolar ECG Leads Bipolar ECG leads measure the potential difference between two electrodes placed at different points on the body, with one electrode serving as a positive pole and the other as a negative pole. The axis of these leads is defined by an imaginary line joining these two electrodes. In contrast, unipolar ECG leads involve modifying the negative terminal into an indifferent electrode that senses near-zero potential through multiple connections and resistances.

The Concept of Indifferent Electrode in Unipolar Leads Unipolar leads are essentially modified bipolar systems where the negative terminal becomes an indifferent or virtual electrode sensing almost zero potential. This modification allows for measuring electrical activity using only one active (positive) exploring electrode while maintaining stability with minimal interference from surrounding potentials. The creation of uniporal lead was driven by medical advancements aiming to simplify measurements without losing accuracy.

Einthoven's Triangle and Bipolar Leads Around 1900, Mr. Einthoven developed the concept of bipolar limb leads forming a triangle with three points: right arm (negative terminal), left arm (positive terminal for lead I), and left leg. This configuration allows observation of heart electrical activity from different angles during ventricular depolarization.

Wilson's Unipolar System Innovation In the 1930s, Dr. Wilson introduced unipolar leads to fill gaps in viewing angles between existing bipolar leads by creating an 'indifferent electrode.' He achieved this by connecting electrodes at three equidistant points on an imaginary equilateral triangle around the heart, summing their potentials to approximate zero voltage at its center.

Creating Indifferent Electrodes Using Geometry Wilson used geometric principles to create a virtual central electrode without invasive methods. By placing resistors in each wire connected to right arm, left arm, and left leg electrodes—forming nearly equal sides—the summed potential approached zero volts effectively simulating placement within the heart’s center.

'Common Terminal' Concept Implementation 'Indifferent terminals' were integrated into electrocardiograph machines as common terminals receiving combined signals from multiple body sites through high-resistance connections ensuring minimal current flow while maintaining near-zero potential difference across them; thus enabling comprehensive cardiac monitoring without direct internal access.

Wilson's Triangle and the Creation of a Virtual Terminal Wilson developed what is known as Wilson's triangle by connecting negative electrodes to the right arm, left arm, and left leg. This configuration created a virtual terminal at near zero potential in the center of this triangle or heart. By converting these multiple negative terminals into an indifferent electrode, he established a unipolar lead system where positive electrodes could explore potentials relative to this central point.

Unipolar Leads: VF Lead Formation The creation of unipolar leads involved placing positive electrodes on different parts of the body while referencing them against Wilson’s common terminal. For instance, positioning it on the foot resulted in VF (voltage from foot), providing a unique view straight up from bottom to top through the heart. Despite reduced voltage sensing compared to bipolar systems due to using an indifferent electrode for reference, this method allowed new perspectives without adding additional physical electrodes.

Expansion with Nine Unipolar Leads Building upon his initial success with limb leads like VR (right arm) and VL (left arm), Wilson expanded his work by creating nine total unipolar leads—three limb-based and six chest-based ones—all utilizing his common terminal concept. These innovations enabled comprehensive cardiac monitoring without needing extra hardware beyond existing connections used previously by Einthoven’s setup.

Bipolar leads have one positive and one negative pole, while unipolar leads use a single positive electrode with an indifferent (near zero potential) reference point. There are three bipolar limb leads and three augmented unipolar limb leads, along with six chest electrodes making up the 12-lead system. Wilson's method places the indifferent electrode at the heart’s center to achieve near-zero potential for accurate readings. However, early recordings had low voltage issues until Dr. Goldberger introduced augmentation techniques in the 1940s.

Goldberger's Innovation in Augmenting ECG Leads In the 1940s, Dr. Goldberger and his colleagues sought to enhance the voltage picked up by unipolar leads created by Wilson. They identified that low voltages were due to partial cancellation between positive and negative electrodes placed at the same location. Goldberger proposed a solution: modify Wilson’s common terminal so that when placing a positive electrode, associated negatives are deactivated or disconnected, creating an augmented virtual electrode with increased voltage.

