The 12-lead ECG is a vital tool for EMT’s and paramedics in both the prehospital and hospital setting. It is extremely important to know the exact placement of each electrode on the patient. Incorrect placement can lead to a false diagnosis of infarction or negative changes on the ECG.
|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 level as V4|
|V6||Midaxillary line at the same level as V4 and V5|
|RL||Anywhere above the ankle and below the torso|
|RA||Anywhere between the shoulder and the elbow|
|LL||Anywhere above the ankle and below the torso|
|LA||Anywhere between the shoulder and the elbow|
- Up to 50% of cases have the V1 and V2 electrodes in a more superior location, which can mimic an anterior MI and cause T wave inversion.
- Up to 33% of cases have the precordial electrodes (V1-V6) inferiorly or laterally misplaced, which can alter the amplitude and lead to a misdiagnosis.
- RA/LA Reversal: Lead I is inverted, Lead II and III are reversed, aVR and aVL are reversed
- RA/RL Reversal: Lead II shows isolated asystole, aVF and aVR are identical
- LA/LL Reversal: Lead III is inverted, aVL and aVF are reversed
The ECG leads are grouped into two electrical planes. The frontal leads (Lead I-III, aVR-F) view the heart from a vertical plane, while the transverse leads (V1-V6) view the heart from a horizontal plane.
|Lead||(-) Electrode||(+) Electrode||View of Heart|
|aVR||LA + LL||RA||None|
|aVL||RA + LL||LA||Lateral|
|aVF||RA + LA||LL||Inferior|
One of the most common questions regarding a 12-lead ECG is why there are only 10 electrodes. It’s important to fully understand what the term “lead” actually means. A lead is a view of the electrical activity of the heart from a particular angle across the body. Think of a lead as a picture of the heart and the 10 electrodes give you 12 pictures. In other words, a lead is a picture that is captured by a group of electrodes.
The heart’s electrical signal is very small and unfortunately this can be combined with other signals of similar frequency to create artifact. It’s not uncommon for 12-lead ECG’s to have some form of artifact; however, it’s important to try to reduce any interference to ensure an accurate ECG. Below is a list of guidelines that will help reduce artifact when performing ECG’s.
- Place the patient in a supine or semi-Fowler’s position. If the patient cannot tolerate being flat, you can do the ECG in a more upright position.
- Instruct the patient to place their arms down by their side and to relax their shoulders.
- Make sure the patient’s legs are uncrossed.
- Move any electrical devices, such as cell phones, away from the patient as they may interfere with the machine.
- Dry the skin if it’s moist or diaphoretic.
- Shave any hair that interferes with electrode placement. This will ensure a better electrode contact with the skin.
- Rub an alcohol prep pad or benzoin tincture on the skin to remove any oils and help with electrode adhesion.
- Check the electrodes to make sure the gel is still moist.
- Do not place the electrodes over bones.
- Do not place the electrodes over areas where there is a lot of muscle movement.
Visit the ECG section to learn how to interpret 12-lead ECG's and identify the different types of heart rhythms.