Frequently Asked Questions

faq

 

1. Why do you use the short axis out-of-plane technique for catheter placement? It seems more logical to use the short axis in-plane technique. With this approach, one can place the catheter more precisely through visualisation of the needle shaft/tip and catheter?

 

Several reasons:

1. The short axis out-of-plane technique facilitates catheter threading as the needle and catheter are orientated in the direction of the nerve and surrounding fascia/muscle.

2. Orientation of the needle more parallel to the nerve should result in the catheter staying alongside the nerve even if threaded several cm beyond the needle tip. In contrast, with the short axis in-plane technique (needle perpendicular to nerve), threading the catheter several cm beyond the needle tip may result in the catheter deviating away from the nerve. 

3. With the catheter positioned more parallel to the nerve, a multi-orifice catheter will have all orifices lined up alongside the nerve (the most proximal orifice can be 1.5 cm from the catheter tip).

4. It can be particularly challenging visualising the catheter both in real time, and "indirectly" by visualising catheter injectate. Neither of these two methods have been strongly validated in a clinical setting.

5. Level 1 evidence (at least one RCT) supports the short axis out-of-plane approach.

 

A good analogy is vascular cannulation. Anesthesiologists almost always use a short axis out-of-plane technique. Cannulating a vessel with the needle approaching from perpendicular can be particularly challenging.

 

2. Why do you not attempt catheter visualisation - either directly (the catheter itself) or indirectly (catheter injectate) as a way of positioning the catheter more precisely?

 

1. Both methods can be technically challenging for both out-of-plane and in-plane techniques. Furthermore, the short axis in-plane technique has its own set of problems for catheter placement (see FAQ immediately above). For operators experienced in UGRA, studies have shown that in 25% of patients, catheters cannot be visualised despite the in-plane technique. 

2. A catheter advanced blindly a short distance (< 3cm) beyond the tip of a needle orientated in the direction of the nerve (or < 2 cm beyond a perpendicularly orientated needle) simply cannot deviate significantly away from the nerve.

3. What does the operator do if they can't visualise the the catheter/injectate when the needle has been removed? If they remove the catheter and reinsert, they will be removing a good proportion of satisfactorily sited catheters. 

 

A good analogy is the stimulating catheter. Stimulating catheters (analogous to catheters placed using direct ultrasound visualisation) do not perform any better than catheters blindly advanced a short distance (< 3cm) beyond the needle tip.