· The electrical impulses originating from the heart can be transmitted to the body surface because the body contains fluids and chemicals that can conduct electricity. These electrical impulses can be recorded by placing electrodes to the different areas of the body. Thus, if a left arm electrode is connected to the positive pole of a galvanometer and a right arm electrode is connected to the negative pole, the magnitude as well as the direction of the electrical impulse can be measured.
o Any flow of current directed toward the positive (left arm) electrode is conventionally recorded as an upright deflection (Fig. 3.1A).
o Any flow of current away from the positive electrode is recorded as a downward deflection (Fig. 3.1B).
o The height of the electrocardiogram (ECG) deflection represents the difference in potential between the two electrodes.
Figure 3.1: Lead. The direction and magnitude of the electrical impulse can be measured with a galvanometer (G). The left arm electrode is conventionally attached to the positive pole of the galvanometer and the right arm electrode to the negative pole. An imaginary line connecting the two electrodes is called a lead. Any flow of current directed toward the positive electrode will be recorded as an upright deflection (A). Any current moving away from the positive electrode is recorded as a downward deflection (B).
Bipolar Leads I, II, and III
· Bipolar Leads: An imaginary line connecting any two electrodes is called a lead. A lead is bipolar when both positive and negative electrodes contribute to the deflection in the ECG. The positive and negative electrodes are placed at an equal distance away from the heart and the resulting ECG deflection is the sum of the electrical forces going in opposite directions. Leads I, II, and III are examples of bipolar leads.
o Lead I: Lead I is conventionally constructed such that the left arm electrode is attached to the positive pole of the galvanometer and the right arm to the negative pole (Fig. 3.2A). If the direction of the impulse is toward the left arm, an upward or positive deflection is recorded. If the direction of the impulse is toward the right arm, a negative or downward deflection is recorded.
Figure 3.2: Bipolar Leads I, II, and III. A-C represent the location of the electrodes for leads I, II, and III, respectively. Any flow of current toward the positive electrode will record a positive deflection. The imaginary line connecting the two electrodes is the ECG lead.
o Lead II: The left leg is attached to the positive pole and the right arm to the negative pole (Fig. 3.2B). When the direction of the impulse is toward the left leg, a positive deflection is recorded. When the direction of the impulse is toward the right arm, a downward deflection is recorded.
o Lead III: The left leg is attached to the positive pole and the left arm to the negative pole (Fig. 3.2C). When the direction of the impulse is toward the left leg, an upward deflection is recorded. When the direction of the impulse is toward the left arm, a downward deflection is recorded.
· Lead I transects an imaginary line from one shoulder to the other shoulder and represents an axis of 0° to 180° (Fig. 3.3A).
· Lead II transects an imaginary line between the left leg and right arm and represents an axis of +60° to -120° (Fig. 3.3B).
· Lead III transects an imaginary line between the left leg and the left arm and represents an axis of +120° and -60° (Fig. 3.3C).
· Leads I, II, and III can be arranged to form the Einthoven triangle (Fig. 3.3D, E) as shown. The leads can also be superimposed on each other by combining all three leads at their mid-points to form a triaxial reference system representing the frontal plane of the body (Fig. 3.3F).
Figure 3.3: Bipolar Leads I, II, and III. A, B, and C represent leads I, II, and III, respectively. The leads form an Einthoven triangle(D, E), which can be rearranged to form a triaxial reference systemby combining all three leads at each midpoint as shown (F).
· Unipolar Leads: When one electrode is capable of detecting an electrical potential (exploring electrode) and the other electrode is placed at a distant location so that it will not be affected by the electrical field (indifferent electrode), the lead that is created is a unipolar lead. A unipolar lead therefore has only one electrode that contributes to the deflection in the ECG. The other electrode serves as a ground electrode and is theoretically neutral.
o The exploring electrode: Only the exploring electrode is capable of measuring the flow of current. This electrode is connected to the positive pole of the galvanometer. If the flow of current is directed toward the exploring electrode, an upward deflection is recorded. If the flow of current is away from the exploring electrode, a downward deflection is recorded. The exploring electrodes of the three unipolar limb leads are conventionally placed in the right arm, left arm and left foot and were originally called VR, VL, and VF respectively.
o The ground electrode: The ground electrode is constructed by placing a resistance of 5,000 ohms to each of the three limb electrodes and connecting them together to form a central terminal (Fig. 3.4).
The central terminal is connected to the negative pole of the galvanometer. This serves as the ground electrode, which has a potential of zero or near zero.
Figure 3.4: Unipolar Leads VR, VL, and VF. Diagram shows the original construction of the unipolar limb leads VR, VL, and VF. Each limb lead is connected to a resistance of 5,000 ohms to form a central terminal. The central terminal serves as the ground electrode and is connected to the negative pole of the galvanometer. The exploring electrode, which in this example is in the left foot, is connected to the positive pole. R, right arm; L, left arm; F, left foot.
