An electrocardiogram (ECG) is a non-invasive procedure that measures the electrical activity of the heart, providing a visual representation of its rhythm and function. A standard 12-lead ECG uses ten electrodes placed on the body to create twelve different electrical perspectives, or views, of the heart’s activity. This multi-angle approach helps detect various heart conditions, such as irregular heartbeats or signs of a heart attack. Obtaining an accurate reading depends entirely on the precise placement of electrodes and careful preparation.
Preparing the Patient and Equipment
Before beginning the procedure, communicate with the patient to ensure they understand the test and to obtain consent. The patient should be positioned supine, or lying flat, on the examination table, with arms resting at their sides and legs uncrossed. A comfortable room temperature is important because shivering can cause muscle tremors that interfere with the electrical signal, creating artifact on the recording.
All metal jewelry, watches, and electronic devices should be removed to prevent electrical interference with the sensitive equipment. Skin preparation is crucial for ensuring good conductivity between the skin and the electrode’s conductive gel. The sites where electrodes will be placed must be cleaned to remove oils, lotions, or sweat using an alcohol wipe or mild abrasive pad, followed by air drying.
If a patient has significant chest hair, it may need to be shaved or clipped away at the electrode sites, as hair impedes adhesion and signal transmission. Finally, check the ECG machine to confirm it is properly calibrated. Ensure it has an adequate supply of recording paper or that the digital system is ready to acquire the trace.
Locating and Applying the Limb Leads
The first four electrodes applied are the limb leads, typically placed on the fleshy parts of the arms and legs, avoiding bony prominences. These four electrodes provide six electrical views of the heart’s activity in the frontal plane (Leads I, II, III, aVR, aVL, and aVF). Placement must be symmetrical to ensure the accuracy of the resulting trace.
The right arm (RA) electrode is placed on the right arm or wrist, and the left arm (LA) electrode is placed symmetrically on the left arm or wrist. The left leg (LL) electrode is placed on the left lower leg or ankle. The right leg (RL) lead is a ground electrode that stabilizes the electrical signal and reduces interference, placed on the right lower leg or ankle.
Placing the electrodes on the upper arms and thighs instead of the wrists and ankles is acceptable, provided the placement is uniform on both sides. Once the four electrodes are firmly adhered, connect the corresponding lead wires.
Placing the Precordial (Chest) Leads
The six precordial leads (V1 through V6) provide a cross-sectional view of the heart’s electrical activity in the horizontal plane. Accurate placement is crucial, as a misplacement of as little as two centimeters can lead to diagnostic errors. Placement begins by identifying the sternal angle, the horizontal ridge felt at the junction between the upper and main parts of the sternum.
The placement sequence is as follows:
- V1 is placed in the fourth intercostal space (ICS) immediately to the right of the sternum.
- V2 is placed in the fourth ICS immediately to the left of the sternum.
- V4 is placed in the fifth ICS at the midclavicular line (running vertically down from the center of the clavicle).
- V3 is placed midway between V2 and V4.
- V5 is placed on the same horizontal level as V4, along the fifth ICS, at the anterior axillary line.
- V6 is placed on the same horizontal level as V4, along the fifth ICS, at the mid-axillary line.
For female patients, V3 through V6 must be placed underneath the breast where the tissue meets the chest wall.
Running the ECG and Troubleshooting Artifact
With all ten electrodes connected, the technical procedure of acquiring the trace can begin by confirming the machine’s operational settings. Standard ECG recordings are typically run at a paper speed of 25 millimeters per second (mm/s). The voltage setting, or gain, is calibrated so that one millivolt (mV) of electrical signal produces a ten-millimeter deflection.
The machine should utilize a filter setting, such as a 60-Hertz notch filter, to minimize electrical interference from surrounding equipment. Before recording, instruct the patient to remain completely still and breathe normally, as movement introduces signal interference, known as artifact.
Common Artifacts and Troubleshooting
Baseline wander appears as a slow, undulating curve on the trace, often caused by patient movement, deep breathing, or poor electrode contact. This issue is resolved by re-prepping the skin or securing a loose electrode.
Somatic tremor presents as a rapid, jagged baseline caused by muscle activity, such as shivering or tremors. Warming the patient or ensuring they are relaxed can help mitigate this. Sixty-cycle interference appears as a thick, fuzzy line and is resolved by ensuring power cords are not near the patient and that the notch filter is active.
If artifact is present, the technician must troubleshoot the connection and re-record the trace to ensure the final report is diagnostically reliable.