Why can large R waves in leads V2-3 be indicative of a ST-Elevation Myocardial Infarction (STEMI)?

Large R Waves in V2-3 as STEMI Equivalents

Large R waves in leads V2-3 can indicate a posterior myocardial infarction, which is a STEMI equivalent requiring immediate reperfusion therapy despite the absence of classic ST-segment elevation on standard 12-lead ECG. 1, 2

Pathophysiology and ECG Findings

• Posterior myocardial infarction (PMI) occurs due to occlusion of the left circumflex coronary artery or right coronary artery, affecting the posterior wall of the left ventricle 2
• In posterior MI, the ECG shows characteristic findings in the anterior precordial leads (V1-V3) that are actually reciprocal changes to what would be ST elevation if leads were placed on the posterior chest wall 1
• These reciprocal changes include:
  • Horizontal ST-segment depression in leads V1-V3 1
  • Dominant R waves (R/S ratio >1) in V2 1
  • Abnormally increased R wave amplitude and width (>30 ms) in leads V1 and V2 1
  • Upright T waves in anterior leads when they should be inverted 1

Clinical Significance

• Isolated posterior STEMI accounts for up to 7% of all STEMIs but is frequently missed due to the absence of classic ST-segment elevation on standard 12-lead ECG 2
• PMI is considered a "STEMI equivalent" requiring the same urgent reperfusion therapy as traditional STEMI 3
• Delayed or missed diagnosis leads to increased morbidity and mortality due to lack of timely reperfusion 2

Confirmation of Posterior MI

• When posterior MI is suspected based on large R waves in V1-V2 and ST depression in V1-V3, additional posterior leads should be recorded 1
• Posterior leads are placed at:
  • V7 (left posterior axillary line in same plane as V6)
  • V8 (left mid-scapular line)
  • V9 (left paraspinal border) 1
• Diagnostic criteria for posterior MI in these leads is ST-segment elevation ≥0.5 mm in at least one of leads V7-V9 1, 2

Common Pitfalls and Challenges

• Many clinicians are unfamiliar with posterior MI ECG patterns, with studies showing only 38% of doctors and 10% of paramedics correctly identifying potential posterior MI 2
• Only 20% of doctors correctly position posterior leads, and only 19% know the diagnostic criteria for posterior MI 2
• Posterior MI is often misdiagnosed as non-STEMI or unstable angina due to the ST depression in V1-V3, leading to delayed or inappropriate treatment 2
• Other conditions that can cause prominent R waves in V1-V2 include:
  • Right ventricular hypertrophy
  • Right bundle branch block
  • Wolff-Parkinson-White syndrome
  • Normal variant in some individuals 1

Clinical Implications

• When a patient presents with chest pain and shows dominant R waves in V2-V3 with ST depression in V1-V3, posterior leads should be immediately recorded 1
• If posterior leads show ST elevation ≥0.5 mm, the patient should receive the same emergent reperfusion therapy as traditional STEMI patients 1, 4
• Primary percutaneous coronary intervention (PCI) is the preferred reperfusion strategy; if PCI cannot be performed within 120 minutes, fibrinolytic therapy should be considered 4
• Failure to recognize posterior MI can result in up to 7% of STEMIs being missed, significantly impacting patient outcomes 2