Bipolar recordings utilize standard limb lead configurations depicted in the figure. By convention, lead I has the positive electrode on the left arm, and the negative electrode on the right arm, and therefore measures the potential difference between the two arms.
In this and the other two limb leads, an electrode on the right leg serves as a reference electrode for recording purposes. In the lead II configuration, the positive electrode is on the left leg and the negative electrode is on the right arm. Lead III has the positive electrode on the left leg and the negative electrode on the left arm.
These three bipolar limb leads roughly form an equilateral triangle with the heart at the center that is called Einthoven's triangle in honor of Willem Einthoven who developed the electrocardiogram in the early s. Whether the limb leads are attached to the end of the limb wrists and ankles or at the origin of the limb shoulder or upper thigh makes little difference in the recording because the limb can simply be viewed as a long wire conductor originating from a point on the trunk of the body.
A lead composed of a single positive electrode and a reference point is a unipolar lead. Chest leads: , , , , , and. It is a normal tracing shows normal sinus rhythm. This course is meant to give the nurse a practical education concerning the lead EKG. We will present the basic clinical aspects of the lead.
We will demonstrate how and where to attach the leads in certain places. We will also demonstrate the basics of interpreting the results. Then all you need is practice. Each lead EKG machine will have its own instructions for use.
Be sure you are familiar with the machines at your facility. The instructions for the machine will show you how to attach the leads to the patient. In most instances, the patient electrodes will be attached with the use of either flat elastic straps or by cloth Velcro straps. In most cases, the metal electrodes will also need to be coated with conductive gel prior to attachment to the patient.
Be sure to clean the electrodes before and after each use, as gel will tend to build up. Be sure to read the entire instruction manual for the machine including: how to start the machine, how to load the paper, how to calibrate the machine if needed , and any other pertinent information needed to safely and accurately run the machine.
Once you know how to operate the machine at your facility the next concept to understand is the placement of the leads. The limb leads are usually first. As the name implies, the limb leads are attached to the four limbs. This is usually accomplished by attaching the leads, according to instructions, on each wrist and each ankle. As mentioned earlier, the electrodes will be sensing the electrical impulses from the heart muscle at ht various locations and with various voltages, either positive or negative.
The lead EKG tracing below was obtained with universal lead placement. These marks are for the purpose of showing the leads as they are changed. Every time you see a blip mark, the next lead is being recorded on the tracing. Location of the Frontal Plane Axis: In order to accurately interpret the lead EKG, you must have an understanding of the electrical activity of the heart.
The direction in which the impulses flow in the heart is important. It is also important to understand that 12 different leads pick up those impulses as they travel in many different directions through the heart.
Remember that the normal conduction of the heart begins in the SA node. The wave of depolarization moves across the atria, through the AV node, into the Bundle of HIS, down the Bundle Branches, and finally through the Purkinje fibers which conduct the electrical impulses throughout the ventricles. The activity of the heart produces electrical potentials that can be measured on the surface of the skin. Using the galvanometer EKG machine , differences between electrical potentials at different sites of the body can be recorded.
Three of these leads are easy to understand, since they are simply the result of comparing electrical potentials recorded by two electrodes; one electrode is exploring, while the other is a reference electrode. In the remaining 9 leads the exploring electrode is still just one electrode but the reference is obtained by combining two or three electrodes. At any given instant during the cardiac cycle all ECG leads analyze the same electrical events but from different angles.
For some purposes e. On the other hand, for the purpose of diagnosis morphological changes e. The lead ECG is a trade-off between sensitivity, specificity and feasibility. Obviously, having leads which has been tested in several studies on acute myocardial infarction would improve sensitivity for many conditions, at the expense of specificity and certainly feasibility.
The other extreme, using only one lead would allow for diagnosing several arrhythmias but certainly not all and more importantly it would not allow one to diagnose morphological changes in the heart.
Later on it will become clear as to why multiple leads are necessary to diagnose morphological changes. The electrocardiograph presents one diagram for each lead. Voltage is presented on the vertical Y axis and time on the horizontal X axis of the diagram. The ECG paper has small boxes thin lines and large boxes heavy lines.
Small boxes are squares of 1 mm 2 and there are 5 small boxes inside each large box. Refer to Figure With normal gain calibration 10 mm on the vertical axis corresponds to 1 mV. Thus, 1 mm corresponds to 0. Anyone aiming to become proficient in ECG interpretation must master any paper speed.
