ECG Leads

eCg leads

The standard 12-Lead ECG uses ten wires connected to ten electrodes on the skin to generate 12-Leads (or views) of the electrical activity of the heart. There are six leads that show electrical fluctuations within the frontal plane (leads I, II, III aVR, aVL, and aVF). There are also six chest leads (V1-V6) which show electrical fluctuations within the horizontal plane. Together the ECG tracings from these 12 leads form the standard 12-Lead ECG.

The Limb Leads (I, II, III)

Lead II is one of the six electrical views of the heart within the frontal plane. These additional views are available by electronically manipulating the polarity of the electrodes on the three limbs; the left arm (LA), the right arm (RA) and the left leg (LL). An additional electrode is placed on the right leg (RL) to provide an earth signal. These different electrical views are collectively referred to as the ‘limb leads’ because they are traditionally acquired by placing the electrodes on the limbs.

Leads I, II and Ill (previously referred to as the ‘bipolar’ leads) measure the electrical fluctuations from different angles between two electrical poles placed on or near the limbs; one positive and the other negative.

The location of the positive electrodes for each lead can be likened to the position from which the heart’s electrical activity is viewed.

The Augmented Limb Leads

The same three limb electrodes used to produce limb leads I, II and III, can also provide us with another three views of the heart. These are known as augmented views. Like leads I, II and III, these augmented views are also within the frontal plane. The augmented leads are also termed ‘unipolar’ by the way they are obtained. There is a single positive electrode that is referenced against a combination of the other limb electrodes (Wilson’s Central Terminal). The positive electrodes for these augmented leads are located on the left arm (aVL), the right arm (aVR), and the left leg (aVF). In practice, these are the same electrodes used for leads I, II and III. (The ECG machine does the actual switching and rearranging of the electrode designations.) The three augmented leads, along with the three standard bipolar limb leads, are depicted as shown using the axial reference system.

Figure 1.2 Augmented Leads

Recall that the location of the positive electrode for each lead can be likened to the position from which the heart’s electrical activity is viewed. An impulse travelling toward a positive electrode provides an upright or positive deflection on the ECG and an impulse travelling away from a positive electrode produces a downward or negative deflection on the ECG.

Figure 1.3: Positive Deflection

aVR

The normal conduction of a healthy heart is directly away from the positive electrode used for aVR, (located on the right arm) and hence the resulting deflections are predominantly negative. In fact, aVR is almost an inverted image of lead II, as it is sampling the electrical fluctuations from almost the opposite direction to lead II. In aVR the P-wave, QRS complex and T wave are all normally seen as negative deflections. A positive P wave and QRS complex in aVR is likely to indicate incorrect limb lead placement.

Figure 1.4: Normal 12 Lead ECG The orientation of lead aVR measures the electrical activity of the heart across the relatively small muscle mass of the right atrium, and apart from aiding in cardiac axis determination, or assisting in detecting incorrect limb lead placement (such as the RA electrode being place inadvertently on the left arm etc.), aVR provides no other particularly useful purpose for routine ECG analysis in Paramedic practice. For this reason aVR has been referred to as the ‘orphan’ lead.

aVL

Lead aVL (augmented Voltage Left), is created by making the left arm electrode positive and combining the negative electrical signal from both the left leg and right arm electrodes to create the central terminal. Lead aVL measures the electrical activity of the heart across the lateral aspect of the relatively large muscle mass of the left ventricle. The aVL’s view is from the left arm. Notice in Figure 1.1 that aVL views the heart from a direction adjacent to lead I.Together, lead I and aVL provide useful additional information about the electrical activity through the left lateral aspect of the heart.

aVF

Lead aVF (augmented Voltage Foot) uses the combined and then amplified signal from the right arm (RA) electrode and the left arm (LA) electrode as the negative reference point (the central terminal), and the left leg (LL) electrode as the positive electrical pole. Like leads II and III, lead aVF keeps the LL as the positive terminal. Accordingly, each of these three leads has a similar, yet slightly different view of the inferior aspect of the heart.

Limb Lead Summary

Together, the limb leads (I, II, III, aVR, aVL and aVF) provide six views of the electrical activity of the heart, but they are all confined to the frontal (vertical) plane, as represented in Figure 1.1.

