Which ECG (electrocardiogram) leads are most likely to show changes in the event of an occlusion in the left circumflex artery?

ECG Changes in Left Circumflex Artery Occlusion

An occlusion in the left circumflex artery would most likely show changes in ECG leads I, aVL, V5-V6 (option A). 1

Pathophysiological Basis

Left circumflex artery (LCx) occlusion produces a characteristic pattern of ECG changes that reflects the anatomical territory supplied by this vessel:

• The LCx typically supplies the lateral and posterolateral walls of the left ventricle
• When occluded, the ST-segment vector shifts toward the lateral wall
• This creates a specific pattern of ST-segment elevation and depression across different lead groups

Characteristic ECG Findings in LCx Occlusion

Primary ST-Segment Elevations

• Lateral leads (I, aVL, V5-V6): ST-segment elevation is commonly observed in these leads as they directly face the lateral wall of the left ventricle 1
• The ST elevation in V6 is particularly frequent (67.7% of cases) 2

Associated ST-Segment Depressions

• Anterior leads (V1-V3): ST-segment depression is frequently observed (up to 80% in V3) 2
• These ST depressions in V1-V3 represent "posterior ischemia" and are considered "STEMI-equivalent" patterns 3

Key Differentiating Features

• When comparing LCx occlusion to right coronary artery (RCA) occlusion (which would show changes in leads II, III, aVF):
  • LCx occlusion typically shows ST elevation in lateral leads with an isoelectric or elevated ST segment in lead I 4
  • The ratio of ST depression in V3 to ST elevation in III > 1.2 strongly suggests LCx occlusion 5

Clinical Implications

The correct identification of LCx occlusion on ECG is crucial for several reasons:

• LCx occlusions are often missed or delayed in diagnosis because they may not present with classic ST elevations in the standard 12-lead ECG
• Posterior leads (V7-V9) may be necessary to capture ST elevations in LCx occlusion 1
• Delayed recognition leads to increased morbidity and mortality due to myocardial damage

Common Pitfalls in Diagnosis

1. Misdiagnosis as non-STEMI: LCx occlusions may present with ST depression only, leading to inappropriate delay in reperfusion therapy
2. Overlooking lateral lead changes: Subtle ST elevations in I, aVL, V5-V6 may be missed
3. Focusing only on inferior leads: While some LCx occlusions can cause inferior changes, the lateral lead changes are more specific

Diagnostic Algorithm for LCx Occlusion

1. Look for ST elevation in lateral leads (I, aVL, V5-V6)
2. Check for ST depression in anterior leads (V1-V3)
3. If suspicious but non-diagnostic, consider:
   • Recording posterior leads (V7-V9)
   • Looking for the "N wave" (notch in terminal QRS complex) in leads II, III, aVF or I, aVL 6

In conclusion, while LCx occlusion can sometimes produce a complex pattern of ECG changes, the most characteristic and reliable changes occur in leads I, aVL, V5-V6, making option A the correct answer.