04 October 2017

Standard 12-lead ECG

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As a most valuable non-invasive diagnostic tool, the 12-lead ECG records the heart’s electrical activity as waveforms. It’s important to properly attach each electrode to a patient, because incorrect placement can lead to a false diagnosis and an unnecessary treatment or a lack of one.

Electrode placement

In a 12-lead ECG, there are 12 leads calculated using a 10 electrodes.

Limb (extremity) electrodes

  • RA (right arm) - between the right shoulder and right wrist
  • RL (right leg) - below the right torso and above the right ankle
  • LA (left arm) - between the left shoulder and the left wrist
  • LL (left leg) - below the left torso and above the left ankle
Standard placement of the limb electrodes in electrocardiography
Standard placement of the limb electrodes in electrocardiography (source: OpenStax College).

The positions of the upper limb electrodes can vary from wrist to shoulder, but they should be uniformly distributed. For instance, if one electrode is attached to the right wrist, the other one should be on left wrist, not shoulder. The same rule applies to the positions of the lower limb electrodes.

Chest (precordial) electrodes

  • V1 - 4th intercostal space to the right of the sternum
  • V2 - 4th intercostal space to the left of the sternum
  • V3 - midway between V2 and V4
  • V4 - 5th intercostal space at the midclavicular line
  • V5 - anterior axillary line at the same horizontal level as V4
  • V6 - midaxillary line at the same horizontal level as V4 and V5
Standard placement of the precordial electrodes in electrocardiography
Standard placement of the precordial electrodes in electrocardiography (source: OpenStax College).

When placing electrodes on female patients, electrodes V3- V6 should be placed under the breasts rather than on. Also, patient’s nipples shouldn’t be used as reference points for locating the electrode position, because their locations vary from patient to patient. Up to 50% of ECGs have V1 and V2 electrodes in a more superior location, which can cause T wave inversion; and up to 33 % have chest electrodes inferiorly or laterally misplaced, which can alter the amplitudes.

Lead Groups

In order to provide 12 different perspectives on the heart activities, the ECG leads are grouped into two electrical planes: vertical and horizontal.

Vertical plane (frontal leads)

Using a 4 limb electrodes, you get 6 frontal leads that provide informations about the heart’s vertical plane. They are explained by Einthoven’s triangle on Figure 3. Bipolar leads are: I goes from RA to LA, II from RA to LL and III from LL to LA. Unipolar leads (augmented vectors) are: aVF points to feet, aVL to LA and aVR to RA.

Graphical representation of Einthoven's triangle
Graphical representation of Einthoven's triangle (source: OpenStax College).

Horizontal Plane (transverse leads)

Using a 6 chest electrodes, you get 6 transverse leads that provide informations about the heart’s horizontal plane: V1, V2, V3, V4, V5 and V6. They are unipolar and require only a positive electrodes, the negative pole of all 6 leads is at centre of the heart.

Spatial orientation of ECG leads
Spatial orientation of ECG leads (source: OpenStax College).

So, the main purpose of the 12-lead ECG is to screen patients for the possible cardiac ischemia, especially for the acute ST-elevation myocardial infarction, i.e. the heart attack. The quick identification and appropriate medical performances are crucial.

Sources:

  • https://www.cablesandsensors.com/pages/12-lead-ecg-placement-guide-with-illustrations
  • http://www.emtresource.com/resources/ecg/12-lead-ecg-placement/
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