An electrocardiogram (EKG or ECG) is a non-invasive procedure that registers the electrical activity of the heart over a period of time. This test provides information about the heart’s rhythm, rate, and the condition of the cardiac muscle tissue. The term “leads” refers not to the wires themselves, but to the electrical viewpoints created by placing small, conductive sensors, known as electrodes, on the patient’s skin. The standard 12-lead EKG uses ten electrodes to capture a comprehensive three-dimensional view of the heart’s electrical pathway. Accurate placement of these electrodes is fundamental to acquiring a reliable tracing for diagnostic purposes.
Preparing for Electrode Placement
Before any electrode is applied, preparing both the patient and the skin at the application sites is necessary to ensure optimal signal transmission. The patient should be positioned comfortably, typically lying flat on their back, which is known as the supine position. This relaxed posture minimizes muscle tension and movement that can interfere with the electrical recording.
Proper skin preparation is essential because the skin naturally acts as an electrical insulator. Oils, lotions, sweat, and dead skin cells all increase the electrical resistance, or impedance, between the skin and the electrode. Preparing the skin involves cleaning the site with mild soap and water or a non-alcoholic wipe, followed by vigorous drying to promote capillary blood flow and reduce impedance.
If excessive hair is present at the electrode sites, it should be clipped or shaved, as hair prevents the electrode’s adhesive and conductive gel from making secure contact with the skin. In some cases, a mild abrasive preparation, such as rubbing the area with gauze, is used to remove the outermost layer of dead skin cells. A poor connection can introduce artifacts, which are unwanted electrical signals that obscure the true cardiac rhythm.
Positioning the Limb Electrodes
The four limb electrodes are responsible for generating six of the 12 views, known as the frontal leads, which monitor the heart’s activity in the vertical plane. The American Heart Association (AHA) recommends placing these electrodes on the fleshy part of the limbs, specifically on the arms and legs distal to the shoulders and hips. Placement over bony prominences or joints should be avoided, as this can affect the signal and cause discomfort.
While the wrist and ankle are traditional placement sites, modern practice often involves placing the electrodes on the upper arms and thighs to reduce motion artifact. It is important that the electrodes on corresponding limbs are placed symmetrically to maintain consistent electrical axes for the frontal plane leads. Placing one electrode on the wrist and the other on the upper arm, for example, would introduce variability in the recorded signal. The cables are often color-coded, but regardless of the convention used, the right arm (RA), left arm (LA), left leg (LL), and right leg (RL) electrodes must be connected to their correct anatomical locations.
Positioning the Chest Electrodes
The placement of the six precordial, or chest, electrodes (V1 through V6) demands the greatest precision because these leads provide the six transverse views of the heart’s horizontal plane. These electrodes must be placed in specific intercostal spaces (ICS) and along defined anatomical lines to ensure the accuracy of the reading.
Correctly locating the fourth intercostal space is the foundational step, which begins by identifying the Angle of Louis. This is the slight, palpable horizontal ridge where the manubrium joins the body of the sternum, and this landmark aligns with the second rib. By locating this landmark, the technician can accurately count down to the fourth intercostal space.
Chest Electrode Placement
The six chest electrodes are placed as follows:
- V1 is placed in the fourth intercostal space immediately to the right of the sternum.
- V2 is placed in the fourth intercostal space immediately to the left of the sternum.
- V4 is positioned in the fifth intercostal space at the mid-clavicular line.
- V3 is placed exactly halfway between V2 and V4.
- V5 is placed on the same horizontal plane as V4, at the anterior axillary line.
- V6 is placed at the mid-axillary line, ensuring V4, V5, and V6 form a continuous horizontal line across the left chest.
For female patients, these electrodes should be placed underneath the left breast tissue at the appropriate anatomical landmarks, as the nipple is not a reliable reference point.
Ensuring Quality and Completing the Procedure
After ten electrodes are placed, a quality check of the electrical tracing is performed to identify and mitigate any artifacts that could interfere with the reading.
Common Artifacts
One common issue is a wandering baseline, which appears as a slow, undulating movement on the tracing, often caused by patient movement, respiration, or a loose electrode. Securing the electrodes or asking the patient to briefly hold their breath can resolve this type of interference.
Muscle tremor, or somatic artifact, presents as a fine, irregular jaggedness on the tracing and is caused by involuntary muscle contractions like shivering or tension. Ensuring the patient is warm and relaxed is the primary way to minimize this noise. Electrical interference appears as a thick, regular baseline and is caused by nearby electrical devices or poor machine grounding.
Once a clean, diagnostic-quality tracing has been successfully recorded, the procedure is complete. The electrodes should be removed gently, pulling the adhesive off in the direction of hair growth to minimize skin trauma. Any remaining gel or adhesive residue is wiped away, and the patient is assisted back to a comfortable position.