To find it on yourself, place your fingers gently at the base of your throat in a central position and move your fingers downward until you can feel the top of the sternum, or rib cage.
From this position, continue to move your fingers downward until you feel a boney lump. This is the "angle of Louis". The angle of Louis is most easily found when the patient is lying down as the surrounding tissue is tighter against the rib cage. From the angle of Louis, move your fingers to the right and you will feel a gap between the ribs. This gap is the 2nd Intercostal space.
From this position, run your fingers downward across the next rib, and the next one. The space you are in is the 4th intercostal space. Where this space meets the sternum is the position for V1. 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. The equations follow:. WCT is computed by connecting all three limb electrodes via electrical resistance to one terminal. This terminal will represent the average of the electrical potentials recorded in the limb electrodes. WCT serves as the reference point for each of the six electrodes which are placed anteriorly on the chest wall.
The chest leads are derived by comparing the electrical potentials in WCT to the potentials recorded by each of the electrodes placed on the chest wall. There are six electrodes on the chest wall and thus six chest leads Figure Each chest lead offers unique information that cannot be derived mathematically from other leads. Since the exploring electrode and the reference is placed in the horizontal plane, these leads primarily observe vectors moving in that plane.
Hair on the chest wall should be shaved before placement of electrodes. This improves quality of the registration. The ECG leads may be presented chronologically i. Chronological order does not respect that leads aVL, I and -aVR all view the heart from a similar angle and placing them next to each other can improve diagnostics.
The Cabrera system should be preferred. In the Cabrera system, the leads are placed in their anatomical order. As mentioned earlier, inverting lead aVR into —aVR improves diagnostics additionally. All modern ECG machines can display the leads according to the Cabrera system, which should always be preferred. Note the clear transition between the waveforms in neighbouring leads. There are conditions that may be missed when utilizing the lead ECG.
Fortunately, researchers have validated the use of additional leads to improve diagnostics of such conditions. These are now discussed. Infarction of the right ventricle is unusual but may occur if the right coronary artery is occluded proximally. None of the standard leads in the lead ECG is adequate for diagnosing right ventricular infarction. 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.
See illustration below:. In picture A above, the negative electrode is on the right arm and the positive electrode is on the left arm. This is lead I. Lead I records electrical difference between the left and right arm electrodes. In picture B above, the negative electrode is on the right arm and the positive electrode is on the left leg left lower chest. This is lead II.
Lead II records electrical differences between the left leg and right arm electrodes. In picture C, the negative electrode is on the left arm and the positive electrode is on the left leg left lower chest. Picture C depicts lead III. Lead III records electrical difference between the left leg and the left arm electrodes.
The other three frontal plane limb leads are called the augmented Vector leads. The Galvanometer EKG machine records potential differences and, therefore, the technique is Bipolar potential site A minus potential site B.
However, if the potential of B is zero the recorder records only the potential site A. As mentioned earlier, unipolar leads measure the electric impulses at only one point, instead of across two points, as the first three leads. With these V leads, the second site is so there is noneed to measure from two pointes, only one point is needed.
The machine automatically makes the needed connection to measure the voltage from these areas. As the above illustrations point out, the six limb leads measure the electrical activity of the heart from the frontal plan.
0コメント