Access, wires, catheters and sheaths

Why this matters

You walk into the angio suite and within a minute someone asks for a Cope wire through the 21, then a five French, then a Glide. Nothing in diagnostic radiology training teaches you that sentence. The gap is not intelligence or effort — it is vocabulary, and it is the single fastest thing to close. Trainees entering IR consistently lack familiarity with the appearance, properties and nomenclature of basic equipment, and with the relationships between the French, inch and gauge systems, which slows procedures down and makes it hard to be useful in a case.

This guide covers the equipment and the numbers. It is not a technique guide, and reading it does not make you competent to select or use any of these devices. What it should do is let you follow a case, hand over the right thing when asked, and read a procedure guide without stalling.

Three measurement systems, one trolley

The first source of confusion is that the three device families on the trolley are measured in three different units, and one of them runs backwards.

Needles

The needle makes the first entry and determines what wire you can pass next. Needle hubs are colour-coded by gauge, which is how people identify them at a glance on a crowded trolley — though the colour scheme is a manufacturer convention, so learn the one your department actually stocks.

Single-wall (bevelled)

A hollow needle with one angled cutting edge and a notch on the hub marking which way the bevel faces. The most commonly used needle in IR. It is steerable — a bevelled needle tends to deviate away from the bevel as it advances, which the operator uses to adjust course.

Trocar

A two-part system: an outer cannula with a removable inner three-sided sharp stylet (trois carré — three-sided). The symmetrical tip means it does not steer, but it is stiff, easy to see, and commonly 18 or 21 gauge in 10, 15 or 20 cm lengths.

Chiba

A fine two-part needle with a bevelled, and therefore steerable, tip. Commonly 20 or 22 gauge, 15 or 20 cm. The standard choice for biliary and renal access.

Echo-tip

Any of the above with a roughened tip that scatters ultrasound and so shows up far better on the screen. Worth asking for whenever the target is being approached under ultrasound.

• 18, 19 and 21 gauge are the usual sizes for arterial and venous access.
• 22 and 21 gauge suit small targets — a small abscess, a calyx, a small vessel — where a miss costs less.
• 18 gauge and larger suit big, accessible targets and biopsies, and take a 0.035-inch wire directly without an exchange.

Guidewires

Every wire is described by three properties — diameter, stiffness, and whether it is hydrophilic — and it is worth learning wires by what they are for rather than by brand name. Departments stock different wires; the three jobs are the same everywhere.

Access wire

Short and simple. Its only job is to hold the path you have just made while you swap the needle for a sheath. Usually exchanged out within a minute or two.

Manoeuvre (selective) wire

Floppy, often curved and hydrophilic at the tip. Used to find and enter the vessel you actually want. Frequently paired with a torque device — a small handle clamped onto the wire so it can be rotated precisely.

Rail (support) wire

Stiff. Its job is to be a stable track. Once a rail wire is parked somewhere safe, you can exchange catheters over it, or pass a balloon, stent or drain, without losing your position.

Hydrophilic wires have a coating that becomes slippery when wet, which is what lets them slide through tight or tortuous segments with less resistance. The same coating makes them behave badly when they dry out — they become tacky and drag. Keep them wet and flush them; a hydrophilic wire left dry on the drape is a wire that will not run. They are also harder to grip, which is the other reason for a torque device. Non-hydrophilic wires are easier to hold and to control by hand, at the cost of more resistance through anything narrow.

What fits through what

This is the practical core of the whole subject, and it is two rules.

Which raises the obvious problem: most vascular access is taken with a 21-gauge needle, because a smaller puncture is safer and more forgiving — but a 21-gauge needle will not pass a 0.035-inch wire. The micropuncture set exists to solve exactly this, and it is why the sequence below is the backbone of almost every vascular case.

