What is the correct placement for limb leads in an electrocardiogram (ECG)?

Standard Limb Lead Placement for ECG

For a standard diagnostic 12-lead ECG, place the four limb electrodes on the wrists and ankles with the patient supine, as this is the traditional standard position endorsed by the American Heart Association. 1

Specific Electrode Positions

Standard Limb Lead Configuration

• Right arm (RA): Place on the right wrist or distal right arm 1, 2
• Left arm (LA): Place on the left wrist or distal left arm 1, 2
• Left leg (LL): Place on the left ankle or distal left leg 1, 2
• Right leg (RL): Place on the right ankle or distal right leg (serves as ground/reference) 1

How These Create the Limb Leads

The electrode pairs generate the following electrical views: 1

• Lead I: Measures potential difference between left arm and right arm (LA-RA)
• Lead II: Measures potential difference between left leg and right arm (LL-RA)
• Lead III: Measures potential difference between left leg and left arm (LL-LA)
• Augmented leads (aVR, aVL, aVF): Derived mathematically from the same four electrodes

Critical Distinction: Diagnostic vs. Monitoring ECGs

When Standard Placement is MANDATORY

Use wrist and ankle placement for: 1, 2

• Diagnostic 12-lead ECGs
• Serial comparison with previous ECGs
• Assessment of chamber enlargement or left ventricular hypertrophy
• Precise ST-segment analysis for acute coronary syndromes
• Any ECG that will be compared to prior tracings

When Modified Torso Placement is Acceptable

For continuous cardiac monitoring only (not diagnostic ECGs), you may use the Mason-Likar position: 1, 2

• Arm electrodes: Infraclavicular fossae medial to deltoid insertions, or over outer clavicles
• Left leg electrode: Midway between costal margin and iliac crest in left anterior axillary line
• Right leg electrode: Similar position on right side

This modified placement reduces motion artifact during exercise testing, ambulatory monitoring, or prolonged bedside monitoring. 1

Major Pitfall: Never Mix Standard and Modified Placement

ECGs recorded with torso placement of limb electrodes cannot be considered equivalent to standard ECGs and must never be used interchangeably for serial comparison. 1, 2, 3 This is a critical error that leads to:

• False-positive infarction patterns 1
• False-negative infarction patterns (particularly inferior infarcts may disappear) 1, 4
• Altered QRS morphology and voltage 1, 3
• Misdiagnosis when comparing to previous standard ECGs 5

How to Avoid This Error

• Always label any ECG using modified electrode placement clearly as "Mason-Likar" or "torso leads" 1, 2
• Never compare a torso-lead ECG to a standard ECG for serial changes 1, 5
• Mark electrode positions with indelible ink for consistent placement in serial ECGs 6
• Use standard placement whenever diagnostic accuracy is the priority over motion artifact reduction 3

Practical Considerations

Patient Positioning

• Patient should be supine for standard diagnostic ECGs 2, 7
• Body position changes (sitting, left lateral, upright) alter ECG parameters and should be avoided for diagnostic tracings 7

Electrode Placement Precision

Technicians require periodic retraining in proper electrode positioning, as even small variations affect ECG interpretation. 1 Specifically:

• Placement along the limbs (proximal vs. distal) affects ECG voltages and durations 1, 3
• Validity of diagnostic algorithms depends on consistent placement matching the positions used for criteria development 1

Special Populations

• Infants and young children: Torso leads sometimes used to reduce motion artifact, but clinical significance of resulting differences remains unclear 1
• Amputees or patients with limb injuries: Place electrodes as distally as possible on remaining limb tissue 2