An electrocardiogram (EKG or ECG) is a non-invasive diagnostic tool that records the electrical activity of the heart as waveforms. This electrical mapping assesses the heart’s rhythm and rate, offering insights into its function and identifying conditions such as arrhythmias or coronary artery disease. The standard 12-lead ECG is a widely used method that captures 12 distinct views of the heart’s electrical activity. These views are derived from placing ten physical electrodes on the patient—four on the limbs and six on the chest. Accurate placement is necessary, as improper positioning can lead to distorted data and misinterpretation.
Preparing the Patient and Equipment
Before applying electrodes, preparation ensures a clear, artifact-free recording. The patient should be placed in a supine position, or a semi-Fowler’s position if lying flat is not possible. The patient must be relaxed, with arms resting at their sides and legs uncrossed, as muscle tension or shivering can introduce interference (artifact) into the tracing.
Skin preparation minimizes electrical resistance between the skin and the electrode. Oils, sweat, or lotions should be removed by cleaning the sites with an alcohol wipe or by lightly abrading the skin surface. Excessive body hair must be shaved or clipped to ensure the adhesive electrode patch makes full contact with the skin.
The equipment requires a check to confirm the ECG machine is functioning and that the ten electrodes are fresh and connected to their corresponding lead wires. The 12 leads recorded include three bipolar limb leads (I, II, III), three augmented unipolar limb leads (aVR, aVL, aVF), and six unipolar precordial leads (V1-V6).
Placing the Limb Electrodes
The four limb electrodes (RA, LA, RL, LL) are placed on the fleshy parts of the limbs, avoiding bony prominences like the wrists or ankles. Placement on the upper arms and thighs is preferred to reduce muscle movement artifact. Positioning should be uniform across corresponding limbs.
The RA electrode is placed on the right upper extremity, and the LA electrode goes on the left upper extremity. The LL electrode is positioned on the left lower extremity. The RL electrode is placed on the right lower extremity and functions as the electrical ground reference.
These four electrodes establish the frontal plane leads (I, II, III, aVR, aVL, aVF), providing a vertical view of the heart’s electrical axis. Although less sensitive to minor shifts than the chest leads, inconsistent positioning can still alter recorded voltages and affect interpretation.
Mapping the Precordial (Chest) Electrodes
The six precordial electrodes (V1 through V6) provide the horizontal plane view of the heart and require precise anatomical landmark identification. The process begins with locating the sternal angle (Angle of Louis), a horizontal ridge felt where the manubrium meets the sternum. This bump marks the location of the second rib.
By sliding a finger down from the second rib, the technician counts down to the fourth intercostal space (ICS). Electrode V1 is placed in the fourth ICS immediately to the right of the sternum, and V2 is placed in the fourth ICS immediately to the left of the sternum.
Next, the technician locates the fifth intercostal space and the midclavicular line, a vertical line running down from the middle of the collarbone. Electrode V4 is placed where the fifth intercostal space intersects this midclavicular line. The nipple is not a reliable landmark and should not be used for placement.
Electrode V3 is then placed directly between the positioned V2 and V4 electrodes. This placement is often slightly diagonal, positioning it midway on the chest wall between the two established points.
The final two electrodes, V5 and V6, must be placed on the same horizontal level as V4, following the fifth intercostal space. V5 is placed on the anterior axillary line (vertical line from the front fold of the armpit). V6 is placed on the midaxillary line (vertical line from the middle of the armpit). Ensuring V4, V5, and V6 form a straight horizontal line is important, as misaligning them can distort the views of the heart’s lateral wall.