Measurements
Gauge
The sizes of needle-based instruments (such as Venflons and needles themselves) are measured in 'Gauge' (ie. how many can be fitted into a tube of a fixed diameter). In Gauge, larger numbers, mean smaller diameter lumens.
French
Catheters use the 'French' system. This is the circumference of the tube in millimetres, eg. a 30F catheter has a circumference of 30mm. In French, larger numbers therefore mean larger diameter lumens. Bear in mind that catheters with greater French are often more 'stiff'. Paradoxically, it can therefore sometimes be easier to try a greater sized catheter if insertion is proving difficult (eg. in elderly men with enlarged prostates). Remember, however, never to force the catheter - it should pass smoothly.
