Risk Stratification
Integrated Risk Assessment¶
Risk is determined from the entire study:
- Symptoms
- Exercise capacity
- Stress ECG
- Hemodynamic response
- Perfusion defect burden
- LV function and volumes
- High-risk ancillary findings
Historical Risk Categories¶
| Risk | Annual cardiac mortality | Typical findings |
|---|---|---|
| Low | <1% | Normal or small defect, preserved LVEF, good exercise capacity |
| Intermediate | 1%–3% | Moderate defect or mild-to-moderate LV dysfunction |
| High | >3% | Large/multivessel ischemia, severe LV dysfunction, or multiple high-risk markers |
These categories are estimates and should be integrated with the patient's overall clinical risk.
Perfusion Burden¶
| Defect size | Approximate LV involvement |
|---|---|
| Small | <10% |
| Moderate | 10%–20% |
| Large | >20% |
Increasing ischemic burden generally predicts increasing risk. Defect severity, location, and number of involved vascular territories also matter.
Low-Risk Findings¶
- Normal perfusion
- Small perfusion defect
- Preserved LVEF
- Normal LV volumes
- Duke Treadmill Score ≥5
- Exercise capacity ≥10 METs
- No high-risk ECG or hemodynamic findings
A normal MPI predicts a low event rate, but its reassurance is reduced in patients with known CAD, diabetes, CKD, poor exercise capacity, or persistent symptoms.
High-Risk Findings¶
- Large or multiple reversible defects
- Ischemia in multiple coronary territories
- Transient ischemic dilation
- Stress-induced fall in LVEF
- Post-stress stunning
- Increased lung uptake
- Increased RV uptake or RV dilation
- LVEF <35%
- Severe coronary calcification
- Marked ischemic ECG changes
- Hypotension or ventricular tachycardia during stress
Exercise Risk Markers¶
| Finding | Interpretation |
|---|---|
| <5 METs | Poor functional capacity |
| Ischemia at low workload | High risk |
| ≥2 mm ST depression | Greater ischemic risk |
| Prolonged ST depression in recovery | Greater ischemic burden |
| Fall in SBP | Consider severe ischemia or LV dysfunction |
| Ventricular tachycardia | High risk |
| Duke score ≤−11 | High risk |
Balanced Ischemia¶
Normal relative perfusion does not exclude left main or three-vessel disease.
Suspect balanced ischemia when a normal-appearing study is accompanied by:
- TID
- Reduced stress LVEF
- Increased lung uptake
- Severe coronary calcium
- Ischemic ECG changes
- Typical angina
- Globally reduced PET myocardial flow reserve
Discordant Findings¶
| Perfusion | Stress findings | Interpretation |
|---|---|---|
| Normal | Low risk | Reassuring |
| Abnormal | High risk | Concordant ischemia |
| Normal | High risk | Consider balanced ischemia or false-negative MPI |
| Abnormal | Low risk | Consider mild disease or artifact |
Board Pearls¶
- Perfusion burden and LVEF are major prognostic variables.
- TID is most concerning when accompanied by abnormal perfusion.
- Exercise capacity may be more prognostic than the ECG alone.
- A normal scan is not automatically low risk if other findings are high risk.
- PET flow can identify diffuse disease missed by relative perfusion imaging.