The Einthoven triangle shows the relationship of the three bipolar extremity leads (I, II, and III). Similarly the triaxial shows the relationship of the three unipolar extremity leads (aVR, aVL, and aVF). For convenience, we can combine these six extremity leads intersect at a common point. The result is the hexaxial lead. The hexaxial shows the spatial orientation of the six extremity leads (I, II, III, aVR, aVL, and aVF).
The exact relationships among the three unipolar extremity leads can also be describved mathematically. However, for present purposes, the following simple guidelines allow you to get an overall impression of the similarities between these two sets of leads.
As you might expect by looking at the hexaxial diagram, the pattern in lead aVL uusually resembles that in lead I. Lead aVR and lead II, on the other hand, point in opposite directions. Therefore, the P-QRS-T pattern recorded by lead II. (For example, when lead II shoes a qR pattern lead aVR usually shows an rS pattern.) Finally, the pattern shown by lead aVF usually but not always resemble that shown by lead III.
CHEST (PRECORDIAL) LEADS
The chest leads (V1 to V6) show the electrical currents of the heart detected by electrodes placed at different positions on the chest wall. The chest leads used today are also unipolar leads in that they measure the voltage in any one location relative to zero potential. By convention the six leads are placed as follows:
Lead V1 is recorded with the electrode in the fourth intercostal space just to the right of the
Lead V2 is recorded with the electrode in the fourth intercostal space just to the lest of the sternum.
Lead V3 is recorded on a line midway between leads V2 and V4.
Lead V4 is recorded in the midclavicular line in the fifth interspace.
Lead V6 is recorded in the midaxillary line at the same level as lead V4.
The chest leads are recorded simply by means of electrodes (usually attached to suction cups to hold them in place on the chest) at six designated locations on the chest wall .
TAKING AN ECG
Now you are ready to take an ECG. The extremity electrodes are attached toe the patient. First, the machine is standardized. Then the dial in the electrocardiograph is turned to lead 1. Several P-QRS-T cycles are run. Nest, the dial is advanced to lead II and the ECG records a few more cycles. This is repeated for leads III, aVR, aVL, and aVF. Next the dial is turned to the V position for the chest leads. The suction cup is placed in the V1 position, and so on until the six V leads have been recorded. The result is a long ECG strip showing the 12 leads recorded sequentially.
THE 12-LEAD ECG: FRONTAL AND HORIZONTAL PLANE LEADS
You may now be wondering why we use 12 leads in clinical electrocardiograph; why not 12 or 22? The reason for exactly 12 leads is partly historical, a matter of the way the ECG had evolved over the years since Einthoven’s original three bipolar extremity leads. There is nothing sacred about the electrocardiographer’s dozen. In some cases, for example, we do record additional leads by placing the chest electrode at different positions on the chest wall. There are good reasons for using multiple leads. The heart, after all, is a three-dimensional structure, and its electrical currents spread out in all directions across the body. Recall that we described the ECG leads as being like photographs by which we can see the electrical activity of the heart from different locations. To a certain extent, the more points we record from the more accurate will be our representation of the heart’s electrical activity.
The importance of multiple leads is illustrated in the diagnosis of myocardial infarction (MI). An MI typically affects one localized portion of the left ventricle. The ECG changes produced by an anterior MI are usually best shown by the chest leads, which are close to and face the injured anterior surface of the heart, while the changes seen with an inferior MI usually appear only in leads such as II, III and aVF, which face the injured inferior surface of the heart. The 12 leads therefore provide a three-dimensional view of the electrical activity of the heart.
Specifically, the six extremity leads (I, II, III, aVR, aVL, aVF) will show electrical voltages transmitted onto the frontal plane of the body. For example, if you walk up to and face a large window, the window will be parallel to the frontal plane of your body. Similarly heart voltages directed upward and downward and to the right and left will be presented by the frontal plane leads.
The chest leads (V1 through V6) present heart voltages from a different viewpoint, on the horizontal plane of the body. The horizontal plane cuts your body into an upper and a lower half. Similarly the chest leads present heart voltages directed anteriorly (front), posteriorly (back), and to the right and left.
We therefore have two sets of ECG leads six extremity leads (three unipolar and three bipolar), which record voltages on the frontal plane of the body, and six chest (precordial) leads, which record voltages on the horizontal plane. Together these 12 leads provide a three-dimensional picture of atrial and ventricular depolarization and repolarization.
Let us begin our description of the normal ECG with the first waveform seen in any cycle, the P wave, which represents atrial depolarization. Atrial depolarization is initiated by the sinus node, in the right atrium. The atrial depolarization path therefore spreads from right to left and downward toward the AV junction. Therefore we can represent the spread of atrial depolarization by an arrow that points downward and to the patient’s left.
Notice that the positive pole of lead aVR points upward in the direction of the right shoulder. The normal path of atrial depolarization, as described, spreads downward toward the left leg (away from the positive pole of lead aVR). Therefore, with normal sinus rhythm, lead aVR will always show a negative P wave. Conversely, lead II is oriented with its positive pole pointing downward in the direction of the left leg. Therefore the normal atrial depolarization path will be directed toward the positive pole of lead II. When normal sinus rhythm is present, lead II will always record a positive (upward) P wave.
When the AV junction is pacing the heart atrial depolarization will have to spread up the atria in a retrograde direction, just the opposite of what happens in normal sinus rhythm. Therefore an arrow representing the spread of atrial depolarization in AV junctional rhythm will point upward and to the right, just the opposite of normal sinus rhythm. Spread of atrial depolarization upward and to the right will result in a positive P wave in lead aVR, since the stimulus is spreading toward the positive pole of lead aVR. Conversely, lead II will show a negative P wave . we will discuss AV junctional rhythms in detail in Part II. The topic was introduced here simply to show how the polarity of the P wave in lead aVR and lead II depends on the direction of the atrial depolarization and how the patterns can be predicted using simple basic principles.