An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that measures the electrical activity of the heart. This test uses small, sticky sensor pads called electrodes placed on the skin, which detect the heart’s electrical impulses. Recording a 12-lead ECG provides a comprehensive view of the heart’s function from twelve different angles, aiding in the diagnosis of conditions like irregular heart rhythms or signs of a heart attack. Although the name suggests twelve electrodes, a standard 12-lead ECG uses only ten electrodes placed on the limbs and the chest to generate the twelve electrical viewpoints.
Patient and Equipment Preparation
A proper recording begins with preparing both the patient and the necessary equipment to ensure the best possible signal quality. The procedure should be explained to the patient, and consent obtained to help alleviate anxiety and encourage cooperation. The patient should be positioned lying flat on their back (supine), with their arms resting comfortably at their sides and their legs uncrossed, as movement or muscle tension can interfere with the reading.
Skin preparation is necessary because the skin’s natural oils and dead cells increase electrical resistance, leading to poor tracing quality. The areas where the electrodes will be placed should be cleaned, often with an alcohol wipe, to remove any lotions, powders, or oils. If the patient has significant body hair, shaving the area may be necessary to ensure secure contact. The equipment must be checked, ensuring the ECG machine is plugged in, the electrode pads are fresh, and the standard calibration settings are confirmed.
Placing the Limb Electrodes
The first four electrodes are the limb electrodes, which provide the electrical viewpoints of the heart in the vertical plane. These four electrodes are designated for the right arm (RA), left arm (LA), right leg (RL), and left leg (LL). The limb electrodes are typically placed on the fleshy part of the arms and legs, such as the forearms and lower legs, while avoiding bony prominences.
It is important to place the limb leads symmetrically, meaning the right and left leads should be at similar locations on their respective limbs. The right leg (RL) electrode acts as the ground for the machine, helping to stabilize the electrical baseline. Proper placement of these peripheral electrodes is necessary to accurately record the three standard limb leads (I, II, and III) and the three augmented limb leads (aVR, aVL, and aVF).
Placing the Chest Electrodes
The six chest electrodes, known as the precordial leads (V1 through V6), are sensitive to placement and provide a view of the heart’s electrical activity in the horizontal plane. Correct positioning requires identifying specific anatomical landmarks on the chest wall, starting with the fourth intercostal space (the space between the fourth and fifth ribs).
V1 is placed in the fourth intercostal space immediately to the right of the sternum, while V2 is placed immediately to the left of the sternum. V4 is placed in the fifth intercostal space at the mid-clavicular line, which is an imaginary vertical line running down from the middle of the collarbone. V3 is then positioned exactly halfway between V2 and V4.
The remaining two leads, V5 and V6, are placed along the same horizontal line as V4, following the curve of the ribs. V5 is placed at the fifth intercostal space on the anterior axillary line. V6 is placed at the fifth intercostal space on the mid-axillary line. For female patients, the electrodes for V3 through V6 should be placed underneath the breast tissue to ensure correct anatomical positioning.
Connecting, Recording, and Verification
Once all ten electrodes are securely placed, the final step involves connecting the patient cable wires to their corresponding electrodes and running the recording. The machine connections are typically color-coded to match the lead names, and it is imperative to ensure the correct wire connects to the correct electrode site to prevent lead reversal errors. Before starting the recording, the patient should be instructed to remain still, relax their shoulders, and breathe normally, as muscle movement can introduce electrical interference known as artifact.
The machine is then activated to capture the heart’s electrical signals, typically printing or displaying the 12-lead tracing within a few moments. The operator must then immediately verify the quality of the tracing, looking for a steady baseline, clear waveforms, and the absence of significant artifact. Common issues like baseline drift or erratic signals often indicate poor skin contact or a loose electrode, which requires re-preparation of the skin or re-application of the electrode.
The standard calibration settings are 10 millimeters per millivolt (mm/mV) for amplitude and a paper speed of 25 millimeters per second (mm/s). These settings should be visible on the final printout for technical assessment.