ECG: 12-lead ECG

Principle

Fig. 1 Placement and vector representation of the precordial leads.

Unipolar chest leads, the precordial leads

When unipolar leads are recorded from the chest wall, the exploring electrode is connected to the positive pole of the ECG and the negative to the central terminal CT of Wilson (see ECG: augmented limb leads) at the sternum. By convention, the following sites are normally selected (Fig. 1):

• V1, the fourth intercostal space just to the right of the sternum

• V2, the fourth intercostal space just to the left of the sternum

• V3, midway between V2 and V4

• V4, the fifth intercostal space in the midclavicular line

• V5, the left anterior axillary line at the same horizontal level as V4

• V6, the left midaxillary line at the same horizontal level as V4.

Since the precordial leads V1, V2, V3 (the right precordial leads) and V4, V5, and V6 (left precordial leads) are placed directly on the chest, close to the heart, they do not require augmentation. CT is used as reference (negative input of the ECG machine), and consequently these leads are considered to be unipolar. The precordial leads view the heart's electrical activity in the so-called horizontal plane, in which the electrical Z-axis is located.

The QRS complex should be negative in lead V1 and positive in lead V6. The QRS complex should show a gradual transition from negative to positive between leads V2 and V4.

The precordial leads are always recorded together with the three basic Einthoven leads I, II and III (see ECG: basic electrocardiography) and the augmented leads (see ECG: augmented limb leads). Fig.2 presents them all 12 for a healthy subject.

Fig.2. Normal 12-lead ECG

Ground electrode

In modern four-lead (the augmented leads and V5) and twelve-lead ECGs, an additional electrode is the ground electrode (usually green). This electrode is placed on the right leg by convention, although in theory it can be placed anywhere.

With a three-lead ECG, when one dipole is viewed, the remaining lead becomes the ground lead by default. Notice, that the ground lead is not the reference electrode (generally CT, see also ECG: augmented limb leads). The ground electrode suppresses 50 or 60 Hz hum of the mains.

Application

The equiphasic (or isolectric or biphasic, see bottom configuration Fig. 2 of ECG: basic electrocardiography) of lead is referred to as the transition lead. When the transition occurs earlier than lead V3, it is referred to as an early transition. When it occurs later than lead V3, it is referred to as a late transition. There should also be a gradual increase in the amplitude of the R wave between leads V1 and V4. This is known as R wave progression. Poor R wave progression is a nonspecific finding. It can be caused by conduction abnormalities, myocardial infarction, cardiomyopathy, and other pathological conditions.

More info

The twelve leads, each recording the activity from a different perspective, which also correlates to the area of identifying acute coronary injury, are classified as follows.

The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior or diaphragmatic wall of the left ventricle.

The lateral leads (I, aVL, V₅ and V₆) look at the electrical activity from the vantage point of the lateral wall of left ventricle. Because the positive electrode for leads I and aVL are located on the left shoulder, leads I and aVL are sometimes referred to as the high lateral leads. Because the positive electrodes for leads V5 and V6 are on the patient's chest, they are sometimes referred to as the low lateral leads.

The septal leads, V₁ and V₂ look at electrical activity from the vantage point of the septal wall of the left ventricle. They are often grouped together with the anterior leads.

The anterior leads, V₃ and V₄ look at electrical activity from the vantage point of the anterior wall of the left ventricle.

In addition, any two precordial leads that are next to one another are considered to be contiguous. In other words, even though V4 is an anterior lead and V5 is a lateral lead, they are contiguous because they are next to one another.

Lead aVR offers no specific view of the left ventricle, but views the endocardial wall from the right shoulder.

The modern ECG machine is completely integrated with an analog front end, a 12- to 16-bit analog-to-digital (A/D) converter, a computational microprocessor, and dedicated input-output (I/O) processors. These systems compute the 12 lead signals and analyze them with a set of rules. Fig. 3 shows the ECG of a heartbeat and the types of measurements that might be made on each of the component waves of the ECG and used for classifying each beat type and the subsequent cardiac rhythm.

Fig. 3 The numerous ECG measurements that can be made with computer-based algorithms, with for instance artificial neural networks, wavelet analysis, component analysis. The measurements are primarily durations, amplitudes, and areas.