Catheter Inner and Outer Diameter: Reading the Dimensions
How to read neurosurgical catheter sizes such as 1.3/2.5 and 0.8/1.6 mm: inner and outer diameter, wall thickness, French conversion and specifications.
A catheter size such as 1.3/2.5 mm gives two numbers: the inner diameter (ID), the width of the channel that carries CSF, and the outer diameter (OD), the overall width of the tube. DESU neurosurgical catheters mainly use four pairs: 1.3/2.5 ventricular, 1.1/2.5 peritoneal, 0.8/1.6 lumbar and 1.5/3.0 for the EVD ventricular catheter.
Key takeaways
- Inner diameter is the width of the channel; outer diameter is the overall width of the tube.
- DESU catheters use four main pairs: 1.3/2.5, 1.1/2.5, 0.8/1.6 and 1.5/3.0 mm.
- Half the difference between the two numbers gives the nominal wall thickness.
- Write ID, OD and length separately in millimetres and centimetres when specifying a catheter.
Theatre teams and buyers comparing catheters on paper need to know what each figure affects and how to write it in a specification. Which catheter suits a patient is a decision for the surgeon, following the instructions for use (IFU).
What do the two numbers in a catheter size mean?
The inner diameter describes the lumen: the channel that CSF travels through, and the space that a stylet occupies during insertion. The outer diameter describes what the outside world meets: the tissue track, a needle, a connector.
Sizes here are written ID first, then OD, so 0.8/1.6 mm means 0.8 mm inner and 1.6 mm outer diameter. Not every document uses that order, so when a size appears without labels, check which is which before comparing two products. Diameters are in millimetres while catheter length is usually given in centimetres, so a line such as “1.3 mm, 2.5 mm, 23 cm” mixes two units.
What are the dimensions of each DESU catheter?
| Catheter | Inner diameter | Outer diameter | Length | Wall (calculated) |
|---|---|---|---|---|
| Ventricular | 1.3 mm | 2.5 mm | 23 cm | 0.6 mm |
| Peritoneal | 1.1 mm | 2.5 mm | 120 cm | 0.7 mm |
| Lumbar shunt | 0.8 mm | 1.6 mm | 100 cm | 0.4 mm |
| Lumbar drainage | 0.8 mm | 1.6 mm | 80 cm | 0.4 mm |
| EVD-010 ventricular | 1.5 mm | 3.0 mm | 31 cm | 0.75 mm |
The wall column is calculated: OD minus ID, divided by two. It is useful for comparison, but it is a nominal figure. If your specification needs tolerances, ask our product specialists.
Standard and antibiotic-impregnated versions share the same dimensions, so a change of version does not change these figures. The ventricular catheter kit, peritoneal catheter kit and lumbar catheter kit pages list each set of figures in full. Our article on ventricular catheter design covers the holes and marks that sit alongside them.
What does inner diameter affect?
In any tube, resistance to flow rises as the channel narrows and as the tube gets longer, and narrowing has the stronger effect. That is why ID and length are best read together. A 120 cm peritoneal catheter with a 1.1 mm lumen and a 23 cm ventricular catheter with a 1.3 mm lumen are different conduits, even with the same outer diameter.
Keep that principle in proportion. In a shunt, the valve is the part designed to govern when CSF passes. In external drainage, the level set on the scale plays that role. Catheter dimensions alone do not rank products by drainage performance.
Inner diameter also sets what fits inside the catheter. The stylet supplied with a ventricular kit travels in the lumen, so use the stylet that comes with the kit rather than one from elsewhere.
What does outer diameter affect?
Outer diameter matters at the points where the catheter meets something else:
- Introduction: a lumbar catheter is placed through a needle, so its OD has to suit the needle. The DESU lumbar catheter kit pairs the 1.6 mm catheter with a 14 G Tuohy needle and a guidewire.
- Connections: a catheter has to seat securely on a valve port or connector. Pair components by kit and by the IFU rather than by nominal OD.
- Component mixing: two catheters with a 2.5 mm OD from different makers are not automatically interchangeable on the same connector. Check before combining them.
The ventricular and peritoneal catheters share a 2.5 mm OD but differ in ID, so their walls differ too. The two lumbar catheters are the reverse case: identical diameters, different lengths, and different jobs. The lumbar drainage catheter kit is for external drainage, the lumbar catheter kit for shunting.
How does the EVD catheter differ from the shunt ventricular catheter?
The EVD-010 catheter is 1.5/3.0 mm and 31 cm, against 1.3/2.5 mm and 23 cm for the shunt ventricular catheter. An external drain often has to handle CSF containing blood or debris, and a wider channel gives more room for that. The two are also different products: one is connected to an external drainage system, the other to an implanted valve.
The 1.5/3.0 mm pair is not the only EVD size. DESU also makes antibiotic-impregnated EVD catheters at 1.5/3.0 mm and 35 cm, and at 1.9/3.4 mm and 35 cm. Those codes show why it pays to read the description: EVDACK-1530350 is 1.5 mm ID, 3.0 mm OD and 35 cm, while other codes carry no dimensions at all.
How do millimetres convert to French?
DESU specifies catheters in millimetres, but some hospital systems use the French scale, where 1 Fr equals one third of a millimetre of outer diameter. To convert, multiply the OD in millimetres by three:
| Outer diameter | French equivalent | DESU catheters |
|---|---|---|
| 1.6 mm | About 4.8 Fr | Lumbar shunt, lumbar drainage |
| 2.5 mm | 7.5 Fr | Ventricular, peritoneal |
| 3.0 mm | 9 Fr | EVD-010 ventricular |
French describes the outside only and says nothing about the lumen, so two 7.5 Fr catheters can have different IDs, as the ventricular and peritoneal catheters do. Gauge is a separate scale again: the “14 G” in a lumbar kit describes the needle, not the catheter. Definitions for these terms are in our neurosurgical catheter glossary.
How should dimensions be written in a specification?
- State ID, OD and length as three separate values, with units, rather than as a single pair of numbers.
- Use millimetres for diameters and centimetres for length, and add French only as a reference.
- State the catheter’s role (ventricular, peritoneal, lumbar shunt, lumbar drainage, EVD), because lumbar catheters with the same diameters differ in length.
- State the version: standard or antibiotic-impregnated.
- Ask for tolerances if the evaluation depends on them, because the figures above are nominal.
Talk to a DESU product specialist
For a full dimension table, product codes or samples for evaluation, write to sales@desu.tr, use the contact page or request a quote.
Frequently asked questions
Is a larger inner diameter always better?
Not automatically. For a given outer diameter, a wider lumen leaves a thinner wall, and the catheter also has to suit the route and the components it connects to. The surgeon weighs these together with the IFU.
Do antibiotic-impregnated catheters have different diameters?
No. DESU antibiotic-impregnated catheters keep the dimensions of the standard catheter. They are told apart by colour: impregnated catheters are orange, standard ones white.
Why do the two lumbar catheters have different lengths?
They serve different routes. The 100 cm catheter is for lumboperitoneal shunting and the 80 cm catheter is for external lumbar drainage. Both are 0.8 mm inner and 1.6 mm outer diameter.
Are the DESU peritoneal and ventricular catheters the same tube at different lengths?
No. They share a 2.5 mm outer diameter, but the ventricular catheter has a 1.3 mm lumen and the peritoneal catheter a 1.1 mm lumen. They also differ in length, length marks and drainage section.
Related standards
- ISO 13485: Medical devices — Quality management systems
- ISO 7197: Neurosurgical implants — Sterile, single-use hydrocephalus shunts and components