Implementation of Augmented Leads for Better Cardiac Views By disconnecting certain negative terminals while recording from specific points on the body (e.g., foot), Goldberger shifted virtual electrodes' positions without changing lead axes but enhancing voltages significantly—by about 50%. This innovation led to clearer cardiac views such as AVF (augmented voltage from foot) which provided better visualization of heart activity compared to traditional methods.

Understanding Unipolar and Bipolar Leads Unipolar leads detect potential differences at a site referenced against an indifferent virtual electrode, while bipolar leads measure the difference between two electrodes. Wilson's system created three unipolar limb leads but had low voltage amplitude issues due to partial cancellation by negative electrodes on limbs. Goldberg later augmented these voltages for better readings in chest lead recordings.

Wilson vs. Goldberger Systems: Fixed vs Variable Electrodes Wilson’s fixed central terminal is used only for chest plates now because it neutralizes voltages when applied to limbs, causing inaccuracies. In contrast, Goldberger's variable central terminal adjusts based on which limb has the positive electrode, providing more accurate measurements without manual adjustments needed today thanks to smart ECG machines.

Understanding ECG Patterns and Lead Placement ECG patterns change based on the position of positive electrodes, which detect electrical activity from different heart views. Unipolar augmented limb leads (AVL, AVR) are compared with bipolar limb leads to understand these variations. For instance, lead II shows maximum QRS deflection when vectors move down and left; AVF has less amplitude than lead II but more than III due to its intermediate positioning.

Orientation of Electrical Vectors in Frontal Plane The mean QRS vector typically moves downward and leftward at around 60 degrees in a healthy heart. Leads positioned inferiorly or on the left side will show positive deflections as they align with this vector direction. Conversely, AVR displays negative deflections because it is oriented opposite to the main cardiac electrical axis.

Comparing Amplitudes Across Different Leads Lead II generally exhibits the tallest QRS complex due to optimal alignment with ventricular depolarization vectors moving downward-leftward. Lead III shows smaller amplitudes while AVL's pattern resembles that of lead I but slightly differs depending on electrode placement angles relative to each other’s axes.

'Summation Law' for Augmented Limb Leads Voltage Relationship 'Summation law' states that adding voltages from AVL (+1), AVF (+5), and AVR (-6) results in zero net voltage: +1 +5 -6 = 0 millivolts—demonstrating their interrelated nature similar yet distinct from Einthoven's law applied among bipolar limbs where sum equals another specific value like one plus three equaling two

The triaxial diagram is used to illustrate the spatial orientation of bipolar limb leads and augmented unipolar limb leads in the frontal plane. For bipolar limb leads, lead I is at 0 degrees, lead II at +60 degrees, and lead III at +120 degrees. The positive ends are solid lines while negative ends are dotted lines on this diagram. In contrast, augmented unipolar limb leads have different orientations: aVF is vertical at +90 degrees; aVL sits around -30 degrees; and aVR lies near -150 degrees when using Lead I as reference.

Understanding the Hexaxial Diagram and Lead Orientation The hexaxial diagram combines six limb leads—three bipolar and three augmented unipolar—to illustrate their orientation relative to each other. By arranging these leads around a circle, one can determine the angles of orientation: lead I at 0 degrees, AVF at 90 degrees, etc., with positive poles designated every 30 degrees clockwise and negative poles counterclockwise. This setup helps in understanding both the angle of orientation and polarity for each lead.

Application of Hexaxial Diagram in Determining Heart's Electrical Axis The primary use of this hexaxial system is to ascertain whether the heart has a normal electrical axis or deviations such as right or left axis deviation. Knowing how to read this diagram allows clinicians to identify specific orientations (e.g., +60° for lead II) and polarities (e.g., AVR’s positive pole at -150°). Mastery over these details aids significantly when interpreting clinical ECGs.

Bipolar leads reference the positive electrode against a negative one, while unipolar leads use an indifferent electrode with near-zero potential. The Wilson system created in the 1930s used resistances to form a central terminal at zero potential, forming what is known as the Wilson triangle. Goldberger later modified this by augmenting voltage through cancellation of negatives when placing positives, leading to augmented limb leads (aVF, aVL, and aVR). These systems help map electrical activity around different orientations: bipolar lead I at 0 degrees; II at 60 degrees; III at 120 degrees; with augmented limb leads oriented differently.