Augmented Unipolar Leads AVR, AVL, and AVF
· Augmented unipolar leads aVR, aVL, and aVF: When the exploring electrode was disconnected from the central terminal, the size of the ECG deflection increased by 50%. Thus, the augmented unipolar leads VR, VL, and VF were renamed aVR, aVL, and aVF and became the standard unipolar limb leads.
o Lead aVR: The unipolar electrode is positioned over the right arm and is capable of detecting the flow of electrical impulse directed toward the right shoulder. The location of aVR is -150° (Fig. 3.5A).
o Lead aVL: The unipolar electrode is positioned over the left arm and is capable of detecting potentials directed toward the left shoulder. The location of aVL is -30° (Fig. 3.5B).
o Lead aVF: The unipolar electrode is positioned over the left leg and is capable of detecting potentials directed toward the left groin. The location of aVF is +90° (Fig. 3.5C).
Figure 3.5: Augmented Unipolar Leads aVR, aVL, and aVF. The upper panel shows the position of the exploring electrode for leads aVR, aVL, and aVF. The lower panel shows the connection of the exploring electrode and the central terminal. R, right arm; L, left arm; F, left foot.
Unipolar Leads AVR, AVL, and AVF
· The three augmented unipolar leads aVR, aVL, and aVF (Fig. 3.6A), and the three standard bipolar leads I, II, and III (Fig. 3.6B) complete the six leads representing the frontal plane of the body. These six leads can be rearranged to form a hexaxial reference system as shown (Fig. 3.6C). All unipolar leads are identified with a letter V.
· The location of each lead as well as the position of the positive and negative terminals of each lead is crucial in understanding the 12-lead ECG.
The Precordial Leads
· Precordial leads: Six precordial leads were later added to the six frontal leads to complete the 12-lead ECG. All six precordial leads are unipolar and are identified with a letter V. When the lead is unipolar, only the exploring electrode contributes to the generation of the electrical complex.
o Exploring electrode: The location of the exploring electrodes in the chest is universally standardized. The electrodes are labeled V1 to V6. The standard universal position of V1 to V6 are as follows:
§ V1 is located at the 4th intercostal space immediately to the right of the sternum.
§ V2 is located at the 4th intercostal space immediately to the left of the sternum.
§ V3 is located between V2 and V4.
§ V4 is located at the 5th intercostal space, left midclavicular line.
§ V5 is located at the same horizontal level as V4, left anterior axillary line.
§ V6 is located at the same level as V5, left mid axillary line.
o Ground electrode: The ground electrode is the central terminal similar to Figure 3.4 and is constructed by placing a resistance of 5,000 ohms to each of the three limb electrodes (Fig. 3.7). The central terminal is connected to the negative pole of the galvanometer and serves as the ground electrode, which has a potential of zero.
Figure 3.6: Hexaxial Reference System Representing the Frontal Plane. The standard bipolar leads I, II, and III and the augmented unipolar limb leads aVR, aVL, and aVF make up the hexaxial reference system representing the frontal plane. (A) The location of these leads in relation to the body. (B, C) How these six leads are related to each other.
Figure 3.7: The Precordial Leads. The construction of the unipolar chest leads is shown diagrammatically. The precordial electrode is the exploring electrode. Six exploring electrodes are positioned in V1 to V6. The ground electrode consists of three limb electrodes individually attached to a 5,000-ohm resistance and connected together to form a central terminal. R, right arm; L, left arm; F, left foot.
The 12-Lead ECG
· The six frontal or limb leads and the six horizontal or precordial leads complete the 12-lead ECG (Fig. 3.8A, B).
Standard and Special Leads
· Standard 12-lead ECG: The 12-lead ECG is recorded with the patient supine and a pillow supporting the head. The latest recommendations of the American Heart Association regarding the ECG include the following:
Figure 3.8: The 12-Lead Electrocardiogram. (A) The position of the six limb leads in the frontal plane. (B) The six precordial leads in the horizontal plane.
o Until further studies are available, the extremity electrodes are placed distal to the shoulders and hips and not necessarily at the wrists or ankles. This reduces motion artifacts and ECG voltage and duration are less affected than if the leads are placed more distally.
o All leads in the standard 12-lead ECG are effectively bipolar. This is based on the principle that all leads consist of an electrode that is paired to another electrode. This includes the standard limb leads as well as the leads where the exploring electrode is paired with an indifferent electrode consisting of the central terminal or its modification. Thus, standard leads I, II, and III; augmented limb leads aVR, aVL, and aVF; and the six precordial leads V1 to V6 are all effectively bipolar and the use of bipolar and unipolar to describe these leads is discouraged.
o Misplacement of the precordial leads is a common cause of variability in the ECG, especially when serial tracings are being interpreted. The position of V4 should be followed horizontally; thus, V5 and V6 should be in the same horizontal position as V4 rather than at a lower position if the course of the 5th intercostal space is followed laterally. In women, it is recommended that the precordial electrodes should be placed under rather than over the breast, thus allowing V5 and V6 to follow the horizontal position of V4. If the anterior axillary line is not well defined, V5 is positioned midway between V4 and V6. The position of V1 and V2 is at the 4th intercostal space at the right and left sternal borders respectively. When V1 and V2 are erroneously placed higher at the 2nd intercostal space, the following changes may occur:
§ A smaller r wave is recorded from V1 to V3. The R wave reduction is approximately 1 mm per interspace, causing poor R wave progression, which can be mistaken for anterior myocardial infarction (MI).
