Peripheral Arterial Disease
- Peripheral arterial disease (PAD) is plumbing failure in the leg arteries — atherosclerotic narrowing that starves muscle of blood, especially when it asks for more (walking).
- The hallmark symptom is claudication: cramping leg pain that comes on with walking and quits with rest. The scary end of the spectrum is critical limb ischemia — rest pain, ulcers, or gangrene.
- The cheap, brilliant bedside test is the ankle-brachial index (ABI) — a pressure ratio that needs nothing but a cuff and a Doppler probe.
- Imaging (ultrasound, CTA, MRA) doesn't diagnose PAD so much as map it — where the blockages are, how bad, and whether you can fix them.
- The artery rarely fails alone: a leg full of plaque usually means a heart and brain full of it too.
Think of the arteries in your leg as a garden hose feeding a sprinkler. When you're sitting still, even a kinked, half-clogged hose delivers enough water. But crank the sprinkler up — start jogging — and suddenly the muscle demands a firehose while the clogged pipe can only offer a trickle. The muscle cramps, you stop, the demand drops, and the pain melts away. That on-with-exercise, off-with-rest pattern is peripheral arterial disease in a nutshell, and the cramping it causes has a fancy name: claudication (from the Latin for "to limp").
What's actually clogging the pipe
PAD is almost always plain old atherosclerosis — the same cholesterol-and-calcium gunk that builds up in coronary arteries, just plumbed into the legs. Plaque narrows the lumen, blood flow downstream drops, and the muscle gets shortchanged whenever it works hard.
The favorite locations are predictable: the superficial femoral artery in the thigh (especially down where it ducks behind the knee), the iliac arteries in the pelvis, and the smaller runoff vessels below the knee. Knowing where matters, because a blockage high up in the pelvis and a blockage down at the ankle cause similar symptoms but call for very different fixes.
PAD is a "canary in the coal mine" disease. If atherosclerosis has clogged the leg arteries badly enough to hurt, it's almost certainly busy elsewhere — coronary arteries, carotids, kidneys. The leg pain is often the first thing the patient notices, not the most dangerous thing happening.
The spectrum: from annoying to limb-threatening
Not all PAD is created equal. It runs along a spectrum:
| Stage | What the patient feels | What it means |
|---|---|---|
| Asymptomatic | Nothing | Narrowing exists but the leg copes at rest and with light activity. |
| Claudication | Cramping with walking, gone with rest | The classic, predictable mismatch between supply and demand. |
| Rest pain | Burning/aching in the foot at rest, worse lying flat | Supply is now too low even for a resting foot. Ominous. |
| Tissue loss | Non-healing ulcers, gangrene | Critical limb ischemia — the limb is at risk. |
That bottom row is the emergency end. Critical limb ischemia means the trickle is no longer enough to keep tissue alive, and the clock starts ticking on the limb. This is where imaging stops being academic and becomes a roadmap for saving a foot.
The test that costs almost nothing: the ABI
Before any fancy scanner, there's the ankle-brachial index (ABI) — and it's genuinely elegant. You measure the blood pressure at the ankle and divide it by the pressure in the arm. In a healthy person the ankle pressure is at least as high as the arm's, so the ratio sits around 1. When leg arteries are clogged, ankle pressure sags and the ratio drops. The lower it falls, the worse the disease.
One honest caveat: in patients with heavily calcified vessels (think long-standing diabetes or kidney disease), the arteries can be too stiff to squash with a cuff. The pressure reads falsely high, and the ABI can look reassuring — or even impossibly elevated — while the leg is starving. A normal-looking number in the wrong patient deserves suspicion, not relief.
Don't trust a "normal" or sky-high ABI in a diabetic or dialysis patient. Stiff, calcified arteries resist the cuff and fake a good number. When the clinical picture and the ABI disagree, believe the foot.
Where imaging earns its keep
Here's the key mindset shift: imaging usually isn't there to prove PAD exists — the history and ABI often did that already. Imaging is there to map the battlefield before anyone intervenes: which segment is blocked, how long the blockage is, and what healthy vessel exists above and below to reconnect to.
Ultrasound with Doppler is the gentle first look. Grayscale shows the plaque; the Doppler part listens to the blood and tells you how fast it's racing through a narrowing — speed jumps where the pipe pinches. It's cheap, radiation-free, and great for a focused question, but tedious for mapping a whole leg.
CT angiography (CTA) is the workhorse for mapping. A bolus of contrast lights up the whole arterial tree from the aorta to the toes, and you get a beautiful 3D map in minutes. Its weakness is dense calcium, which can bloom and hide whether the lumen underneath is open — particularly cruel in the small below-knee vessels.
MR angiography (MRA) maps the same territory without ionizing radiation and without calcium blooming, which makes it attractive for the calcified-vessel crowd. The trade-offs are longer scans and contrast considerations. The deeper comparison of how these map vessels lives in CTA & MRA.
Why the map matters
The treatment hinges entirely on the picture. A short, isolated narrowing in a big pelvic vessel is a tidy target for a balloon and stent. A long, diffusely diseased segment with poor vessels below it may instead need a surgical bypass — or, if the limb is too far gone, frank conversation about amputation. The angiography and endovascular team lives in this world, threading wires through the very blockages the CTA revealed.
So if you remember one thing: PAD is a supply-and-demand problem in the leg's plumbing. The history tells you it exists, the ABI grades it, and imaging draws the map that decides whether it's a balloon, a bypass, or a hard conversation.