An electrocardiogram (ECG) is a non-invasive diagnostic tool that records the heart’s electrical activity. The standard 12-lead ECG uses ten electrodes placed on the skin to provide twelve views of the heart’s electrical function. These views are translated into waveforms, allowing healthcare providers to assess cardiac rhythm, rate, and potential damage. Accurate placement of these electrodes is essential, as misplacement can significantly alter the resulting waveforms and lead to misinterpretation.
Essential Preparation Steps
Before attaching electrodes, the patient should be positioned comfortably, typically lying supine with arms resting at the sides. Muscle tension or movement can introduce artifact, so asking the patient to relax and remain still minimizes electrical interference known as somatic tremor.
The skin surface must be prepared thoroughly, as oils, sweat, and dead skin cells create high electrical resistance that impedes signal transmission. Preparation involves cleaning the sites with an alcohol wipe or soap and water to remove surface residue. Gentle abrasion further lowers impedance by removing the outermost layer of dead skin cells. If the patient has excessive hair, it should be clipped or shaved to ensure firm, conductive contact.
Placement of the Limb Electrodes
Four electrodes are placed on the limbs to generate the six frontal plane leads (I, II, III, aVR, aVL, and aVF). These electrodes should be placed on the fleshy, non-bony parts of the limbs, ideally distal to the torso but proximal to the wrists and ankles. Consistent placement is important for serial ECGs, as moving the electrodes significantly alters the recorded voltage amplitudes.
The four limb electrodes are labeled Right Arm (RA), Left Arm (LA), Left Leg (LL), and Right Leg (RL). RA and LA are positioned on the upper limbs, and LL is placed on the lower left limb. The RL electrode, placed on the lower right limb, serves as the neutral or ground electrode to stabilize the baseline and reduce electrical interference from external sources.
Placement of the Precordial Electrodes
The six precordial electrodes (V1 through V6) are placed across the chest wall to provide six views of the heart’s electrical activity in the horizontal plane. Accurate placement requires locating the angle of Louis, a horizontal ridge on the sternum that marks the level of the second rib. Counting down from this point allows identification of the intercostal spaces (the gaps between the ribs).
V1 and V2 Placement
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. These septal leads look directly at the interventricular septum.
V4 and V3 Placement
The next electrode to be placed is V4, which must be positioned before V3 to ensure proper spacing. V4 is located in the fifth intercostal space at the midclavicular line, which is an imaginary vertical line running down from the center of the clavicle. This lead provides a view of the anterior wall of the left ventricle. V3 is then placed midway in a straight line between the already positioned V2 and V4 electrodes.
V5 and V6 Placement
The final two electrodes are positioned horizontally at the same level as V4, maintaining the fifth intercostal space level. V5 is placed on the anterior axillary line. V6 is positioned on the mid-axillary line. These lateral leads complete the horizontal sweep, providing views of the lateral wall of the left ventricle.
Verifying the Tracing and Common Errors
After all ten electrodes are placed, the resulting 12-lead tracing must be verified immediately for quality and correctness before interpretation. A clean tracing displays a stable, straight baseline without excessive wandering or rapid, fine oscillations. Unstable baselines are often caused by patient movement or poor skin contact, while a thick, noisy baseline may indicate 60-cycle interference from nearby electrical equipment.
The most serious and common error is lead reversal, which can mimic a cardiac pathology, leading to misdiagnosis. Reversing the Right Arm (RA) and Left Arm (LA) electrodes is a frequent mistake that creates a distinct and recognizable pattern. When RA and LA are switched, the electrical polarity of Lead I is inverted, resulting in negative P waves, negative QRS complexes, and negative T waves in that lead.
A quick check of Lead I is a reliable final confirmation of correct limb lead placement; if the P wave and QRS complex are predominantly upright, the leads are likely in their proper positions. If the tracing shows the specific pattern of RA/LA reversal, the correction involves checking the lead wires and reapplying the electrodes correctly. Errors in precordial lead placement, such as placing V1 and V2 too high, can also alter the tracing, often by changing the morphology of the QRS complex in those leads.