Contiguous Leads in ECG for Myocardial Infarction
Contiguous leads are anatomically adjacent ECG leads that view neighboring regions of the heart, and ST-segment elevation or depression must appear in at least two contiguous leads to diagnose acute myocardial infarction, ensuring the ECG changes represent true regional ischemia rather than artifact or normal variant. 1
Definition of Contiguous Lead Groups
The concept of contiguous leads refers to specific anatomical groupings that reflect adjacent myocardial territories 1:
Anterior leads: V1 through V6 (sequential precordial leads) 1
Inferior leads: II, III, and aVF 1
Lateral/apical leads: I and aVL 1
More precise frontal plane contiguity can be established using the Cabrera display sequence: aVL, I, -aVR, II, aVF, and III 1. This arrangement recognizes that lead -aVR (inverted aVR) sits between leads I and II in spatial orientation 1.
Clinical Significance for MI Diagnosis
Why Two Contiguous Leads Matter
The requirement for ST-segment changes in two or more contiguous leads distinguishes true regional myocardial ischemia from artifact, lead misplacement, or normal variants. 1 This criterion increases specificity for acute coronary occlusion while maintaining adequate sensitivity 1.
The magnitude of ST-segment shift must meet specific thresholds 1, 2:
- ≥0.1 mV (1 mm) in standard leads (except V2-V3) 2
- ≥0.2 mV (2 mm) in leads V2-V3 for men ≥40 years 2
- ≥0.25 mV (2.5 mm) in leads V2-V3 for men <40 years 2
- ≥0.15 mV (1.5 mm) in leads V2-V3 for women 2
Important Clinical Caveat
Occasionally acute myocardial ischemia creates sufficient ST-segment shift to meet criteria in one lead but produces slightly less than the required threshold in an adjacent contiguous lead—this should not exclude the diagnosis of acute MI when clinical suspicion is high. 1 In such cases, serial ECGs at 15-30 minute intervals are essential 2.
Territory-Specific Patterns
Standard Territory Recognition
Anterior MI: ST elevation in contiguous precordial leads V1-V4 or V2-V5 indicates left anterior descending artery occlusion 1, 2
Inferior MI: ST elevation in contiguous leads II, III, and aVF indicates right coronary artery or left circumflex occlusion 1, 2
Lateral MI: ST elevation in contiguous leads I and aVL (and possibly V5-V6) indicates left circumflex or diagonal branch occlusion 1
Extended Lead Considerations
Posterior MI requires additional posterior leads V7-V9, as ST depression in V1-V3 with positive terminal T-waves serves as an "ST elevation equivalent" 1, 2. Confirmation requires ST elevation ≥0.05 mV in V7-V9 (≥0.1 mV in men <40 years) 2.
Right ventricular MI requires right precordial leads V3R-V4R, with ST elevation ≥0.05 mV considered significant (≥0.1 mV in men <30 years) 2. This is particularly important in inferior MI where right ventricular involvement dramatically alters management 2.
Reciprocal Changes Enhance Diagnostic Certainty
ST-segment elevation in one territory typically produces reciprocal ST-segment depression in leads whose positive poles are directed approximately 180° opposite to the leads showing elevation. 1 For example:
- Inferior MI (ST elevation in II, III, aVF) commonly shows reciprocal ST depression in aVL and sometimes I 1
- Anterior MI may show reciprocal ST depression in inferior leads 1
The presence of reciprocal changes, particularly when associated with prolonged ST elevation >20 minutes, strongly indicates acute coronary occlusion and increases diagnostic specificity. 1, 2
Common Pitfalls to Avoid
Do not dismiss isolated ST changes in a single lead when clinical suspicion is high—obtain serial ECGs at 15-30 minute intervals, as evolving changes may subsequently meet the two-contiguous-lead criterion 1, 2.
Recognize that the standard 12-lead ECG may miss posterior and right ventricular infarctions—record V7-V9 for suspected left circumflex occlusion and V3R-V4R for suspected right ventricular involvement in inferior MI 2.
Be aware of conditions that confound ECG interpretation: acute pericarditis, left ventricular hypertrophy, left bundle branch block, Brugada syndrome, and early repolarization patterns can all produce ST elevation 1. Comparison with prior ECGs is invaluable 2.
In left bundle branch block, concordant ST elevation (ST elevation in leads with positive QRS deflections) strongly suggests acute MI and warrants immediate reperfusion therapy. 2
Practical Algorithm for ECG Interpretation
Identify ST-segment elevation or depression ≥ threshold values 1, 2
Confirm changes appear in at least two anatomically contiguous leads from the same territory 1
Look for reciprocal changes in opposite territories to increase diagnostic certainty 1, 2
If initial ECG shows changes in only one lead but clinical suspicion is high, obtain serial ECGs at 15-30 minute intervals 1, 2
Consider extended leads (V7-V9 for posterior, V3R-V4R for right ventricular) when standard leads are non-diagnostic but suspicion remains high 2
Compare with previous ECGs when available to identify new changes 2
Do not delay reperfusion therapy in STEMI while waiting for cardiac biomarkers 2