What are the correct lead positions for an electrocardiogram (ECG)?

Correct Lead Positions for an Electrocardiogram (ECG)

The standard 12-lead ECG requires precise electrode placement with limb leads on the extremities and precordial leads at specific anatomical landmarks to ensure accurate cardiac assessment and diagnosis. 1

Standard Limb Lead Placement

• Right Arm (RA): Placed on the right arm, preferably at the wrist
• Left Arm (LA): Placed on the left arm, preferably at the wrist
• Right Leg (RL): Placed on the right leg, serves as ground/reference electrode
• Left Leg (LL): Placed on the left leg, preferably at the ankle

These four electrodes create the six limb leads:

• Lead I: LA-RA (potential difference between left arm and right arm)
• Lead II: LL-RA (potential difference between left leg and right arm)
• Lead III: LL-LA (potential difference between left leg and left arm)
• aVR: Augmented vector right
• aVL: Augmented vector left
• aVF: Augmented vector foot

Precordial Lead Placement

• V1: 4th intercostal space at the right sternal border 1
• V2: 4th intercostal space at the left sternal border 1
• V3: Midway between V2 and V4 1
• V4: 5th intercostal space at the midclavicular line 1
• V5: Anterior axillary line at the same level as V4 (horizontal plane) 1, 2
• V6: Mid-axillary line at the same level as V4 (horizontal plane) 1, 2

Common Placement Errors and Their Clinical Impact

• V1-V2 placement errors: Superior displacement of V1 and V2 electrodes often results in rSr' complexes with T-wave inversion, resembling the complex in lead aVR 2
• V5-V6 placement errors: Inferior placement of V5 and V6 (in the sixth intercostal space or lower) can alter amplitudes used in diagnosing ventricular hypertrophy 2
• Precordial lead displacement: Variation in precordial lead placement of as little as 2 cm can result in significant diagnostic errors 1
• Common error pattern: The most common errors are placing V1 and V2 leads too superiorly, and V5 and V6 leads too medially 3

Modified Lead Placements for Special Situations

Mason-Likar Lead Position (for exercise or ambulatory monitoring)

• Arm electrodes: Placed in the infraclavicular fossae medial to the deltoid insertions (or over the outer clavicles in more recent applications) 2
• Left leg electrode: Placed midway between the costal margin and iliac crest in the left anterior axillary line 2
• Precordial electrodes: Remain in standard positions 2

Important Considerations for Modified Lead Placements

• ECGs recorded with torso placement of extremity electrodes cannot be considered equivalent to standard ECGs 2
• Tracings using torso limb lead placement must be clearly labeled as such 2
• Monitoring electrodes placed on the trunk do not provide standard limb leads and distort the central terminal 2
• These differences can include false-negative and false-positive infarction criteria 2

Best Practices for Accurate Lead Placement

• Medical personnel responsible for ECG recording should receive periodic retraining in skin preparation, proper electrode positioning, and proper patient positioning 2, 1
• For V5 placement, defining it as midway between V4 and V6 is more reproducible than using the anterior axillary line 2
• For V6 placement, attention should be directed to the definition of the midaxillary line as extending along the middle or central plane of the thorax 2
• For female patients with large breasts, electrodes are most commonly placed beneath the breast to reduce amplitude attenuation 2

Clinical Implications of Improper Lead Placement

• Inaccurate lead placement can result in misdiagnosis of conditions such as poor R-wave progression, anteroseptal infarction, or ventricular hypertrophy 1, 3
• Lead switches can mimic or mask ischemic changes, resulting in false-positive or negative signs of ischemia 1
• Standardization of the procedure is essential to achieve comparable results for serial ECG comparison 4

Proper lead placement is fundamental to obtaining an accurate ECG recording, which is essential for correct diagnosis and appropriate treatment decisions. Regular training and adherence to standardized placement protocols are crucial for maintaining high-quality ECG recordings.