The Chest Leads (V1 - V6)

Recall that the limb leads view the electrical fluctuations of the heart from different points within the frontal vertical plane. Because these limb leads are confined to the vertical plane, they can only sense electrical impulses from angles within that plane. The limb leads cannot measure the heart’s electrical activity from the anterior or posterior perspectives, as these angles do not lie in the vertical plane. Therefore a different set of electrodes within the horizontal plane is required. Leads used within this plane are often referred to as the ‘chest leads’, the ‘precordial leads’ or the V-Leads.

Six electrodes placed in particular locations across the chest are used in conjunction with ‘Wilson’s Central Terminal’ to generate six chest or precordial leads called V1, V2, V3, V4, V5 and V6. The ‘V’ stands for voltage, however some references use ‘C’ for chest electrodes or precordial electrodes. The chest leads view the electrical activity in a horizontal plane through the heart to each respective chest electrode.

As the chest leads use Wilson’s Central Terminal as a common reference point, the location of the limb electrodes can affect the accuracy and quality of the ECG tracing. Therefore, correct limb lead placement is of greater importance than what was previously required for 3-Lead ECG monitoring.

The chest electrodes (V1 - V6) are located at specific anatomical locations around the anterior and left lateral chest wall. Appropriate lead placement for both the limb and chest electrodes is discussed in detail later.

Figure 1.5: The Chest (V) Leads

Recall that the location of the positive electrode for each lead can be likened to the position from which the heart’s electrical activity is viewed. In the Figure 1.5, it is easy to see that the V1 and V2 electrodes ‘view’ electrical activity of the septum, V3 and V4 electrodes will ‘view’ electrical activity from the anterior perspective, while V5 and V6 electrodes would ‘view’ the electrical activity from the left lateral aspect of the heart. Figure 1.6 summarises the lead views, and the location of each of the chest electrodes.

V1: Right 4th IC space

V2: Left 4th IC space

V3: Halfway between V2 and V4

V4: Left 5th IC space, mid-clavicular line

V5: Horizontal to V4, anterior-axillary line

V6: Horizontal to V5, mid-axillary line

Figure 1.6: Placement of V-Lead Electrodes

Lead V1 measures the electrical activity across the horizontal plane from the front of the chest towards the central terminal (Wilson’s Central Terminal). From this perspective most electrical current is moving away from the V1 electrode with normal depolarisation. The deflections seen in V1 are thus predominantly negative, i.e. the P wave QRS complex and T wave are often seen inverted normally in V1.

Figure 1.7: View in the Horizontal Plane

Comparatively, Lead V6 measures the electrical activity across the horizontal plane from the left lateral aspect of the chest towards the central terminal. From this perspective most of the current of normal depolarisation is moving towards the V6 electrode and so the deflections seen on the ECG in V6 are predominantly positive or upright.

These perspectives are illustrated in Figure 1.7. The QRS deflections in V1 are predominantly negative and opposite to those seen in V6 which are predominantly positive. Now look at the R-Waves differences between the V-Leads. Notice that the R-Wave changes progressively from being small in leads viewing from the anterior of the chest to being tall in leads viewing from that lateral side of the chest. This is known as R-Wave progression.

Figure 1.8: Standard 12 Lead ECG Layout

The standard 12-Lead ECG (Figure 1.8 above) shows the grouping and order of the leads. Leads in the frontal plane are grouped on the left hand side of the ECG, whereas the leads of the horizontal plane are on the right.

It is useful to be aware that not all ECGs are printed out in this standard format, and in fact the positions of respective leads can be altered by the user. This is, however, the standard accepted layout that should be used by paramedics.

Many hospital acquired 12-Lead ECGs include an additional extended rhythm strip across the bottom of the page. Whilst this strip is usually lead II, this can in fact be set to display any lead of interest. In each case, the respective lead is labelled to aid identification. To avoid ECG interpretation errors, it is essential that the paramedic look at the label beside each ECG strip to ensure that the appropriate lead is being viewed.

Figure 1.9 Lead Grouping by Anatomical Location