This figure should be studied carefully and attention should be paid to differences on the X-axis there are no difference with respect to the Y-axis. The reader should know these differences as it is often necessary to manually measure the time duration of various waves and intervals on the ECG.
Every lead represents differences in electrical potentials measured in two points in space. The simplest leads are composed using only two electrodes.
The electrocardiograph defines one electrode as exploring positive and the other as reference negative electrode. In most leads, however, the reference is actually composed of a combination of two or three electrodes. Regardless of how the exploring electrode and the reference is set up, the vectors have the same impact on the ECG curve. Please refer to Figure The electrical activity of the heart can be observed from the horizontal plane and the frontal plane. The ability of a lead to detect vectors in a certain plane depend on how the lead is angled in relation to the plane, which in turn depend on the placement of the exploring lead and the reference point.
For pedagogical purposes, consider a lead with one electrode placed on the head and the other electrode placed on the left foot. The angle of this lead would be vertical, from the head to the foot. This lead is angled in the frontal plane and it will primarily detect vectors traveling in that plane. Refer to Figure 17 panel A. Now consider a lead with an electrode placed on the sternum and the other electrode placed on the back on the same level.
This lead will be angled from the back to the anterior chest wall, which is the horizontal plane. This lead will primarily record vectors traveling in that plane. A schematic illustration is provided in Figure Refer to Figure 17 panel B. These leads are therefore excellent for detecting vectors traveling in the frontal plane.
The chest precordial leads V1, V2, V3, V4, V5 and V6 have the exploring electrodes located anteriorly on the chest wall and the reference point located inside the chest. Hence, the chest leads are excellent for detecting vectors traveling in the horizontal plane. The remaining nine leads use a reference which is composed of the average of either two or three electrodes. This will be clarified shortly.
Given the electrode placements, in relation to the heart, these leads primarily detect electrical activity in the frontal plane. Figure 18 shows how the electrodes are connected in order to obtain these six leads. Considering lead I the electrode on the right arm serves as the reference, whereas the electrode on the left arm serves as the exploring electrode.
This means that a vector moving from right to left should yield a positive deflection in lead I. In lead aVF the electrode on the left leg serves as exploring electrode and the reference is actually composed by computing the average of the arm electrodes.
The average of the arm electrodes yields a reference directly north of the left leg electrode. Thus, any vector moving downwards in the chest should yield a positive wave in lead aVF. The same principles apply to lead aVR and lead aVL. Lead II, aVF and III are called inferior limb leads , because they primarily observe the inferior wall of the left ventricle Figure 18, coordinate system in upper panel.
Lead aVL, I and —aVR are called lateral limb leads , because they primarily observe the lateral wall of the left ventricle. Note that lead aVR differs from lead —aVR discussed below. All six limb leads are presented in a coordinate system, which the right hand side of Figure 18 panel A shows. To eliminate this gap, lead aVR can be inverted into lead —aVR.
It turns out that this is actually meaningful, as it facilitates ECG interpretation e. Whether lead aVR or —aVR is presented depends on national traditions.
Lead I compares the electrode on the left arm with the electrode on the right arm, of which the former is the exploring electrode. Lead II compares the left leg with the right arm, with the leg electrode being the exploring electrode. Lead III compares the left leg with the left arm, with the leg electrode being the exploring one.
In clinical electrocardiography this means that the amplitude of, for example, the R-wave in lead II is equal to the sum of the R-wave amplitudes in lead I and III. It follows that we need only know the information in two leads in order to calculate the exact appearance of the remaining lead.
Hence, these three leads actually carry two pieces of information, observed from three angles. These leads were originally constructed by Goldberger. In these leads the exploring electrode is compared with a reference which is based on an average of the other two limb electrodes. The letter a stands for augmented, V for voltage and R is right arm , L is left arm and F is foot. In aVR the right arm is the exploring electrode and the reference is composed by averaging the left arm and left leg.
Lead aVR can be inverted into lead —aVR which means that the exploring and reference point has switched positions , which is identical to aVR but upside-down. There are three advantages of inverting aVR into —aVR:. Despite these advantages lead aVR is unfortunately still used in the United States and many other countries. We recommend the use of —aVR but for the purpose of this course we will frequently present both leads.
If only one of these leads is shown, the reader may simply turn it upside-down to get a view of the desired lead. In lead aVF the exploring electrode is placed on the left leg, so this lead observes the heart directly from south.
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