What is the difference in lead placement between a normal electrocardiogram (ECG) and a treadmill test (TMT)?

Differences in Lead Placement Between Standard ECG and Treadmill Test (TMT)

The primary difference between standard ECG and treadmill test (TMT) lead placement is that TMT uses torso placement of limb electrodes (Mason-Likar position) rather than the standard distal limb placement to reduce motion artifact during exercise, but these modifications significantly alter ECG morphology and should not be used interchangeably with standard ECGs. 1

Standard 12-Lead ECG Electrode Placement

• In standard ECG recording, limb lead electrodes are traditionally attached at the wrists and ankles with the patient in a supine position 1
• The AHA guidelines recommend placement of the 4 limb electrodes on the arms and legs distal to the shoulders and hips, not necessarily on the wrists and ankles 1
• Precordial leads (V1-V6) are placed in specific anatomical positions:
  • V1: Fourth intercostal space at the right sternal border 1
  • V2: Fourth intercostal space at the left sternal border 1
  • V3: Midway between V2 and V4 1
  • V4: Fifth intercostal space in the midclavicular line 1
  • V5: In the horizontal plane at the anterior axillary line 1
  • V6: In the horizontal plane at the midaxillary line 1

Treadmill Test (TMT) Electrode Placement

• In treadmill testing, the Mason-Likar lead position is commonly used to reduce motion artifact 1
• Arm electrodes are placed in the infraclavicular fossae medial to the deltoid insertions or over the outer clavicles 1
• The left leg electrode is placed midway between the costal margin and iliac crest in the left anterior axillary line 1
• Precordial electrodes remain in the standard positions as used in conventional ECG 1
• An alternative modification for bicycle ergometry places arm electrodes on the upper outer arm and leg electrodes on the anterior iliac crest 1

Clinical Implications of Modified Lead Placement

• ECGs recorded with torso placement of limb electrodes cannot be considered equivalent to standard ECGs and should not be used interchangeably for serial comparison 1
• Torso lead placement produces important amplitude and waveform changes associated with a more vertical and rightward shift of the QRS frontal axis 2
• These changes can generate false ECG abnormalities in patients with normal standard ECGs, suggesting "heart disease of electrocardiographic origin" 2
• Mason-Likar and other alternative lead placements may affect QRS morphology more than repolarization compared with standard ECG 1
• Significant diagnostic differences include:
  • False-negative and false-positive infarction criteria 1, 2
  • Appearance of new possible myocardial infarcts or disappearance of inferior infarcts 2
  • Important T wave or QRS frontal axis changes 2

Best Practices for ECG Recording

• Any modification of electrode placement must be clearly labeled on the ECG (e.g., "torso-positioned limb leads" or "non-standard") 2
• Marking modified lead placement alerts clinicians to potential limitations for clinical interpretation 2
• When torso lead monitoring is justified, standardization of one universally adopted method would reduce confusion and minimize invalid ECG comparisons 3
• The convenience of modified lead placement should be weighed against the risk of misdiagnosis resulting from serial comparison of non-equivalent ECGs 3

Pitfalls to Avoid

• Never compare ECGs with different lead placements to determine changes in an individual over time, as this may lead to misdiagnosis 3, 2
• Avoid placing precordial electrodes without reference to underlying bony landmarks, as this can result in an erroneously vertical orientation 1
• Be aware that electrode placement along the limbs can affect ECG voltages and durations, particularly in the limb leads 1
• Remember that different proprietary reduced lead configurations are inherently different and should not be compared with each other or with standard ECGs 3