Deep Vein Thrombosis
- Deep vein thrombosis (DVT) is a clot inside a deep vein — usually a leg — that should be carrying blood back to the heart.
- The first-line test is compression ultrasound: a normal vein squishes flat under the probe; a clotted one won't.
- The headline danger isn't the leg — it's the piece that breaks off and sails to the lungs as a pulmonary embolism.
- Acute clot is soft, expands the vein, and is poorly compressible; chronic clot is shrunken, scarred, and stuck to the wall.
Imagine the veins in your leg as the return-trip lane of a highway, quietly shuttling blood back up to the heart. A DVT is a traffic jam in that lane — a clot parked where blood should be flowing. Annoying locally, sure. But the real fear is that a chunk of that jam dislodges, rides the express lane straight to the lungs, and causes real trouble. That's the whole reason we chase these clots so hard.
What a DVT actually is
"Deep" is the operative word. Legs have two plumbing systems: superficial veins near the skin and deep veins buried with the arteries. A clot in a superficial vein is usually a nuisance; a clot in a deep vein is the one that can throw emboli. So when we say DVT, we mean the deep system — most commonly the leg veins from the calf up through the thigh.
Clots love three conditions, classically remembered as the trio of sluggish flow, vessel-wall injury, and blood that's too eager to clot. In plain terms: blood that sits still (long flights, hospital beds, casts), a vein that's been irritated, and a clotting system turned up too high (surgery, cancer, pregnancy, certain medications). Stack a few of those and you've built the perfect parking spot for a clot.
The test: compression ultrasound
The workhorse here is ultrasound, and the trick is delightfully low-tech. A healthy vein is a floppy, low-pressure tube. Press the probe down and a normal vein collapses flat — the walls kiss together like squeezing an empty drinking straw. A vein packed with clot is stuffed; press all you like and it won't squish shut.
That's the single most useful sign in the whole exam: loss of compressibility. Everything else is supporting evidence.
A normal deep vein compresses completely and disappears under gentle probe pressure. A non-compressible vein that stays round is a clot until proven otherwise.
We add Doppler to listen to flow. Normal leg-vein flow rises and falls gently with breathing and surges when you squeeze the calf. A clotted segment is quiet — no flow where there should be, or flow detouring around the blockage.
Acute versus chronic: reading the clot's age
Not all clots are fresh, and telling new from old changes what you do next.
| Feature | Acute clot | Chronic clot |
|---|---|---|
| Vein caliber | Expanded, bulging | Shrunken, contracted |
| Echotexture | Soft, often dark/anechoic | Brighter, organized, scarred |
| Wall attachment | Loosely filling the lumen | Stuck to the wall, retracted |
| Compressibility | Absent (spongy, non-compressible) | Absent, but vein is small and firm |
Think of acute clot as a fresh, plump jelly filling the pipe, and chronic clot as that same jelly left out for weeks — dried, shriveled, and welded to the wall. The distinction matters because old scarred clot can leave a vein permanently non-compressible, which fakes out the unwary.
A chronically scarred vein never fully compresses again, so on a later scan it can look like a "DVT" forever. Without an old study to compare, a small, shrunken, brightly echogenic vein points to old disease — not a fresh clot needing urgent treatment. Compare with priors whenever you can.
When ultrasound isn't the answer
Ultrasound owns the legs, but it can't see everywhere. Clot in the pelvic veins or the inferior vena cava sits too deep for a good squeeze, so we lean on CT or MR venography there. And if a patient turns up short of breath, the lungs become the priority — the question quietly shifts from "is there a clot in the leg?" to "did a piece already reach the lungs?"
A negative leg ultrasound does not rule out a pulmonary embolism. The clot may have already left the leg, or it may live in veins the probe can't reach. If the clinical concern is the lungs, image the lungs.
Why we care so much
Treatment is usually blood thinners — not to dissolve the existing clot so much as to stop it growing and buy time for the body to organize and stabilize it. The goal is preventing that breakaway piece. When thinners can't be used safely, an IVC filter can act as a literal strainer in the big return vein, catching clots before they reach the lungs.
So the one thing to carry out of here: a DVT is a parked clot in a deep vein, the ultrasound question is simply "does this vein squish flat?", and the reason the whole machine springs into action is the trip that clot might take to the lungs.