What qualifies as significant ST elevation or depression in a 12-lead Electrocardiogram (EKG), in terms of millimeter changes in limb and precordial leads, and the number of consecutive leads required for diagnosis of myocardial pathology?

Criteria for Significant ST Elevation or Depression in 12-Lead ECG

The correct answer is B: (i) 1 mm in a limb lead, (ii) 2 mm in precordial lead, (iii) and must be present in 2 consecutive leads.

ST Elevation Criteria for Myocardial Pathology

According to the American Heart Association (AHA)/American College of Cardiology Foundation (ACCF)/Heart Rhythm Society (HRS) recommendations for ECG interpretation, significant ST elevation is defined as follows:

• Limb Leads: ≥1 mm (0.1 mV) ST elevation 1
• Precordial Leads: ≥2 mm (0.2 mV) ST elevation in leads V2-V3 for men 40 years and older 1
• Consecutive Leads: Changes must be present in at least 2 anatomically contiguous leads 1

The American College of Emergency Physicians (ACEP) clinical policy on reperfusion therapy confirms these thresholds, stating that the best evidence-based recommendations for ECG eligibility for fibrinolytic therapy are "≥1 mm ST-segment elevation in 2 contiguous limb leads, and ≥2 mm in 2 contiguous precordial leads" 1.

Understanding Lead Contiguity

Anatomically contiguous leads represent adjacent areas of the heart:

• Inferior leads: II, III, aVF
• Lateral leads: I, aVL, V5, V6
• Anterior leads: V1-V4

The concept of contiguity is important because it helps identify the affected region of myocardium. The AHA/ACCF/HRS recommendations suggest that leads should be displayed in their anatomically contiguous sequence to better visualize the extent of ischemia/infarction 1.

Clinical Implications

These criteria are critical for the diagnosis of ST-elevation myocardial infarction (STEMI) and decisions regarding reperfusion therapy:

• ST elevation meeting these criteria in 2 contiguous leads is a key indicator for immediate reperfusion therapy 1, 2
• The presence of reciprocal changes (ST depression in leads opposite to those with elevation) significantly improves the positive predictive value for myocardial infarction 3
• For posterior wall infarction, ST depression in leads V1-V3 with positive terminal T waves may represent posterior STEMI and should be confirmed with posterior leads (V7-V9) 2

Common Pitfalls and Caveats

• ST elevation can occur in conditions other than myocardial infarction, including pericarditis, early repolarization, and left ventricular hypertrophy 1
• Age and gender affect normal ST segment levels - younger patients and males may have higher normal ST elevation, particularly in leads V2-V3 1
• ST depression alone (except in suspected posterior MI) should not prompt fibrinolytic therapy, as it may increase mortality 1, 2
• The sensitivity of standard 12-lead ECG for detecting acute MI is approximately 85% for anterior MI but only 46-61% for left circumflex coronary artery occlusions 4

Optimization of ECG Interpretation

• Additional right precordial leads (V3R-V4R) should be considered for suspected right ventricular infarction 2, 4
• Posterior leads (V7-V9) may help diagnose posterior infarction when ST depression is seen in V1-V3 2, 4
• Body surface mapping studies suggest that leads placed on a horizontal strip in line with V1 and V2 may provide optimal placement for diagnosis of anterior and lateral STEMI 5

In conclusion, the criteria for significant ST changes that warrant diagnosis of myocardial pathology are 1 mm elevation in limb leads, 2 mm elevation in precordial leads, present in at least 2 contiguous leads.