What is the reliability of the association between the South African Flag Sign on a 12-lead Electrocardiogram (ECG) and occlusion of the 1st diagonal branch of the Left Anterior Descending (LAD) artery?

The South African Flag Sign and First Diagonal Branch Occlusion

The South African Flag Sign (SAFS) is a specific but not entirely reliable ECG pattern for first diagonal (D1) branch occlusion, as it can occasionally represent more extensive LAD disease including proximal occlusions. While the pattern is classically associated with D1 occlusion, recent case reports demonstrate important exceptions that mandate urgent angiography regardless of the presumed culprit vessel.

ECG Pattern Definition

The South African Flag Sign consists of:

• ST-segment elevation in leads I, aVL, and V2 1, 2
• ST-segment depression in lead III 2
• This creates a visual pattern resembling the South African flag on the ECG 1

Evidence for D1 Association

The SAFS pattern does correlate with first diagonal branch occlusion in many cases:

• In a 2025 case series, 2 of 4 patients with SAFS had confirmed acute diagonal branch occlusion requiring stent placement 1
• The pattern is considered a "key indicator" of acute D1 occlusion in the appropriate clinical context 1
• All patients with SAFS demonstrated varying degrees of left ventricular dysfunction and troponin elevation, confirming myocardial injury 1

Critical Limitations and Pitfalls

The association is not absolute, and SAFS can represent more dangerous lesions:

• A 2023 case report documented SAFS in a patient with 100% proximal LAD occlusion (the "widow-maker" lesion) plus severe multivessel disease (90% PLV stenosis, 80% LCx stenosis) 3
• This demonstrates that SAFS cannot reliably exclude proximal LAD or multivessel disease 3
• The AHA/ACCF guidelines describe that occlusion between the first septal and first diagonal branches produces ST elevation in aVL with ST depression in lead III, which overlaps with the SAFS pattern but represents a different anatomic location 4

Clinical Management Algorithm

Regardless of whether SAFS represents D1 or proximal LAD occlusion, the urgency of management is identical:

1. Treat SAFS with the same urgency as other acute occlusion patterns (e.g., new LBBB with chest pain) 1
2. Proceed immediately to invasive coronary angiography to definitively identify the culprit lesion 1
3. Do not delay reperfusion therapy based on assumptions about the specific vessel involved 1, 2
4. Consider pharmacological reperfusion if PCI is not immediately available, followed by rescue angioplasty 2

Diagnostic Accuracy Considerations

The reliability of SAFS for specifically identifying D1 occlusion is moderate at best:

• Only 50% (2 of 4) of SAFS cases in the most recent series had confirmed D1 occlusion requiring intervention 1
• The other cases showed angiographic changes without requiring stent placement 1
• Cardiac MRI revealed aborted infarction in some cases, suggesting variable degrees of ischemic injury 1

Key Clinical Takeaway

The South African Flag Sign should be recognized as an acute coronary occlusion pattern requiring immediate catheterization, but it cannot reliably differentiate between first diagonal branch occlusion and more proximal LAD disease 3, 1. The pattern's value lies in triggering urgent intervention rather than in precise anatomic localization. Increased awareness prevents these cases from being overlooked and ensures timely reperfusion therapy 1.