1. 01Puncture with the 21-gauge needle under ultrasound or fluoroscopic guidance.
2. 02Pass a 0.018-inch access wire through the needle.Decision point: The operator confirms the tip is genuinely in the intended vessel or space — by flow, aspiration and imaging — before any wire is advanced.
3. 03Remove the needle over the wire, leaving the wire in place.
4. 04Advance the micropuncture sheath (commonly 4 or 5 F) over the 0.018-inch wire.
5. 05Remove the 0.018-inch wire and place the 0.035-inch wire you actually want through the sheath.

Sheaths

A sheath holds the access open so instruments can be exchanged repeatedly through one puncture. It arrives as an assembly: a tapered dilator sits inside the sheath, and the two go over the wire together — the dilator gradually opens the tract so the blunter sheath can follow. Flush the sheath before use, and never advance a sheath without its dilator in place.

• Sheath French is the inner diameter — it tells you the largest catheter that will fit through it.
• Sheaths come straight or curved; a curved sheath helps hold a direction in a difficult arch or aorta.
• The sheath size you need for a balloon or stent is not a guess. It is printed on the device's package insert — read it before the sheath goes in, not after.
• Sheath hubs are colour-coded by size (a common convention runs 4 F red, 5 F grey, 6 F green, 7 F orange, 8 F blue, 9 F and above black). Useful shorthand, but manufacturer-specific — check what your own department stocks before relying on it.

Catheters

There are a great many catheters and you do not need to know them all. Four distinctions cover almost every conversation you will hear: end-hole or side-hole, selective or non-selective, hydrophilic ("glide") or not, and shape. Glide catheters track more easily through vessels but are floppier and give less support — the same trade-off as with wires.

Putting it together

The pattern — needle, wire, sheath, catheter, target — is the same whether the target is an artery, a bile duct or an abscess. What changes is which member of each family gets chosen, and that choice follows from the size of the target and what has to be delivered at the end. Two illustrations of how the reasoning runs:

Once you can predict the equipment from the problem, rather than memorising kit lists per procedure, the trolley stops being intimidating.

What goes wrong

Three equipment failures are worth knowing before you ever touch anything, because two of them leave something behind in the patient and all three are avoidable.

Shearing

Pulling a wire back forcefully through a bevelled needle can slice the wire against the needle's cutting edge, leaving a fragment inside the patient. The coating of a hydrophilic wire can strip the same way. The rule that prevents it: if a wire will not advance or will not withdraw freely, take the needle and the wire out together as one unit.

Kinking

Forcing a stiff dilator or catheter over a non-hydrophilic wire at too sharp an angle can put a permanent kink in the wire. It does not straighten out, and the catheter will not pass it. The usual fixes are to exchange the wire, or to draw the kinked segment back inside the catheter or sheath so it is held straight.

Advancing bare

Pushing a catheter with no wire ahead of it, or a sheath with no dilator, exposes a blunt or cut edge to the vessel wall. Scraping, dissection and perforation follow from this more often than from anything exotic.

What confuses registrars

• Gauge runs backwards and French does not. An 18G needle is bigger than a 22G; an 8 F sheath is bigger than a 5 F.
• Sheath French is the inner diameter, catheter French is the outer diameter. This is why a 5 F catheter fits a 5 F sheath.
• Hydrophilic does not mean better. It means slippery when wet, tacky when dry, and harder to grip. Keep it wet.
• Wires are not interchangeable because they share a diameter. A 0.035 Amplatz and a 0.035 Glidewire do completely different jobs.
• Exchanges are not indecision. Moving from a manoeuvre wire to a rail wire once you are in position is the plan, not a correction.
• "Give me a five French" almost always means a sheath, not a catheter. Listen for what is about to be passed through it.

The fastest way to consolidate any of this is to attach it to a case. Before your next list, find out what access and what device sizes are planned and why; during the case, name each item as it comes off the trolley; afterwards, write down what was used against the indication. That note is also the beginning of the logbook habit covered in the research, governance and career pillar.