STS Risk Score Thresholds for High-Risk CABG in Acute Coronary Syndrome
An STS-predicted risk of operative mortality >5% is considered high risk for patients with acute coronary syndrome requiring CABG, while >2% represents an intermediate-to-high risk threshold that may favor PCI over CABG in selected patients.
Risk Stratification Framework
High-Risk Definition
- STS score >5% defines high surgical risk in ACS patients, particularly when considering PCI as an alternative to CABG for unprotected left main disease 1
- STS score >2% represents moderate-to-severe surgical risk and may favor PCI in patients with low-to-intermediate anatomic complexity (SYNTAX score <33) 1
Clinical Application in ACS
For patients with unprotected left main disease and ACS:
- STS >5% with favorable anatomy (SYNTAX <22): PCI is reasonable as Class IIa recommendation 1
- STS >2% with low-intermediate anatomy (SYNTAX <33): PCI may be reasonable as Class IIb recommendation 1
- These thresholds specifically predict significantly increased risk of adverse surgical outcomes 1
Calculation is recommended for all patients with unprotected left main or complex multivessel CAD to guide revascularization decisions 1
Risk Assessment Context
What STS Score Predicts
- Primary outcomes: In-hospital or 30-day operative mortality 1
- Secondary outcomes: In-hospital morbidity including stroke, renal failure, prolonged ventilation, deep sternal infection, and reoperation 1
- The score uses 40 variables to calculate risk 1
Performance in ACS Population
- Both EuroSCORE II and STS score demonstrate excellent discrimination for in-hospital mortality (AUC 0.879-0.900) in CABG patients 2
- Predictive ability remains acceptable for 2-year postoperative mortality but decreases progressively thereafter 2
- Despite higher baseline risk, early CABG in ACS patients shows very low in-hospital mortality (1.1%) when appropriately selected 3
Critical Decision Points
When CABG Remains Preferred Despite High STS Score
- Three-vessel disease with diabetes: CABG superior to PCI regardless of STS score 1
- Complex anatomy (SYNTAX >22): CABG preferred even with elevated surgical risk 1
- Reduced LVEF (<40%): CABG reasonable with STS consideration secondary 1
When to Favor PCI Over CABG
- Severe comorbidity not reflected in scores (advanced age/frailty, reduced life expectancy, restricted mobility) 1
- Anatomical factors: poor quality conduits, severe chest deformation, sequelae of chest radiation, porcelain aorta 1
- Clinical characteristics: moderate-severe COPD, disability from prior stroke, prior cardiac surgery 1
Common Pitfalls to Avoid
Do not use STS score in isolation - it must be integrated with anatomic complexity (SYNTAX score) and clinical judgment through Heart Team discussion 1
Do not delay revascularization in very high-risk ACS presentations (hemodynamic instability, cardiogenic shock, life-threatening arrhythmias) while calculating risk scores 1
Do not assume high STS score mandates conservative therapy - appropriately selected ACS patients undergoing early CABG have lower mortality than those treated conservatively, even with higher baseline risk 3, 4
Recognize that 4.5% of ACS patients undergo CABG during index hospitalization with in-hospital mortality of 3.7%, comparable to non-CABG patients despite higher risk profiles 4