2. IR units of measurement

You will quickly learn that we use wires, catheters and needles that come in a confusing variety of measurement systems. 

Gauge: most of our needles are labeled by Gauge, which is 1/inch , or the number of needles that would fit into an inch.  That means a 20 g needle is 1/20th an inch. That's why an 18 gauge needle is larger than a 22 gauge needle.

French: Our catheters and sheaths are labeled as French.  1 French is 1/3 of a mm.  Remember also that catheters are labeled by their Outer Diameter, while sheaths are labeled by their inner diameter. Therefore a 5 Fr catheter will fit inside a 5 Fr sheath.  The outer diameter of a 5 Fr sheath is usually closer to 7 or 8 Fr. 

Inches: Most guide wires are labeled as inches.  The two most common  sizes are 0.035 inch (Standard guidwire) and 0.018 (Microwires).

A typical 0.018 microwire will fit within a typical 21 gauge access needle (micropuncture).  A standard 035 will fit through a 19 gauge needle.  The needles are labeled by their outer diameter and so its always smart to test a specific access needle to make sure it accepts the wire.