The Leads

We will look at the different segments, points, and waves of the ECG in detail in the following chapters.

For now, it is important to understand that, even though each ECG comprises information from many different leads, each of the leads refers to the same heart and the same electrical heart axis. The only difference between the leads is their perspective: each lead considers electrical conduction in the heart from a different point of view.

Imagine you want to make a movie, and you plan to shoot each scene from multiple perspectives. To do this, you will need several cameras. Clearly, the use of 12 cameras does not mean that 12 different movies are being shot. The 12 ECG leads can be likened to these cameras. During the ECG, heart conduction is “filmed” from 12 different perspectives. Each lead is positioned at a certain point in relation to the heart, and always remains in the same position. When interpreting the readout for a particular ECG lead, you must keep the position of the lead in relation to the heart, and what this means in terms of the positive or negative deflection of waves and segments, in mind.

So, during an ECG examination, all 12 leads are showing the same change in excitation, but from a different angle. Thus, if one or more leads suggest atrial fibrillation, but the other leads suggest cardiac arrhythmias such as ventricular fibrillation or asystolia, this is considered a technical artifact.

Likewise, the presence of clear P-QRS complexes in lead II, but no P waves in the remaining leads, indicates sinus rhythm.

One important scenario must be considered: some ECG machines record limb leads and augmented limb leads at one time-point, and the precordial leads immediately afterwards. This means that, depending on the technical settings of your ECG machine, two different conduction patterns, such as sinus rhythm and ventricular tachycardia, are recorded on the same ECG. In this case, the limb leads will show one situation, and the precordial leads the other.

There are three types of basic ECG leads. These were developed by the cardiologists Einthoven, Goldberger, and Wilson. The first type are called the Einthoven limb leads, and these three leads are denoted by the Roman numerals I, II, and III. The second type are called the Goldberger augmented voltage limb leads, and these three leads are denoted by the abbreviation aV. The position of the aV lead on the patient is denoted by the addition of the letter L, R, or F: “aVL, as in left”; “aVR, as in right”; and “aVF, as in foot”. The third type are called the Wilson chest or precordial leads, and these six leads are denoted as V1 to V6. There are thus 12 basic ECG leads, which explains why we often speak of the 12-lead ECG. The limb leads provide a frontal view of the electrical activity of the heart, while the precordial leads provide a horizontal view. For certain clinical problems, additional precordial leads are used. More on that later.

To get the right camera perspective, we must first position the cameras correctly.

In the case of the Einthoven and aV limb leads, an electrode is attached to each limb. The color code for the placement differs between Europe and North America. In Europe, a red electrode is used for the right arm. We then continue clockwise using the traffic light system: a yellow electrode is placed on the left arm, and a green electrode is placed on the left leg. The grounding lead is attached to a black electrode, which is placed on the right leg. In North America, the white electrode is used for the right arm, black for the left arm, red for the left leg, and green for the right leg.

The electrodes can be attached to any part of the limb, the shoulder or the wrist and the thigh or the ankle.

From these four electrodes, Einthoven and Goldberger developed a total of six leads.

In the Einthoven limb lead system, lead I measures the voltage between the right arm and the left arm, lead II measures the voltage between the right arm and the left leg, and lead III measures the voltage between the left arm and the left leg. Assuming that the electrode on the left leg is exactly caudal to the heart, lead II points diagonally down to the left and lead III points diagonally down to the right. These three limb leads form the points of the so-called Einthoven triangle.

The Goldberger aV lead system uses the same three electrodes as the Einthoven system. However, a combination of inputs from two limb electrodes is used as the negative pole, with a different combination being used for each lead. Thus, the angle of the lead is rotated by 30 degrees in comparison with the Einthoven limb leads.

For the lead aVF, as in “foot”, the two arm electrodes form the negative pole and the left leg electrode is positive. For the lead aVL, as in “left”, the right arm and left leg electrodes form the negative pole and the left arm is positive. For lead aVR, as in “right”, the left arm and left leg electrodes form the negative pole and the right arm is positive.

If the six extremity leads are presented according to the same zero point, the famous Cabrera circle is obtained. This is not complicated at all: it is simply a diagram that is used to demonstrate the electrical axis of the heart, as measured by all six limb leads.

The limb leads provide a frontal view of the electrical activity of the heart. To map the horizontal plane, each complete ECG assessment also includes Wilson’s six precordial leads. Together with the six limb leads, it becomes clear why we often speak of the 12-lead ECG.

As in the Goldberger system, the Wilson precordial lead system uses a central terminal as the negative pole, but here this is derived using the three limb leads. The six chest wall electrodes then act as the positive poles for V1–V6. The precordial electrodes are positioned on the thoracic wall according to the following scheme:

-          V1 and V2 are placed in the 4th intercostal space, to the right and left of the sternum

-          V4 is placed in the fifth intercostal space and in the midclavicular line

-          V3 is placed between V2 and V4

-          V5 is placed at the same height as V4, but at a more lateral position in the anterior axillary line

-          V6 is placed at the same level as V4 and V5, but at a more lateral position in the midaxillary line.

In certain clinical situations, additional precordial leads are used. For example, if the 12-lead ECG is unremarkable but a myocardial infarction is still suspected, chest wall leads V7–V9 are used to assess the posterior cardiac wall, and the right cardiac leads V3r to V6r are used to assess the right ventricular myocardium.

V7 to V9 are positioned at the same level as electrodes V4 to V6.

V7 is placed in the posterior axillary line, V9 is placed in the paravertebral line, and V8 is placed on the scapula between V7 and V9.

The right cardiac leads are positioned on the right side of the chest, in positions that are analogous to those of the left precordial leads.

This chapter has explained that the ECG recording is effectively a short film about the heart conduction system in a given patient. It has also explained the ECG leads, and how the ECG electrodes are placed. In this graph, the 12 standard ECG leads are shown in three-dimensional space.

In the next chapter, we will consider the heart axis.