§ Terminal r' waves with T wave inversion resulting in rSr' pattern in V1 and V2 are recorded similar to the configuration in lead aVR.
§ If the diaphragm is displaced downward and the heart becomes vertically oriented, as when there is chronic obstructive lung disease, the normal location of V3 and V4will place these leads in a relatively higher position than the ventricles; thus, deep S waves will be recorded in these leads, which can be mistaken for anterior MI.
o Finally, when the extremity leads are modified so that the leads are placed in the torso rather than the extremities, the ECG is not considered equivalent to the standard ECG. Similarly, tracings obtained in the sitting or upright position is not equivalent to the standard ECG, which is recorded supine.
o It has also been observed that when V1 and V2 are placed higher than their normal location, small q waves may be recorded in V2 andV3, especially in the presence of left anterior fascicular block.
· Special Electrodes: In addition to the 12 standard ECG leads, special leads can be created by repositioning some electrodes to the different areas on the chest.
o Special leads V7, V8, and V9: V7 is located at the left posterior axillary line at the same level as V6. V8 is located just below the angle of the left scapula at the same level as V7and V9 just lateral to the spine at the same level as V8. These leads supplement the 12-lead ECG in the diagnosis of posterolateral ST elevation MI and should be recorded when reciprocal ST segment depression is present in V1 to V3.
o Right sided precordial leads V3R, V4R, V5R, and V6R: After recording the usual standard 12-lead ECG, special leads V3R, V4R, V5R, and V6R can be added by moving precordial leads V3, V4, V5, and V6 to the right side of the chest corresponding to the same location as that on the left. These leads are very useful in the diagnosis of right ventricular MI, dextrocardia, and right ventricular hypertrophy. These leads should be recorded routinely when there is acute coronary syndrome with ST elevation MI involving the inferior wall (leads II, III, and aVF).
o Other lead placement used for detection of arrhythmias:
§ CF, CL, and CR leads: Bipolar leads have electrodes positioned in the arms or leg, which are equidistant from the heart. When one electrode is moved to the precordium and the other electrode is retained in its original position in the arm or leg, the chest electrode will contribute more to the recording than the remote electrode.
§ Thus, a CL lead is created if one electrode is placed on the chest and the more remote electrode is retained in its original position in the left arm. If the chest electrode is placed in V1 and the remote electrode is at the left arm, the lead is identified as CL1.
§ CR lead is created when the remote electrode is retained in the right arm. If the chest electrode is placed in V1 and the remote electrode is at the right arm, the lead is identified as CR1.
§ CF lead is created when the remote electrode is retained in the left foot. If the chest electrode is placed in V1 and the remote electrode is at the left foot, the lead is identified as CF1.
§ Modified CL1 or MCL1: MCL1 is a lead that resembles V1. The lead is bipolar and is a modified CL1 lead. The positive electrode is placed at V1 and the negative electrode is placed close to the left shoulder. A ground electrode is placed at the other shoulder. It is frequently used for detecting arrhythmias during continuous monitoring of patients admitted to the coronary care unit.
§ Lewis lead: When the P wave is difficult to recognize, the right arm electrode is moved to the 2nd right intercostal space just beside the sternum and the left arm electrode to the 4th right intercostal space also beside the sternum. Lead I is used for recording.
§ Fontaine lead: The Fontaine leads are special leads for recording epsilon waves in patients with arrhythmogenic right ventricular dysplasia. The epsilon waves are usually difficult to record using only the standard 12 leads. The right arm electrode is placed at the manubrium and the left arm electrode at the xiphoid. Additionally, the left foot electrode may be moved to position V4. Leads I, II, and III are used for recording.
§ Other modifications: Other special leads can be created if the P waves cannot be visualized by placing the right arm electrode at V1 position and the left arm electrode anywhere to the left of the sternum or more posteriorly at V7 position. Lead I is recorded.
§ Esophageal and intracardiac electrodes: These electrodes can be connected to any precordial or V lead, usually V1, for recording atrial activity (P waves) if the P wave cannot be visualized in the surface ECG.
§ A pill electrode can be swallowed and positioned behind the left atrium in the esophagus and connected to a precordial lead usually V1. The ECG is recorded in V1.
§ An electrode can also be inserted transvenously and positioned into the right atrium. The electrode is connected to a precordial lead and recorded as above.
§ A central venous catheter, which is often already in place for intravenous administration of medications, is filled with saline. A syringe needle is inserted to the injecting port of the central line and attached with an alligator clamp to a precordial lead, usually V1. The ECG is recorded in V1. This special lead is for recording atrial activity if the P waves are not visible in the surface ECG.
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