Coronary Artery Anatomy & Segments
- There are two coronary arteries off the aorta: the left (which usually splits almost immediately into the LAD and the circumflex) and the right (RCA).
- The LAD feeds the front and the septum, the circumflex wraps around the left side, and the RCA runs down the right and usually feeds the bottom of the heart.
- "Dominance" just answers one question: who supplies the back wall? About 85% of people are right-dominant.
- The 18-segment SCCT map is just a shared address system so two readers can point at the exact same stretch of artery.
- Name the vessel and the segment, every time — "a lesion in the LAD" is fine, "a lesion in the proximal LAD" is what changes management.
The heart is a muscle, and muscle is hungry. The coronary arteries are the takeout delivery service that keeps it fed — and the cruel irony is that they sit right on the outside of the busiest pump in the body and still manage to clog. Learn the layout once and the rest of cardiac imaging stops feeling like alphabet soup.
Here's the friendly version: there are only two front doors off the aorta. Everything else is just branches off those two. If you can hold that in your head, you're already halfway home.
The two main trunks
Just above the aortic valve, the aorta bulges out into little pockets called the sinuses of Valsalva, and two of them have a coronary artery poking out the top.
The left coronary comes off the left sinus as a short, fat stub called the left main — think of it as a hallway about a centimeter long that immediately forks. One fork dives down the front groove between the two ventricles: that's the left anterior descending (LAD). The other fork swings around the left side of the heart like an arm reaching behind someone for a hug: that's the left circumflex (LCx).
The right coronary (RCA) comes off the right sinus and runs down the groove between the right atrium and right ventricle, curving toward the back and bottom of the heart.
That early fork is so consistent that we name it: when the left main splits cleanly into LAD and LCx, fine. When it throws off an extra branch between them, that bonus vessel is the ramus intermedius. It's a normal variant, not a problem — just say its name so nobody mistakes it for an LAD.
What each artery actually feeds
The reason we care which artery is sick is that each one supplies a predictable chunk of muscle. Match the vessel to the wall and you can predict where the heart will fail before the patient ever has the event — the foundation of reading ischemia.
| Artery | Major branches | Territory it feeds |
|---|---|---|
| LAD | Diagonals, septal perforators | Anterior wall, anterior septum, the apex |
| LCx | Obtuse marginals | Lateral wall of the left ventricle |
| RCA | Acute marginal, AV nodal branch | Right ventricle, and usually the inferior wall |
The LAD is the one everyone fears, because it feeds the most muscle. A clot high up in it earns the grim nickname "the widow-maker" — not an official term, but it tells you how much real estate is downstream.
Dominance: who owns the back wall?
This is the single most useful piece of jargon on the page, and it sounds scarier than it is. Dominance answers exactly one question: which artery gives off the posterior descending artery (PDA), the vessel that runs down the back groove and feeds the bottom of the heart?
- Right-dominant — the PDA comes off the RCA. This is most people, roughly 85%.
- Left-dominant — the PDA comes off the circumflex instead.
- Co-dominant — they split the job.
Dominance is decided by the PDA's parentage, not by which artery looks bigger. A left-dominant heart means a circumflex problem can knock out the inferior wall — a twist you won't predict unless you checked dominance first.
The 18-segment address system
Saying "there's a plaque in the LAD" is like telling a friend you're "somewhere on Main Street." The Society of Cardiovascular Computed Tomography (SCCT) map chops the coronary tree into numbered segments — 18 of them — so two readers anywhere in the world can point to the exact same stretch of vessel.
The logic is simple: each major artery is cut into proximal, mid, and distal thirds, plus named side branches get their own numbers. "Proximal LAD" and "distal LAD" are wildly different problems — one threatens a huge territory, the other a sliver — so the segment, not just the vessel, is what drives whether someone goes to the cath lab.
When you dictate, always pair the noun with the address: "calcified plaque, proximal LAD," not just "LAD disease." The treating cardiologist plans the procedure off your segment, and a vague report makes you the bottleneck.
Where normal stops and "variant" starts
Coronary plumbing is gloriously inconsistent, and most of the variety is harmless. A short left main, a chunky ramus intermedius, a left-dominant system — all normal, all worth naming.
The dangerous ones aren't the size differences — they're the anomalous origins, where an artery starts from the wrong sinus and takes a path squeezed between the aorta and pulmonary trunk. That course can be lethal in young athletes, so it graduates to its own page on coronary anomalies. On a normal study, just confirm both arteries arise from their correct sinuses before you call it boring.
If you remember nothing else: two front doors, three main rivers, and one question about the back wall. Get the anatomy reflexive here and the rest — protocols and gating, calcium scoring, all of it — is just describing what's wrong with a map you already know by heart.