What percentage of left‑ventricular myocardium involvement on a stress myocardial perfusion SPECT scan defines a high‑risk defect?

High-Risk Myocardial Perfusion Defect Threshold

A reversible perfusion defect involving ≥10% of the left ventricular myocardium defines a high-risk scan on stress myocardial perfusion SPECT imaging. 1, 2

Evidence-Based Risk Thresholds

High-Risk Criteria (>3% Annual Mortality)

The ACC/AHA guidelines establish that reversible defects involving >20% of myocardial segments are associated with significantly higher risk of perioperative cardiac death or MI, with risk increasing progressively as defect extent increases. 1 However, more recent evidence from the European Society of Cardiology and American College of Cardiology refines this threshold:

• Moderately abnormal defects encompassing ≥10% of myocardium indicate moderate-to-severe ischemia associated with high event rates (annual cardiac death or MI >3%). 1, 2
• Severely abnormal defects encompassing ≥15% of myocardium represent the highest risk category. 1
• The 10% threshold has been consistently reported across multiple prognostic series as the cutpoint where patients may benefit from invasive coronary angiography and revascularization. 1

Intermediate-Risk Criteria

• Reversible defects involving <20% of myocardial segments are associated with small, non-significant increased risk of perioperative death or MI. 1
• Meta-analysis of 1,179 patients demonstrated that defects <20% carried minimal incremental risk. 1

Low-Risk Criteria (<1% Annual Mortality)

• Normal or small myocardial perfusion defects at rest or with stress indicate low risk. 1
• Normal stress MPS is associated with approximately 1% annual cardiac death/MI rate. 2
• Normal scans have approximately 99% negative predictive value for cardiac events. 2

Additional High-Risk Markers Beyond Defect Size

Even when perfusion defects are <10%, the following findings indicate high risk:

• Transient ischemic left ventricular dilation (TID) during stress imaging 1, 2, 3
• Increased lung uptake of thallium-201 1
• Post-stress LVEF <35% or fall in LVEF ≥10% from rest 1
• Multiple perfusion defects of moderate size even if each individual defect is small 1
• Large fixed perfusion defect with LV dilation 1
• Stress-induced wall motion abnormalities involving >2 segments 1

Critical Clinical Context

The 20% vs 10% Threshold Discrepancy

The evidence shows an important evolution in risk stratification:

• Older ACC/AHA perioperative guidelines (2007,2009) used the 20% threshold primarily for perioperative risk assessment in noncardiac surgery patients. 1
• Contemporary cardiology guidelines (2015,2019) from the European Society of Cardiology and American College of Cardiology have refined this to 10% as the threshold for moderate-to-severe ischemia requiring consideration of revascularization. 1, 2
• The 10% threshold is now the accepted standard for identifying patients who may benefit from invasive management in chronic coronary syndromes. 1

Risk is Continuous, Not Categorical

Importantly, perioperative cardiac event risk increases continuously rather than categorically as the extent of reversible defects increases. 1 The 10% and 20% thresholds represent clinically useful cutpoints, but risk stratification should consider the full spectrum of defect size. 1

Left Main Disease Caveat

In patients with left main coronary artery disease, perfusion assessment alone underestimates disease severity in 44% of cases, with 13-15% showing no significant perfusion defect despite critical stenosis. 3 Combining perfusion data with non-perfusion variables (especially TID) identifies high risk in 83% of left main disease patients. 3

Practical Application Algorithm

For stress myocardial perfusion SPECT interpretation:

1. Quantify reversible defect size as percentage of LV myocardium
2. If ≥10% reversible defect: High-risk scan → consider invasive angiography and revascularization 1, 2
3. If <10% but ≥5% reversible defect: Intermediate risk → optimize medical therapy, consider functional capacity and clinical risk factors 1
4. If <5% reversible defect: Low risk → medical management appropriate 1
5. Regardless of defect size, assess for high-risk markers: TID, increased lung uptake, post-stress LVEF <35%, or multiple vascular territories involved 1, 2