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The left main is the single artery that delivers blood to most of your heart muscle. A narrowing here is the most consequential blockage anywhere in your coronary system, because it can starve a large area of heart tissue all at once.
This number reports the worst narrowing along that artery, as a percentage of the original opening. It is a value that can change treatment urgency overnight, which is why understanding what it means before symptoms appear is worth doing.
Most coronary arteries supply one region of the heart. The left main is different. It is a short trunk that branches into two of the three major arteries on the front and side of your heart. A serious narrowing in this segment threatens roughly two thirds of the heart's muscle territory at once, which is why even moderate disease here is treated with more urgency than a comparable narrowing further downstream.
The measurement you receive, Max Stenosis LM (maximum stenosis of the left main coronary artery), captures the worst spot of narrowing along this short artery. It is reported as a percentage, where 0% means no narrowing and 100% means total blockage.
Clinical practice treats a narrowing of at least 50% as significant, because this level of obstruction has clear consequences for blood flow and survival. A narrowing above 70% is considered severe and carries a meaningfully higher risk of death. These thresholds, drawn from decades of cardiology research, are the same whether the measurement comes from a CT scan (computed tomography of the coronaries) or from an invasive angiogram (a direct dye study performed in the catheterization lab).
About one in three left main narrowings are misclassified when the percentage from an angiogram is used by itself. Roughly 23% of left main lesions that look under 50% on imaging are actually functionally significant when blood flow is directly measured, and about 6% of lesions that look over 50% are not flow-limiting at all. This is why a borderline reading often triggers follow-up tests rather than immediate action.
Two follow-up methods are commonly used. Fractional flow reserve, called FFR, measures how much pressure drops across a narrowing during peak blood demand. A value of 0.80 or lower is treated as functionally significant. Intravascular ultrasound, called IVUS, threads a tiny camera into the artery and measures the actual opening size in square millimeters. An opening of 4.5 square millimeters or smaller is treated as significant, while 6 square millimeters or larger is generally considered safe to leave alone.
There is one finding that often surprises people. In a large international registry of more than five thousand adults, narrowings under 50% in the left main were still linked to worse long-term outcomes in women. The takeaway is not that the threshold is wrong, but that the left main is so consequential that even modest disease here deserves attention. A 30% to 40% reading is not a clean bill of health. It is a warning that the underlying process of plaque buildup has reached the most important real estate in your heart, and that aggressive prevention is warranted.
These thresholds come from cardiology guideline bodies and large angiographic and CT-based research cohorts. Different imaging methods can produce slightly different numbers for the same artery, so the practical interpretation depends on which scan was used and how it was read.
| Tier | Narrowing | What It Suggests |
|---|---|---|
| Normal | Under 25% | No meaningful obstruction. Prevention remains the focus. |
| Mild | 25% to 49% | Plaque is present in a high-stakes location. Aggressive risk factor control is warranted. |
| Significant | 50% to 70% | Functional assessment with FFR or IVUS is appropriate. Revascularization may be needed. |
| Severe | Above 70% | High mortality risk. Revascularization is typically recommended. |
Source: synthesis of cardiology guideline reviews and angiographic cohort data including Hamilos et al, Jasti et al, Park et al, and Collet et al.
Atherosclerosis is a slow disease, and a single image is a snapshot of a moving process. Two readings taken a year or two apart tell you something more useful than one reading alone. They tell you whether the narrowing is stable, growing, or shrinking in response to your prevention efforts.
If your baseline scan shows any plaque in the left main, a reasonable cadence is to repeat advanced imaging every one to two years for moderate disease, or sooner if symptoms change or if you are testing whether an aggressive prevention regimen is working. If the baseline shows no disease in the left main, the cadence depends on your overall calcium score and risk profile, typically every three to five years.
What you do next depends on the percentage and on whether you have symptoms. A reading of 50% or higher on a CT scan should prompt an in-person visit with a cardiologist, ideally one with interventional or imaging expertise. The next step is rarely immediate intervention. It is usually a functional assessment to see if the narrowing actually restricts blood flow.
Heavy calcium deposits in the artery wall can make a narrowing on a standard CT scan look worse than it is. Newer photon-counting CT scanners reduce this overestimation by about 11% in heavily calcified segments, which can change whether a borderline lesion needs further investigation. If your scan was done on older equipment and the calcium burden is high, ask whether photon-counting CT or invasive assessment would give a more accurate read.
A second source of confusion is compression from outside the artery. In people with severe pulmonary hypertension (high pressure in the lung circulation), the enlarged main pulmonary artery can press on the left main from the outside. The artery itself may be healthy, but the imaging shows narrowing. This is uncommon but worth knowing, especially if you have a known lung circulation problem.
Finally, percentage stenosis on its own is imperfect. About one in three left main readings disagree with functional flow measurements. A normal-looking left main can still be flow-limiting in a minority of cases, and a borderline-looking left main can sometimes be benign. This is why the percentage is best understood as a starting point, not a final verdict.
Evidence-backed interventions that affect your Max Stenosis LM level
Max Stenosis LM is best interpreted alongside these tests.